CONTENTS Sl.No Lecture Notes Page No. Dynamic Action of ...
Transcript of CONTENTS Sl.No Lecture Notes Page No. Dynamic Action of ...
CONTENTS
Sl.No Lecture Notes Page No.
1Dynamic Action of Fluid and Concept of Velocity
Triangles1
2 Water Turbines 24
3 Gas and Steam Turbines 38
4 Industrial Pumps and its Applications 79
5 Computational Fluid Dynamics 87
6 Dimensional Analysis 106
7 Pnematics 119
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DYNAMIC ACTION OF FLUID ON VANESDynamic force
Consider a stream of fluid entering a machine such as a hydraulic turbine or steam
turbine, or a pump or a fan. The stream of fluid has a more or less defined direction. For a
force to be exerted by the fluid on the machine, the stream of fluid must undergo a
change in its velocity either in its magnitude or direction or both. When the fluid stream
enters the machine, the machine exerts a force on the fluid bringing about a change in the
velocity of fluid either in its magnitude or in its direction. According to Newton’s third
law of motion for every action there is an equal and opposite reaction. Hence, the fluid
stream exerts an equal and opposite force upon the machine that causes the change in
velocity of fluid stream. This force exerted by the virtue of fluid in motion is called the
dynamic force of fluid. The dynamic force of fluid always involves a change in its
velocity and thus a change in its momentum. Hence, the force exerted by the machine on
the fluid is the action and the force, in turn, exerted by the fluid on the machine is the
reaction.
The major problem in hydraulic machinery is to determine the power developed by a
particular machine or the power consumed in a particular machine. For instance, a
hydraulic turbine develops power while a pump consumes power in order to run. The
power consumed or developed by a machine can be determined from the dynamic force
or forces exerted by the flowing fluid on the boundaries of the flow passage and which
are due to the change of momentum. These are determined by applying Newton’s second
law of motion.
Fundamental principle of dynamics
The fundamental principle of dynamics is Newton’s Second Law of Motion. It states that
“the rate of change of momentum (linear momentum) is proportional to the applied force
and takes place in the direction of the force”. More precisely, this statement may be
written as “the resultant external force Fx acting on the particle of mass m along any
arbitrarily chosen direction x is equal to the time rate of change of linear momentum of
the particle in the same direction, i.e., x – direction”.
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Linear momentum of a body is the product of its mass and its velocity. Let m be the mass
of the fluid moving with a velocity v. Let the fluid mass undergoes a change in velocity
dv in time dt.
Hence, change in linear momentum of fluid mass = m.dv
Time rate of change of linear momentum of fluid mass = m.dt
dv
According to Newton’s Second Law of Motion,
Dynamic force applied in x – direction= time rate of change of linear momentum of fluid mass in x – direction
i.e.,dt
dvmF x
x . …… (1)
where the suffix x, denotes the components in x – direction.
Equation (1) is known as linear momentum equation. Equation (1) can also be written as:
xx dvmdtF .. …… (2)
The term on the LHS of equation (2) represents the product of the dynamic force Fx and
the time increment dt during which it acts. This is known as the impulse of applied
dynamic force. The term on the RHS of equation (2) represents the product of mass of
fluid and the change in velocity dvx undergone by the fluid mass in x-direction in time
increment dt. This term represents the change in linear momentum of fluid mass.
Note: As velocity is a vector quantity, any change in magnitude or direction or both will
change the velocity, and hence momentum.
Equation (2) is known as the Impulse-Momentum Equation. It states that the impulse of
the dynamic force is equal to the resulting change in linear momentum of body.
Newton’s Second Law of Motion is generally applied to a system. A system is a definite
mass of fluid (or material) and all other matter around it is known as its surroundings.
The boundaries of the system will form a closed surface and this surface may change
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with time so that it contains the same mass during which the change takes place. When
Newton’s Second Law of Motion is applied to a system, equation (1) may be written as:
dt
dvmF x
x . …… (3)
where m is the constant mass of the system. xF represents the algebraic sum (or
resultant) of all body forces such as gravity as well as surface forces acting on fluid
mass m in any arbitrary direction x. vx is the velocity of the centre of mass of the system
in x-direction. dvx is the change in vx in time dt.
Concept of Control Volume and One-dimensional form of momentum equation
Control volume is a specific region in space, its size and shape being entirely arbitrary.
However, the shape and size of a control volume are made to coincide with solid
boundaries.
Considering a control volume and a fluid entering the control volume with uniform
velocity1x
v in the arbitrary x – direction and leaving the control volume after time t with
a uniform velocity2xv in the x – direction, equation (3) can be written as
12 xxx vv
t
mF …… (4)
Sincet
mrepresents the mass of fluid flowing per unit time, we have,
fluid)offlowof(rate xfluid)ofdensity(mass Qt
m
where = mass density of fluid (kg m-3)Q = rate of flow of fluid or discharge (m3s-1)
t
mhas units of kg s-1.
Hence, equation (4) can be written as
12 xxx vvQF …… (5)
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Equation (5) represents the one-dimensional form of steady flow momentum equation.
This equation is important in the study of hydraulic machines as it enables the
determination of forces developed by the flow of fluid in the machine.
Dynamic force exerted by a fluid jet on a stationary flat plate held normal todirection of jet
Let a fluid jet issued from a nozzle strikes a flat plate with a velocity v. Let the fluid jet
be oriented in x – direction (horizontal direction). The flat plate is held stationary, vertical
and normal (perpendicular) to the direction of the jet. Let the flow rate of fluid issued
from the nozzle and impinging on the plate be Q. Let be the specific weight of fluid and
be the mass density of fluid. Then weight of fluid flowing per second is Q.
Mass of fluid issued from nozzle and striking the plate per second, QQgt
m
Velocity of fluid issued from the nozzle = v
Velocity with which the fluid jet strikes the plate in x – direction = v
The fluid jet after striking the plate gets divided into two equal halves, with each half
deflected by 90 from the original direction of the fluid jet (at the instant of striking the
plate). One half of the jet moves in the vertical upward direction and the other half of the
x x
Fluid jet moving withvelocity v
Nozzle
v
Stationary flat plate heldnormal to fluid jet
v
Figure 1 Fluid jet impinging on a stationary flat plate held normal to jet
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jet moves in the vertical downward direction as shown in Figure 1. Assuming the plate to
be frictionless, the jet leaving the plate will move with the same magnitude v of the jet
incident on the plate. As the direction of the velocity of fluid jet is changed after
impingement on the plate, it is said that the velocity of fluid jet has changed causing a
linear momentum.
Final velocity of fluid jet in x - direction after striking the plate
= component of velocity of fluid jet leaving the plate in x – direction= v cos 90 = v x 0 = 0 ms-1
Hence change in velocity of fluid jet in x – direction
= final velocity of fluid jet in x – direction (after impingement on plate) – initial velocityof fluid jet in x - direction (before impingement)
= 0 – v = - v ms-1
Dynamic force exerted by plate on fluid
= rate of change of momentum of fluid= (mass of fluid striking the plate per second) x
(change of velocity of fluid normal to the plate)Applying Equation (5)
12 xxx vvQF = vQ 0 Qv
- QvFx …… (6)
The negative sign on RHS of above equation represents that the velocity of fluid jet is
decreasing, while the negative sign on the LHS of the equation represents that the force
exerted by the plate on the fluid jet is acting in the negative direction of x – axis.
Equation (6) gives the force exerted by the plate on the fluid jet. Here, the plate is
responsible for effecting a change in the velocity of fluid jet. Therefore, the force is
exerted by the plate on the fluid jet (this is the action). Since the final velocity of fluid jet
is less than the initial velocity, the force exerted by the plate on the fluid jet is a retarding
force, thus it acts in a direction opposite to the direction of flow of fluid.
As per Newton’s Third Law of motion, for every action there is an equal and opposite
reaction. Now, the reaction, in this case, is the force exerted by the fluid jet on the plate.
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The magnitude of the force exerted by the fluid jet on the plate is equal to Qv . The
direction of force exerted by fluid jet on the plate is diametrically opposite to that exerted
by the plate on the fluid jet. As the direction of the force exerted by the plate on the fluid
jet is in the negative x - direction, the direction of the force exerted by the fluid jet on the
plate must be in x - direction. Hence, the dynamic force exerted by the fluid jet on the
plate is given by
QvFx …… (7)
Dynamic force exerted by a fluid jet on a stationary flat plate held inclined todirection of jet
Let a fluid jet of diameter d and cross-sectional area a issued from a nozzle and moving
with a velocity v impinges on a stationary flat plate held inclined to the direction of jet.
The jet is oriented in the horizontal direction (x – direction). The plate makes an angle
with the horizontal. After impingement on the plate, the jet gets divided into two equal
parts, with one part moving upward at an angle to the horizontal and the other part
moving downward at the same angle to the horizontal. Assuming the plate to be
frictionless, the jet leaves the plate with a velocity whose magnitude is equal to v which is
the same as that of the velocity of the jet incident on the plate.
x
Nozzle
Fluid jet moving withvelocity v Stationary flat plate held
inclined to fluid jet
v
v
xv
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The velocity of jet issued form nozzle and incident on the inclined plate can be resolved
into two mutually perpendicular components, one component normal (perpendicular) to
the plate and other component along the plate (parallel to the plate). Let the velocity
component defined normal to the plate be denoted by vn and the component defined along
the plate be defined by the symbol vt.
Here, sin)90cos( vvvn
and cosvvt
As the plate surface is frictionless, the jet leaving the plate has the same magnitude v as
that of the jet incident on the plate. The jet leaving the plate is tangential to the plane
surface of the plate. The component of the velocity of jet leaving the plate in a direction
normal to the plate is .sm00 x90cos -1 vv Hence, the change in velocity of jet in
a direction normal to the plate is given by
Change in velocity of jet in a direction normal to the plate
= final velocity of jet normal to the plate – initial velocity of jet normal to the plate
= sinsin0 vv
Dynamic force exerted by plate on fluid in a direction normal to the plate, Fn
= rate of change of momentum of fluid in a direction normal to the plate
= (mass of fluid striking the plate per second) x
(change of velocity of fluid normal to the plate)
90
v
90 -
vn = v sin vt = v cos
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Replacing the direction x (horizontal) by n (normal) in Equation (5)
12 nnn vvQF = sin0 vQ sinQv
- sinQvFn …… (8)
The negative sign on RHS of above equation represents that the velocity of fluid jet is
decreasing in the direction normal to the jet, while the negative sign on the LHS of the
equation represents that the force exerted by the plate on the fluid jet in a direction
normal to the fluid jet is acting in the negative direction of x – axis.
The dynamic force exerted by the fluid jet on the plate in a direction normal to the plate is
oriented in a direction diametrically opposite to that exerted by the plate on the fluid jet.
It is given by
sin)sin( QvQvFF nn …… (9)
The component of Fn in x – direction (along the direction of jet issued from nozzle) is
given by
2sinsin)sin(sin)90cos( QvQvFFF nnx …… (10)
Dynamic Force exerted by Fluid Jet on a Moving Flat Plate
The plate is held normal to the direction of fluid jet. The fluid jet moving with velocity v
impinges on the plate and after the jet strikes the plate, the plate moves with a velocity u
in the direction same as the direction of the fluid jet incident on the plate. Hence, the
velocity of jet leaving the plate becomes (v - u). as the plate moves away progressively
from the jet with velocity u, the quantity of fluid striking the plate per second is given by
multiplying the cross-sectional area of fluid jet and the velocity of jet relative to the plate,
w.
uvaawQ
where, w = velocity of fluid jet relative to the motion of the plate
v = absolute velocity of fluid jet
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Force exerted by the fluid on the plate (in x – direction)
= (mass of fluid striking the plate per second) x
(change of velocity of fluid jet (in x – direction))
uvQuvQuvuvQFx 090cos
Here, uvaQ
Hence, 2uvauvuvaFx
Here, under the impact of the jet on the plate, the plate starts moving away from the jet at
a velocity u. that is, the distance between the plate and the nozzle from which the jet is
issued increases constantly at u m s-1. Therefore, a single moving plate is not a practical
case. However, if a series of plates (refer Figure below) were so arranged that each plate
appeared successively before the fluid jet in the same position and always moving with a
velocity u in the direction of the jet, the entire flow issued from the nozzle will be utilized
in making impact on all the plates. Thus, the mass of fluid striking the plates per second
would be avQ .
Hence, uF
(mass of water striking the plates per second) x (change in velocity of fluid jet)
= uvavuvavuvuvQ 090cos
Work done by the fluid jet on the plates = uuvQuFu .
x x
Fluid jet moving withvelocity v
Nozzle
Moving flat plate heldnormal to fluid jet
Figure. Fluid jet impinging on a moving flat plate held normal to jet
u
v - u
v - u
v
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This gives the output of the system under consideration
Kinetic energy of the fluid jet = 2
2
1mv
Where m = mass of fluid issued from the nozzle per second = Q
Hence, kinetic energy of the fluid jet (input to the system under consideration)
= 2
2
1vQ
Efficiency of the system,jetfluid theofenergykinetic
plateson thejetfluidby thedonework
input
output
=
2
2
1Qv
uuvQ
=
2
2
v
uuv
For maximum efficiency, max , 0
du
d
v
Figure. Fluid jet impinging on aseries of moving flat plates
u
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i.e.,
02
2
v
uuv
du
d
022
uuvdu
d
v
02 uvudu
d
02 uv
2
vu
Substituting the value of u in the expression for , the value of max can be obtained.
5.02
12222
222
2
2
22max
v
v
v
vv
v
vvv
(or) 50%
Dynamic force exerted by fluid jet on a stationary curved plate
The fluid jet moving with a velocity v1 impinges on a curved plate such that it makes an
angle 1 with the x – axis at the point of impingement (inlet of plate). After impingement
1
2v2
v1
xx
xx
Inlet
Outlet
Figure. Fluid jet impinging on a stationary curved plate withacute discharge angle 2
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at inlet, the jet glides over the surface of the plate and leaves the plate tangentially at its
outlet. The jet leaving the plate makes an angle 2 with the x – axis. The angle 2 also
indicates the angle of curvature of the plate at its outlet as the jet leaves the plate
tangentially to the curvature of the plate. Let v2 be the velocity of fluid jet leaving the
plate. If the surface of the plate along which the jet glides past the plate is frictionless, the
magnitude v2 of the velocity of fluid jet leaving the plate will be the same as the
magnitude v1 of the velocity of the jet incident on the plate at its inlet.
Velocity of fluid jet at inlet in x - direction = 11 cosv
Velocity of fluid jet at outlet in x - direction = 22 cosv
Therefore, force exerted by fluid jet on plate in x - direction is given by
xF (mass of fluid striking the plate per second) x
(change of velocity of fluid jet in x – direction)
= 2211 coscos vvQ
where
inlet)itsatplateon theincidentjetfluidof(velocity xjet)fluidofareasectional-cross(
secondperplate thestrikingfluidofquantity
Q
= av1
Hence, 22111 coscos vvavFx
If the curvature of plate at its outlet is such that the discharge angle, 2, is more than 90
(as shown in Figure below), then cos 2 is negative, hence, 22 cosv is negative. Hence,
in order to get more force, the curvature of the plate should be such that the discharge
angle 2 must be more than 90.
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Dynamic force exerted by a fluid jet on a single moving curved plate
Let a fluid jet of velocity v1 issued from a nozzle impinges at the inlet ‘1’ of a curved
plate which moves as a result of impingement of fluid jet at its inlet. The velocity with
which the fluid jet is issued from the nozzle and impinges on the plate at tits inlet is
termed as the absolute velocity of jet at inlet. The absolute velocity of fluid jet makes an
angle 1 with the X-direction.
1
2
v2
v1
Inlet
Outlet
x x
x x
Figure. Fluid jet impinging on a stationary curved plate withobtuse discharge angle 2
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Once the jet is incident on the plate at inlet, let the plate move in X-direction with a
velocity u. Let the velocity of plate at inlet be referred by u1. It should be noted that as the
entire plate moves in a unique direction (X-direction) with the same velocity, u, we have,
u1 = u. Once the jet strikes the plate at its inlet, the velocity of jet remains no more equal
to v1, but it becomes the velocity of jet relative to the motion of the plate at inlet (that is,
relative velocity of fluid jet) be referred by the symbol w1. The direction of relative
velocity of jet at inlet is tangential to the curvature of the plate at inlet. The tangent to the
curvature of the plate at its inlet may or may not make the same angle as that of the fluid
jet with the X-direction. In Figure drawn above, the angle which the tangent to the
curvature of the plate makes with the X-direction is different from that which the fluid jet
at inlet makes with the X-direction. Let the angle which the relative velocity of jet at inlet,
w1, makes with the reversed direction of motion of plate at inlet, that is, - u2, be 1. That
is, the angle of curvature of the plate at inlet (tangent to the curvature of the plate at inlet)
makes an angle 1 with the reversed direction of motion of plate at inlet.
X
XX1
1w1
v1
u11
1 - Inlet
2 - Outlet
u = u1 = u2
22
w2 v2
u2
X2
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Here, the magnitudes of v1 and u1 (= u) are known. The magnitude of w1 can be
determined by subtracting u1 from v1 vectorially by applying the Law of Parallelogram of
Velocities.
Vecorially speaking, we have, by Law of Parallelogram of Velocities,
111 uvw
Construction of Parallelogram of Velocities at Inlet:
Draw 1vAC to a suitable scale such that 1v makes an angle 1 with the X-direction.
At A, draw 1uAB in X –direction to the same scale as that of v1. Join B and C so that
1wBC . Measure BC which gives the magnitude of 1w to the same scale as those of
11 and uv .
Then, the jet glides past the surface of the plate and leaves the plate at its outlet with a
relative velocity equal to w2. The magnitude of the relative velocity of jet at outlet, w2,
may remain equal to the relative velocity of jet at inlet, w1, provided the surface of the
plate is perfectly smooth, that is, when there is no energy loss due to friction of the
surface of the plate. Let the plate at outlet be inclined at an angle 2. This angle 2 is
measured such that it is the angle between relative velocity of jet at outlet, w2, and
reversed direction of motion of plate at outlet, i.e., - u2.
v1
u1
w1
A
X
B
1XX
1
C
1 - Inlet
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Construction of Parallelogram of Velocities at Outlet:
Draw 2uAB to a suitable scale in X – direction. At B set out angle 2 representing the
direction of 2w . Draw 2wBC along the set direction. Join A and C so that 2vAC .
Measure AC which gives the magnitude of 2v . Vectorially speaking, we have,
222 wuv
The magnitude and direction of 2v can be determined by applying the Law of
Parallelogram of Forces. The angle which the absolute velocity of jet at outlet, 2v , makes
with X – direction is referred by 2. It is the angle between 2v and 2u .
Force exerted by the fluid jet on the plate in X – direction or in the direction of motion of
plate is determined by applying the Linear-Momentum Equation:
direction-Xinjetfluidofyin velocitchange xunit timeperplate thestrikingfluidofmassxF
= 2211 coscos x vvQ
where uvauvaQ 111 , since the plate progressively moves away from the jet
and hence, the velocity of jet falling on the plate is reduced by u, or the velocity with
which the jet falls on the plate is (v1 – u). The quantity of fluid jet issued from the nozzle
(i.e., av1). For 2 <2
, cos 2 > 0, while for 2 >
2
, cos 2 < 0, hence, when cos 2 < 0,
the second term (v2 cos 2) within the parentheses will become negative. Hence, the
quantity 2211 coscos vv will be higher for the same Q, v1, 1 and v2. Hence, in
22
w2 v2
u2X
2
2
w2 v2
u2X
2
2
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order to get more magnitude for force F, the curvature of the plate at outlet should be
such that 2 is obtuse.
Velocity Diagrams for Turbine Blades
At Inlet:
u1 = circumferential or peripheral velocity of vanes at inlet
w1 = velocity of jet relative to motion of vane at inlet
v1 = absolute velocity (i.e., relative to earth) of jet at inlet
1 = angle between v1 and u1, i.e., angle of jet with the direction of motion of vane atinlet
1 = angle between w1 and – u1, i.e., angle of vane tip at inlet. This angle is measuredbetween w1 and u1 reversed
The absolute velocity of jet, v1, can be resolved into two mutually perpendicularcomponents.
1
1
w1v1
u1Inlet
1uvu1
1 1
v1
w1
1mv
AB
C
D
=
Figure. Typical velocity triangle for flow over turbine blade at Inlet
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(i) Tangential component,1uv , equal to v1 cos 1. This component is parallel to
the direction of motion of vane, i.e., u1, and hence is responsible for doing
work. Therefore, this velocity component is referred to as Velocity of Whirl at
inlet.
(ii) Radial component, ,vm1equal to v1 sin 1. This component is perpendicular to
the direction of motion of vane and hence they do not do any work on the
blades (runner). This component causes the water to flow through the turbine
blade and therefore, is called the Velocity of Flow at inlet.
Drawing Velocity Triangle at Inlet:
Step 1: Draw 1uAB , in the horizontal direction (x – direction) to a suitable scale.
Step 2: At A, set out angle 1 downward, to mark the direction of 1v . It should be noted
that the angle between 1v and 1u is 1
Step 3: Along the direction set out at A as outlined in Step 2, set out 1v to the same scale
so that 1vAC
Step 4: Join B and C so that 1wBC
Step 5: Measure ABC = 1, the vane angle at inlet
The relationship between the velocity vectors, 11 u,v and 1w is
111 wuv
111 uvw
The absolute velocity of water jet at inlet, v1, is resolved into two mutually perpendicular
velocity components namely, the Velocity of whirl at inlet,1uv , which is the tangential
component, and the Velocity of flow at inlet, ,vm1which is the normal or radial
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component. It should be noted that the direction of1uv is along the direction of u1. The
direction of1mv is perpendicular (normal) to u1
The arrow heads mark the directions of the respective velocity vectors.
At outlet:
u2 = circumferential or peripheral velocity of vanes at outlet
w2 = velocity of jet relative to motion of vane at outlet
v2 = absolute velocity (i.e., relative to earth) of jet at outlet
2 = angle between v2 and u2, i.e., angle of jet with the direction of motion of vane at
2
u2
v2
w2
2
Outlet
=
2
u2
v2w2
2
2mv
2uv
A B
C
D
Figure. Typical velocity triangle for flow over turbine blade at Outlet
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outlet
2 = angle between w2 and – u2, i.e., angle of vane tip at outlet. This angle is measured
between w2 and u2 reversed
The absolute velocity of jet, v2, can be resolved into two mutually perpendicular
components.
(i) Tangential component,2uv , equal to v2 cos 2. This component is parallel to the
direction of motion of vane at outlet, i.e., u2, and hence is responsible for doing work.
Therefore, this velocity component is referred to as Velocity of Whirl at outlet.
(ii) Radial component, ,vm2equal to v2 sin 2. This component is perpendicular to the
direction of motion of vane and hence they do not do any work on the blades
(runner). This component causes the water to flow through the turbine blade and
therefore, is called the Velocity of Flow at inlet.
Drawing Velocity Triangle at Outlet:
Step 1: Draw 2uAB , in the horizontal direction (x – direction) to a suitable scale.
Step 2: At B, set out angle 2 downward, to mark the direction of 2w . It should be noted
that the angle between 2w and 2u reversed is 2
Step 3: Along the direction set out at B as outlined in Step 2, set out 2w to the same scale
so that 2wBC
Step 4: Join A and C so that 2vAC
Step 5: Measure BAC = 2, the angle which the absolute velocity of jet at outlet, v2,
makes with the circumferential or peripheral velocity of runner at outlet, u2.
The relationship between the velocity vectors, 22 u,v and 2w is
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222 wuv
The absolute velocity of water jet at outlet, v2, is resolved into two mutually
perpendicular velocity components namely, the Velocity of whirl at outlet,2uv , which is
the tangential component, and the Velocity of flow at outlet, ,vm2which is the normal or
radial component. It should be noted that the direction of2uv is along the direction of u2.
The direction of2mv is perpendicular (normal) to u2.
Note: The velocity of whirl at outlet,2uv , may be positive or negative depending upon
whether the angle 2 is acute or obtuse. When 2 is acute, cos 2 is positive, and hence,
222cosvvu , is positive. When 2 is obtuse, cos 2 is negative, and hence,
222cosvvu , is negative.
Fluid jet on moving curved surface of a Pelton turbine blade
Figure: Plan view of a double hemispherical bucket of a Pelton runner
1
2
2
NozzleAbsolute path
Relative path
1 – Inlet (splitter)
2 – Outlet
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Each bucket consists of two approximately hemispherical cups separated by a sharp edge(called ‘splitter’) at the centre. The water jet impinges at the centre of the bucket (splitter)and is divided by the splitter into two equal halves, each half moving sideways inopposite direction. Thus the incident jet at inlet is deflected backward when leaving thebucket at outlet. The theoretical angle of deflection of jet is 180. But due to practicalreasons, the actual angle of deflection of jet is kept less than 180, say, about 165.
The direction of absolute velocity of jet at inlet is tangential to the curvature of blade atinlet. Further, the direction of absolute velocity of jet at inlet is along the direction of
peripheral velocity of runner at inlet, i.e., 1v and 1u are in the same direction. Hence,
01 . Hence, the direction of relative velocity of jet at inlet is also along the same
direction as those of 1v and 1u . Therefore, 1 = 180. There is no formation of velocity
triangle at inlet, as all the three velocity vectors, 1v , 1u and 1w are along the samedirection. Further, we have,
111 uvw
As the outlet of the runner blade is located at the same radial distance (in the same plane)
from the axis of runner, as that of the inlet, the peripheral velocity of blade at outlet, 2u
peripheral velocity of blade at inlet, 1u . The shape of the outlet velocity triangle is shownbelow.
u1
u2
v1
v2
w1
w2
22
1 = 01 = 180
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The direction of absolute velocity of jet at outlet is against the direction of motion of
runner, i.e., 2v is against 2u . Further, as the angle between 2v and 2u , i.e., 2, is obtuse
(more than 90), cos 2 is negative. Hence,2uv = v2 cos 2, is negative. What is the
impact of this condition in development of the dynamic force by the fluid jet on the runnervanes?
u2
v2
w1
w2
2
2
v2
w2
u2
2mv
2uv
2
2
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WATER TURBINESIntroduction:
Water turbines were developed in the 19th century and were widely used for industrial
power prior to electrical grids. Now they are mostly used for electric power generation.
Water turbines are mostly found in dams to generate electric power from water kinetic
energy. Water wheels have been used for hundreds of years for industrial power. Their
main shortcoming is size, which limits the flow rate and head that can be harnessed. The
migration from water wheels to modern turbines took about one hundred years.
Development occurred during the Industrial revolution, using scientific principles and
methods. They also made extensive use of new materials and manufacturing methods
developed at the time.
Turbine is a device in which a mechanical energy is transferred from the flowing liquid
through the machine to its operating member. The inlet energy of the liquid is greater
than the outlet energy of the liquid are referred as Water turbines. It is well known from
Newton’s law that to change of momentum of Fluid, a force is required. Similarly, when
momentum of fluid is changed, a force is generated. This principle is made use in
hydraulic turbine.
It converts energy in the form of falling water into rotating shaft power. The geometry of
turbines is such that the fluid exerts a torque on the rotor in the direction of its rotation.
The shaft power generated is available to derive generators or other devices.
Classification of turbines:According to action of water on moving blades:The two basic types of hydraulic turbines are impulse and reaction turbines.
In impulse turbine the entire pressure energy of water is converted into kinetic energy,
also it converts the kinetic energy of a jet of water to mechanical energy.
The static pressure of water at the entrance to the runner is equal to the static pressure at
exit and the rotation of wheel is caused purely due to tangential force created by the
impact of the jet, and hence it is called as impulse turbine.
Reaction turbines convert potential in pressurized water to mechanical energy. The static
pressure at inlet to the runner is higher than the static pressure at the exit, and there is a
gradual conversion of static pressure into kinetic energy while water is flowing through
the runner. In this type of turbine, the rotation of the runner is partly due to impulse
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action and partly due to change in pressure over the runner blades, hence the turbine is
called reaction turbine.
According to available head:
Turbines can be classified as high head, medium head or low head machines. The
turbines which are capable of working under high heads (i.e greater than 400m) are called
high head turbines. When the gross head lies between 50m to 400m, the turbines are
medium head turbines. Turbines capable of working under heads varying from 50m to
2.5m are called low head turbines.
Type High head Medium head Low head
Impulse turbines pelton
turgo
cross-flow
multi-jet pelton
turgo
cross-flow
Reaction turbines francis propeller
kaplan
According to direction of flow of water:
The turbines are classified into Tangential flow, Radial flow, Axial and mixed flowturbines.
Tangential flow turbines: In this type of turbines, the water strikes the runner in the
direction tangent to the wheel. Example: pelton wheel
Radial flow turbines: In this type of turbines, the water strikes in the radial direction. It is
further classified as inward radial flow turbine and outward radial flow turbine.
Inward radial flow turbine: The flow is inward from periphery to the centre. Example: old
Francis turbine.
Outward radial flow turbine: The flow is outward from centre to periphery. Example:
Fourneyron turbine.
Axial flow turbine: When flow of water is in the direction parallel to the axis of the shaft.
Example: Kaplan and propeller turbine.
Mixed flow turbine: The water enters the runner in the radial direction and leaves in the
axial direction. Example: Modern Francis turbine.
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Selection of a type of turbine:The selection of the best turbine for any particular hydro site depends on the site
characteristics, the dominant ones being the head and flow available. Selection also
depends on the desired running speed of the generator or other device loading the
turbine. Other considerations such as whether the turbine is expected to produce power
under part-flow conditions also play an important role in the selection. All turbines have
a power-speed characteristic. They will tend to run most efficiently at a particular speed,
head and flow combination.
In large scale hydro installation Pelton turbines are normally only considered for heads
above 150 m, but for micro-hydro applications Pelton turbines can be used effectively at
heads down to about 20 m. Pelton turbines are not used at lower heads because their
rotational speeds become very slow and the runner required is very large and heavy. If
runner size and low speed do not pose a problem for a particular installation, then a
Pelton turbine can be used efficiently with fairly low heads. If a higher running speed
and smaller runner are required then there are two further options:
• Increasing the number of jets.
Having two or more jets enables a smaller runner to be used for a given flow and
increases the rotational speed. The required power can still be attained and the
part-flow efficiency is especially good because the wheel can be run on a reduced
number of jets with each jet in use still receiving the optimum flow.
• Twin runners.
Two runners can be placed on the same shaft either side by side or on opposite sides of
the generator. This configuration is unusual and would only be used if number of jets
per runner had already been maximized, but it allows the use of smaller diameter and
hence faster rotating runners.
Impulse Turbine:
A Pelton turbine consists of a set of specially shaped buckets mounted on a periphery of a
circular disc. It is turned by jets of water which are discharged from one or more nozzles
and strike the buckets. The buckets are split into two halves so that the central area does
not act as a dead spot incapable of deflecting water away from the oncoming jet. The
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cutaway on the lower lip allows the following bucket to move further before cutting off
the jet propelling the bucket ahead of it and also permits a smoother entrance of the
bucket into the jet. The Pelton bucket is designed to deflect the jet through 165 degrees
(not 180 degrees) which is the maximum angle possible without the return jet interfering
with the following bucket for the oncoming jet.
Impulse turbines are generally more suitable for micro-hydro applications compared with
reaction turbines because they have the following advantages:
• greater tolerance of sand and other particles in the water,
• better access to working parts,
• no pressure seals around the shaft,
• easier to fabricate and maintain,
• better part-flow efficiency.
Runner of a Pelton wheel. Source: http://www.hydrowest.com/runners1.htm
Components of Pelton Turbine:
The main components of pelton wheel are
1. Nozzle and flow regulating arrangements,
2. Runner with buckets,
3. Casing,
4. Breaking jet.
Working Principle of Pelton Turbine
High speed water jets emerging from the nozzles (obtained by expanding high pressure
water to the atmospheric pressure in the nozzle) strike a series of spoon-shaped buckets
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mounted around the edge of the pelton wheel. High pressure water can be obtained from
any water body situated at some height or streams of water flowing down the hills.
Components of Pelton Turbine. Source: Web image
As water flows into the bucket, the direction of the water velocity changes to follow the
contour of the bucket. These jets flow along the inner curve of the bucket and leave it in
the direction opposite to that of incoming jet. When the water-jet contacts the bucket, the
water exerts pressure on the bucket and the water is decelerated as it does a "u-turn" and
flows out the other side of the bucket at low velocity.The change in momentum (direction
as well as speed) of water jet produces an impulse on the blades of the wheel of Pelton
Turbine. This "impulse" does work on the turbine and generates the torque and rotation in
the shaft of Pelton Turbine.To obtain the optimum output from the Pelton Turbine the
impulse received by the blades should be maximum. For that, change in momentum of
the water jet should be maximum possible. This is obtained when the water jet is
deflected in the direction opposite to which it strikes the buckets and with the same speed
relative to the buckets. For maximum power and efficiency, the turbine system is
designed such that the water-jet velocity is twice the velocity of the bucket. A very small
percentage of the water's original kinetic energy will still remain in the water. However,
this allows the bucket to be emptied at the same rate at which it is filled, thus allowing
the water flow to continue uninterrupted. Often two buckets are mounted side-by-side,
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thus splitting the water jet in half (see photo). The high speed water jets emerging form
the nozzles strike the buckets at splitters, placed at the middle of the buckets, from where
jets are divided into two equal streams. This balances the side-load forces on the wheel,
and helps to ensure smooth, efficient momentum transfer of the fluid jet to the turbine
wheel. Because water and most liquids are nearly incompressible, almost all of the
available energy is extracted in the first stage of the hydraulic turbine. Therefore, Pelton
wheels have only one turbine stage, unlike gas turbines that operate with compressible
fluid.
Bucket shape. Source: http://europa.eu.int/en/comm/dg17/hydro/layman2.pdf
Velocity triangle for pelton Wheel:
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Velocity triangles for the jet striking the bucket . Source VTU Learning
From the impulse momentum equation
Force= (Mass/second)× change in velocity in x direction
Applications of Pelton Wheel:
Pelton wheels are the preferred turbine for hydro-power, when the available water source
has relatively high hydraulic head at low flow rates. Pelton wheels are made in all sizes.
There exist multi-ton Pelton wheels mounted on vertical oil pad bearings in hydroelectric
plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels are only
a few inches across, and can be used to tap power from mountain streams having flows of
a few gallons per minute. Some of these systems utilize household plumbing fixtures for
water delivery. These small units are recommended for use with thirty meters or more of
head, in order to generate significant power levels. Depending on water flow and design,
Pelton wheels operate best with heads from 15 meters to 1,800 meters, although there is
no theoretical limit.
Thus, more power can be extracted from a water source with high-pressure and low-flow
than from a source with low-pressure and high-flow, even though the two flows
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theoretically contain the same power. Also a comparable amount of pipe material is
required for each of the two sources, one requiring a long thin pipe, and the other a short
wide pipe.
Reaction Turbines:
The reaction turbines considered here are the Francis turbine and the propeller turbine. A
special case of the propeller turbine is the Kaplan. In all these cases, specific speed is
high, i.e. reaction turbines rotate faster than impulse turbines given the same head and
flow conditions. This has the very important consequences in that a reaction turbine can
often be coupled directly to an alternator without requiring a speed-increasing drive
system. Some manufacturers make combined turbine-generator sets of this sort.
Significant cost savings are made in eliminating the drive and the maintenance of the
hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while
the propeller is more suitable for low heads.
On the whole, reaction turbines require more sophisticated fabrication than impulse
turbines because they involve the use of larger and more intricated profile blades together
with carefully profiled casings.
Francis turbines can either be volute-cased or open-flume machines. The spiral casing is
tapered to distribute water uniformly around the entire perimeter of the runner and the
guide vanes feed the water into the runner at the correct angle. The runner blades are
profiled in a complex manner and direct the water so that it exits axially from the centre
of the runner. In doing so, the water imparts most of its pressure energy to the runner
before leaving the turbine via a draft tube.
The Francis turbine is generally fitted with adjustable guide vanes. These regulates the
water flow as it enters the runner and are usually coupled to a governing system which
equals flow to turbine loading in the same way as a spear valve or deflector plate in a
Pelton turbine. When the flow is reduced the efficiency of the turbine falls away.
Francis Turbine.(Radial flow turbine)Construction and Working: Figure shows schematic diagram of a Francis turbine.
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The main parts are:
• Penstock: It is a large size conduit which conveys water from the upstream to the
dam/reservoir to the turbine runner.
• Spiral Casing: It constitutes a closed passage whose cross-sectional area
gradually decreases along the flow direction; area is maximum at inlet and nearly
zero at exit.
• Guide Vanes: These vanes direct the water on to the runner at an angle
appropriate to the design, the motion of them is given by means of hand wheel or by a
governor.
• Governing Mechanism: It changes the position of the guide blades/vanes to
affect a variation in water flow rate, when the load conditions on the turbine change.
• Runner and Runner Blades: The driving force on the runner is both due to
impulse and reaction effect. The number of runner blades usually varies between 16
to 24.
• Draft Tube: It is gradually expanding tube which discharges water, passing
through the runner to the tail race.
http://www.ululu.in/first-year/elements-of-mechanical-engineering/img/francis-
turbine.jpg
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Source:VTU LearningWorking: Francis turbine has a purely radiate flow runner. Water under pressure, enters
the runner from the guide vanes towards the center in radial direction and discharges out
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of the runner axially. Francis turbine operates under medium heads. Water is brought
down to the turbine through a penstock and directed to a number of stationary orifices
fixed all around the circumference of the runner. These stationary orifices are called as
guide vanes.
The head acting on the turbine is transformed into kinetic energy and pressure head. Due
to the difference of pressure between guide vanes and the runner (called reaction
pressure), the motion of runner occurs. That is why a Francis turbine is also known as
reactionturbine.
The pressure at inlet is more than that at outlet. In Francis turbine runner is always full of
water. The moment of runner is affected by the change of both the potential and kinetic
energies of water. After doing the work the water is discharged to the tail race through a
closed tube called draft tube.
Draft Tubes:
In radial flow turbines, as the water flows from higher pressure to lower pressure, it
cannot come out of the turbine and hence a divergent tube is connected to the end of the
turbine. This divergent tube, one end of which is connected to the outlet of the turbine
and the other is immersed well below the tail race.
Functions of Draft tube:
1. It is to increase the pressure from inlet to outlet of the draft tube as it flows through it
and hence increase it more than atmospheric pressure.
2. To safely discharge the water that has worked on the turbine to the tail race.
Velocity Triangle for a radial flow Turbines:
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Source VTU Learning
Component of Velocity at inlet ×
Component of Velocity at outlet ×
Now angular momentum per second at inlet and outlet is the product of momentum inlet
and outlet with respect to their radial distance R1 and R2.
Torque=
Work done per second= Torque× ω
= ×ω
Kaplan Turbine (axial flow turbine)
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Kaplan turbine is a low head reaction turbine, in which water flows axially. It was
developed by German Engineer Kaplan in 1916.
In this type of turbine, the water flows parallel to the axis of rotation. The shaft of the
turbine may be either vertical or horizontal. The lower end of the shaft is made of larger
to form the boss or the hub. When the vanes are composite with the boss the turbine is
called as propeller turbine. When the vanes are adjustable the turbine is called as Kaplan
turbine.
All the parts of the Kaplan turbine (viz, spiral casing, guide wheel and guide blades) are
similar to that of the Francis turbine, except the runner blades, runner and draft tube. The
runner and runner blades of the Kaplan turbine resemble with the propeller of the ship.
Hence, Kaplan turbine is also called as Propeller Turbine. The blades of a Kaplan turbine,
three to eight in number are pivoted around the central hub or boss, thus permitting
adjustment of their orientation changes in load and head.
Source: VTU Learning
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Governor of Turbines:
Source: VTU Learning
A Governor is a mechanism to regulate the speed of the turbine. The turbine is coupled to
shaft of the generator, which is generating power. The power generated should have
uniform rating of current and frequency which in turn depends on the speed of the shaft
of the turbine. The above figure shows the oil pressure governor of the turbine.
Specific speed of the turbine: Ns
The specific speed of a turbine is the speed at which the turbine will run when developing
unit power under a unit head.
Consider P as the power developed by the turbine.
Where Ns= specific speed of turbine, H= Head, N= speed of the turbine, and
P= power developed by the turbine.
****************************************
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GAS AND STEAM TURBINESINTRODUCTIONTurbines and compressors are used in electric power generation, aircraft propulsion and a
wide variety of medium and heavy industries. Small and heavy duty fans and blowers
cover a wide range of industrial applications. Though the steam turbine was perfected
much earlier than the gas turbine engine, the last two decades have seen almost parallel
development in aeroengines and steam turbine power plant. As a result today on the one
hand we have the jumbo-jet and their high thrust engines in the aeronautical field, while
on the other there are giant steam turbine plants operating in the “super thermal powerstations”. These developments suggest that the 2000MW steam turbine plant will beoperating in many countries by the turn of century. Along with this the “super jumbo-jet”air liners will also be flying between the major cities of the world.
ROTODYNAMIC MACHINES
A Rotodynamic machine is one in which a fluid flows freely through an impeller of
rotor, the transfer of energy between the fluid and the rotor is continuous and the
change of angular momentum of the fluid causes , or is the result of, a torque on the
rotor. When energy is transferred from the fluid to the rotor the machine is known as
turbine, when the energy is transferred to the fluid from the rotor the machine is known
as fan, pump or compressor.
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COMPRESSIBLE FLOW MACHINES
The pressure, temperature and density changes occurring in fluids passing through steam
and gas turbines, and compressors are appreciable. A finite change in the temperature of
the working fluid is a typical characteristic of this class of machines which distinguishes
them from other turbo machine . These classes of machines with predominantly
compressible flows are refereed to as compressible flow or thermal turbomachnies. They
are characterized by higher temperature and peripheral speeds of rotor. Therefore their
design and operation are influenced by compressible flows, high temperature and speed
problems.
INCOMPRESSIBLE FLOW MACHINES
Hydraulic pumps and turbines are examples of turbo machines working with a liquid. The
fluid or water is incompressible giving a constant volume flow rate for a given mass flow
rate in steady operation. Water and air are considered here as typical working fluid in
turbomachines handling liquids and gases. The density of water is about 800 times that of
atmospheric air. Therefore the force required to accelerate a given quantity of water is
much larger compared to that required for air. This factor largely accounts for much
lower fluid and rotor velocity in hydro -turbomachines.
Turbomachines dealing with gases over a small pressure difference also behave as
incompressible flow machines. This is because of negligible changes in the temperature
and density of the fluid across the machine. Fans, low pressure blowers, airscrews and
windmills are examples of such machines.
Thus a majority of incompressible flow machines work near ambient conditions and are
comparatively low speed and low temperature machines. This makes their running and
maintenance much easier compared to thermal turbomachines
AXIAL STAGES
In an axial flow turbo machine or its stage shown in Figure 1.1 the radial component of
the fluid velocity is negligible. The change in radius between the entry and exit of the
stage is small. The through flow in such machines mainly occurs in the axial direction,
hence the term “axial stage”.
An axial machine can be easily connected with other components. For example in a gas
turbine plant this configuration offers mechanically and aerodynamically a convenient
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connection between the compressor, combustion chamber and turbine. For the same
reason, axial stages are widely employed in multi stage turbo machines. Such a stage is
ideally suited for high flow rates. The area of cross section available to the flow in an
axial stage is
1.1Suitable values of the hub and tip diameters can be chosen to provide required area. For
aircraft propulsion, the axial flow configuration of compressors and turbines has special
advantages of low frontal area resulting in a lower aircraft drag.
The turning of the fluid in axial stages is not too severe and the length of the blade
passages is short. This leads to lower aerodynamic losses and higher stage efficiencies.
On account of the individual blade root fixtures, the root of an axial stage has limited
mechanical strength. This restricts the maximum permissible peripheral speed of the
rotor.
Figure 1.1 An axial flow stage
RADIAL STAGES
In the radial stage of a turbo machine the through flow of the fluid occurs mainly in the
radial direction, i.e. perpendicular to the axis of rotation. Therefore the change of radius
between the entry and exit of the stage is finite. This causes a finite change in the energy
level of the fluid due to the centrifugal energy.
A radial turbo machine may be inward flow type or outward flow type. Since the purpose
of compressors, blowers, fans and pumps is to increase the energy level of the fluid, they
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are of the outward flow radial type as shown in figure1. 2. Radial gas turbines are mostly
of the inward
Figure 1.2 A centrifugal compressor stage
Figure 1.3 An inward-flow radial turbine stage
Figure 1.4 An outward flow radial turbine ( Ljungstrom turbine)
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flow type as shown in figure 1.3; the fluid transfers its centrifugal energy to the rotor in
flowing from a larger to a smaller radius. The Ljungstrom stem turbine shown in
figure1.4 is a double rotation outward flow radial turbine. The outward flow
configuration is chosen here to accommodate the large volume flow rate of the rapidly
expanding steam.
dhddA hti
)(4
22
1.2
111 bdA
1.3
222 bdA
1.4
For given impeller and shaft diameters and change of radius from entry to exit the area at
the entry to the stage is restricted by equation1. 2. At this station a compressible fluid has
the largest volume requiring a correspondingly large area. Conversely, the same is true
for an inward flow gas turbine figure 3. On account of this, radial flow stages do not offer
the best geometrical configuration for high flow rates. In radial flow stages the flow
invariably turns through 90o traversing a much longer blade passage compared to that in
the axial types. This leads to comparatively higher losses and lower efficiencies. In a
multi stage radial machine the flow is required to change its direction drastically number
of times in long interconnecting flow passages. This is obviously an undesirable feature
both mechanically and aerodynamically. Therefore a majority of radial machines are
single stage machines; very few multistage radial machines employ more than three
stages.
Since the power developed is proportional to the mass flow rate, and the number of stages
that can be employed is much smaller compared to axial machine s, radial flow machines
are not suited for large power requirement.
Radial stages employ ‘one piece’ rotors in which the blades are an integral part of the
main body. This makes a radial rotor mechanically stronger than an axial type in which
the blades are separately fixed. Therefore radial machines can employ higher peripheral
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speeds.On account of higher peripheral speeds and additional change in the energy level
of the fluid caused due to centrifugal energy, much higher values of the pressure ratio per
stage are obtained in the radial stage compared to the axial type.
Radial flow compressors and turbines for large power and thrust requirements have a
larger overall diameter of the aero-engine, leading to an unacceptably large frontal area.
Therefore radial machines are unsuitable for the propulsion of large aircrafts.
STEAM TURBINEIntroduction to Steam TurbineA steam turbine is a mechanical device that extracts thermal energy from
pressurized steam, and converts it into rotary motion. Its modern manifestation was
invented by Sir Charles Parsons in 1884.
Definitions of Steam turbine
Turbine in which steam strikes blades and makes them turn
A system of angled and shaped blades arranged on a rotor through which steam is
passed to generate rotational energy. Today, normally used in power stations
A device for converting energy of high-pressure steam (produced in a boiler) into
mechanical power which can then be used to generate electricity.
Equipment unit flown through by steam, used to convert the energy of the steam
into rotational energy.
Principle of Operation
In reciprocating steam engine, the pressure of energy of steam is used to overcome
external resistance and dynamic action of the steam is negligibly small. Steam engine
may be return by using the full pressure without any expansion or drop of pressure in the
cylinder.
The steam energy is converted mechanical work by expansion through the turbine. The
expansion takes place through a series of fixed blades (nozzles) and moving blades each
row of fixed blades and moving blades is called a stage. The moving blades rotate on the
central turbine rotor and the fixed blades are concentrically arranged within the circular
turbine casing which is substantially designed to withstand the steam pressure.
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Classification of Steam Turbines
The first steam turbine, at its time indeed did spark off the industrial revolution through
out the west. However, the turbine at that time was still an inefficient piece of heavy
weighing high maintenance machine. The power to weight ratio of the first reciprocating
steam turbine was extremely low, and this led to a great focus improving the design,
efficiency and usability of the basic steam turbine, the result of which are the power
horses that currently produce more than 80% of today’s electricity at power plants!
Steam Turbines are Classified as
Steam Turbines can be classified on the basis of a number of factors. Some of the
important methods of steam turbine classification are enunciated below:
On the basis of Stage Design:
Steam turbines use different stages to achieve their ultimate power conversion
goal. Depending on the stages used by a particular turbine, it is classified as Impulse
Turbine, or Reaction type.
On the Basis of the Arrangement of its Main Shaft:
Depending on the shaft arrangement of the steam turbine, they may be classified
as Single housing (casing), tandem compound (two or more housings, with shafts
that are coupled in line with each other) and Cross compound turbines (the shafts
here are not in line).
On the Basis of Supply of Steam and Steam Exhaust Condition:
They may be classified as Condensing, Non Condensing, Controlled or Automatic
extraction type, Reheat (the steam is bypassed at an intermediate level, reheated
and sent again) and Mixed pressure steam turbines (they have more than one source
of steam at different pressures).
On the basis of Direction of Steam Flow:
They may be axial, radial or tangential flow steam turbines.
On the Basis of Steam Supply:
Superheated steam turbine or saturated steam turbine.
According to method of governing
Throttle governing turbine, Nozzle governing turbine and By pass governing
turbine.
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Basic types of turbine
The two most basic and fundamental types of steam turbines are the impulse turbine and
the impulse reaction turbine.
The Impulse Turbine
The impulse turbine consists of a set of stationary blades followed by a set of rotor
blades which rotate to produce the rotary power. The high pressure steam flows through
the fixed blades, which are nothing but nozzles, and undergo a decrease in pressure
energy, which is converted to kinetic energy to give the steam high velocity levels. This
high velocity steam strikes the moving blades or rotor and causes them to rotate. The
fixed blades do not completely convert all the pressure energy of the steam to kinetic
energy, hence there is some residual pressure energy associated with the steam on exit.
Therefore the efficiency of this turbine is very limited as compared to the next turbine we
are going to review- the reaction turbine or impulse reaction turbine.
Fig.2.4.1 Diagram of an Impulse Turbine Fig.2.4.1a. An Impulse TurbineStage
Working of Impulse Turbine
The impulse turbine involves striking of the blades by a stream or a jet of high pressure
steam, which causes the blades of the turbine to rotate. The direction of the jet is
perpendicular to the axis of the blade. It was realized that the impulse turbine is not very
efficient and requires high pressures, which is also quite difficult to maintain. The
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impulse turbine has nozzles ,that are fixed, to convert the steam to high pressure steam
before letting it strike the blades.
Impulse turbine mechanism
Impulse turbine mechanism deals with the Impulse force action-reaction.
It works on Newton third law which state that," Every action has equal and
opposite reaction".
As the water falls on the blade of the rotor it generates the impact force on the
blade surface, The blade tends to give the same reaction to the fluid, but the rotor
is attached to the rotating assembly, it absorb the force impact and give the
reaction in the direction of the fluid flow. Thus the turbine rotates.
The rotational speed of the turbine depends on the fluid velocity. More the fluid velocity,
greater is the rotational speed, and greater the speed have the power generation.
The Reaction Turbine
The reaction turbine is a turbine that makes use of both the impulse and the reaction of
the steam to produce the rotary effect on the rotors. The moving blades or the rotors here
are also nozzle shaped (They are aerodynamically designed for this) and hence there is a
drop in pressure while moving through the rotor as well. Therefore in this turbine the
pressure drops occur not only in the fixed blades, but a further pressure drop occurs in the
rotor stage as well. This is the reason why this turbine is more efficient as the exit
pressure of the steam is lesser, and the conversion is more. The velocity drop between the
fixed blades and moving blades is almost zero, and the main velocity drop occurs only in
the rotor stage.
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Fig.2.4.2 Diagram of a Reaction Turbine Fig.2.4.2a. A Reaction Turbine
Stage
WORKING OF REACTION TURBINE
In the reaction turbine, the rotor blades themselves are arranged to form convergent
nozzles. This type of turbine makes use of the reaction force produced as the steam
accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by
the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference
of the rotor. The steam then changes direction and increases its speed relative to the speed
of the blades. A pressure drop occurs across both the stator and the rotor, with steam
accelerating through the stator and decelerating through the rotor, with no net change in
steam velocity across the stage but with a decrease in both pressure and temperature,
reflecting the work performed in the driving of the rotor.
This type of turbine makes use of the reaction force produced as the steam accelerates
through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed
vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the
rotor. The steam then changes direction and increases its speed relative to the speed of the
blades. A pressure drop occurs across both the stator and the rotor, with steam
accelerating through the stator and decelerating through the rotor, with no net change in
steam velocity across the stage but with a decrease in both pressure and temperature,
reflecting the work performed in the driving of the rotor.
Applications of Steam Turbine
The Steam turbines of today are mostly used in the power production field. Steam
turbines are used to efficiently produce electricity from solar, coal and nuclear power
plants owing to the harmlessness of its working fluid, water/steam, and its wide
availability. Modern steam turbines have come a long way in increasing efficiency in
performance and more and more efforts are being made to try and reach the ideal steam
turbine conditions, though this is physically impossible! Almost every power plant in the
world, other than hydro electric power plants, that use turbines that run on water (the
Francis, Pelton turbines also have the influence of steam turbines) , use steam turbines for
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power conversion. With all the scientific advancement in power generation being
attributed to them, steam turbines really have changed the way the world moves!
Steam turbines are devices which convert the energy stored in steam into rotational
mechanical energy. These machines are widely used for the generation of electricity
Utility Steam Turbine Applications:
Applications for utility Steam Turbines are applied for control of straight condensing,
reheat and non-reheat steam turbines up to 300MW. These upgrades may include
integrated generator control for generator protection and excitation/ AVR upgrades,
utilizing the latest commonly available industry-standard digital equipment.
Industrial application of steam turbine:
Applications of Industrial Steam Turbines cover all straight condensing, non-condensing,
and automatic extraction steam turbines. Specific design features are incorporated to
address control issues often unique to process plants including paper mills, oil refineries,
chemical plants, and other industrial applications, generator and mechanical drive.
Some of the world’s largest turbines manufacturing companies that are seeing the
rewards of research and steam turbine advances are coming together to develop highly
efficient turbines. The collaboration of Mitsubishi Heavy Machinery and General Electric
Energy (GE Energy) for the conceptualization and design of a highly efficient “next-
generation” steam turbine for its inception in combined cycle gas turbine power plants
recently has further proved that there is still a lot to be achieved in steam turbine related
research and development, and that the scope for improvement can be much higher.
Compounding:
When expansion of steam takes place from the high initial pressure to the exhaust
pressure in only one stage, the velocity of the steam will be very high and this will set up
excessive blade speeds, far above the normal useful speeds. Further, “the lost velocity or
the leaving loss", namely the kinetic energy of the steam leaving the turbine will also be
high. Therefore, in order to restrict the rotational speed to the turbine and also to
minimize the leaving loss, the exhaust steam from the first ring of moving blades is
diverted to a second ring of moving blades with the help of a ring of stationary or fixed
blades. There may be two or more rings of moving blades keyed to a common shaft and
in between two rings of moving blades there will be a ring of fixed blades usually
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anchored to the turbine casing. This way of reducing rotor speed is known as
“COMPOUNDING ".
Methods of Compounding:
1. Pressure Compounding
2. Velocity Compounding
Pressure Compounding (The Rateau turbine)
The Pressure drop available to the turbine is used in a series of small increments, each
increment being associated with one stage of the turbine. The physical arrangement is
shown in the figure 2.6.1. The nozzles are carried in diaphragms which separate each
stage from next. The steam Pressure in the space between each pair of diaphragms is
constant, but there is a pressure drop across each diaphragm as required by the nozzles.
Precaution must be taken to prevent leakage of the steam from one section to next at the
shaft and outer casing. The steam speeds and hence the blade speeds, are low if the
number of stages is high in (figure 3) the variation of pressure & Velocity through the
turbine are shown. The final pressure being that of the condenser, and the final velocity
that required for the steam to leave the turbine. In fig. 3 only one set of wheels is shown,
but these may be followed by another set with a larger mean radius. Each of the stages
can be analysed by the method used previously for the single stage. A turbine with a
series of simple impulse stage is called a pressure compounding.
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Figure 2.6.1 Pressure –compounded impulse turbineshowing pressure and velocity variations
Velocity compounding (the Curtis turbine):
From, previous considerations it is seen that in the simple impulse stage the optimum
condition of blade speed is hardly practical, and with the speeds actually used only a
small amount of the kinetic energy of the steam can be utilized. The velocity
compounded stage, called the Curtis stage after its designer, is used to employ lower
blade speeds and a higher utilization of the kinetic energy of the steam. In this type all the
expansion takes place in a single set of nozzles, and the steam then passes through a
series of blades attached to a single wheel or rotor. Since the blade moves in the same
direction it is necessary to change the direction of the steam between one set of
moving blades & the next. For this purpose a stationary ring of blades is fitted between
each pair of moving blades. A two row wheel version of this turbine is shown in fig
2.6.2.
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Figure 2.6.2 Two-row velocity-compounded impulse turbineshowing pressure and velocity variations
Axial Turbine stage velocity Triangles -Impulse steam turbine:
The steam supplied to a single wheel impulse turbine expands completely in the nozzles
and leaves with a high absolute velocity. This is the absolute inlet velocity to the blades
as shown in figure 2.7.1(a). The steam is delivered to the wheel at an angle α.i. The
absolute velocity Cai can be considered as the resultant of the blade velocity Cb, and the
velocity of the steam relative to the blade at inlet, Cri. The two points of particular interest
are those at the inlet and exit of the blades. The velocities of these points are as shown in
figure 2.7.1 (b) as Cri a Cre respectively, and the directions are defined by the angles βi &
βe as shown. The velocity triangle for the inlet conditions is drawn in Figure 2.7.2 (a)
from the information of figure 2.7.1. The steam leaves the blade with a velocity, Cre,
relative to the blade, and at the blade exit angle of βe. the absolute velocity at exit Cae is
determined from the velocity triangle of Figure 2.7.2(b) and its direction is α .e. Since both
triangles have the common side OA = Cb, the triangles can be combined to give a single
diagram shown in figure 2.7.2(c).
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Driving force on wheel = m ∆Cw
Power output = m Cb ∆Cw
Figure 2.7.1 Absolute and relative velocities for a simple impulse turbine blade
Figure 2.7.2 Inlet (a) and Outlet (b) blade velocity diagrams for an impulse turbineand a composite diagram (c)
Figure 2.7.3Absolute Velocities at inlet and exit and the forces produced
Axial Turbine stage velocity Triangles - Reaction steam turbine:
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The reaction turbine applies the principle of both the pure impulse and the pure
reaction turbine. Each stage of the reaction turbine consists of a fixed row of blades over
the whole of the circumferential annulus, and an equal number of blades on a wheel.
Admission of fluid in the reaction turbine takes place over the complete annulus, and so
there is full admission. The fixed blade channels are of nozzle shape and there is a
comparatively small drop in pressure accompanied by an increase in velocity. The fluid
then passes over the moving blades and as in the pure impulse turbine, a force is exerted
on the blades by the fluid. There is a further drop in pressure as the fluid passes through
the moving blades, since the moving blade channels are also of nozzle shape, and
therefore there is an increase in the fluid velocity relative to the blades. This is illustrated
in the velocity diagram of figure 2.8.1(a). With a simple impulse type the value of Cre
would be given by AD, but in the reaction turbine this velocity is increased to AC by
further expansion of the fluid in the blade channels. The net change in velocity of the
fluid is given by BC and the resultant force on the blades by m(CB) and shown in figure
2.8.1(b). This force can be resolved into the tangential and axial thrust, m(CE) and m(EB)
as shown in figure2.8.1(b).
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Figure 2.8.1 Blade velocity (a) and thrust diagram (b) for an axial-flow reaction
turbine
Ranking Cycle
A power cycle continuously converts heat (energy released by the burning of fuel) into
work (shaft work), in which a working fluid repeatedly performs a succession of
processes. In the vapour power cycle, the working fluid, which is water, undergoes a
change of phase. Figure 2.9.1 gives the schematic of a simple steam power plant working
on the vapour power cycle. Heat is transferred to water in the boiler from an external
source (furnace, where fuel is continuously burnt) to raise steam, the high pressure, high
temperature steam leaving the boiler burnt) to raise steam, the high pressure, high
temperature steam leaving the boiler expands in the turbine to produce shaft work, the
steam leaving the turbine condenses into water in the condenser (where cooling water
circulates), rejecting heat, and then the water is pumped back to the boiler.
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For each process in the vapour power cycle, it is possible to assume a hypothetical or
ideal process which represents the basic intended operation and involves no extraneous
effects. For the steam boiler, this would be a reversible constant pressure heating process
of water to form steam, for the turbine the ideal process would be a reversible adiabatic
expansion of steam, for the condenser it would be a reversible constant pressure heat
rejection as the steam condenses till it becomes saturated liquid, and for heat pump, the
ideal process would be the reversible adiabatic compression of this liquid ending at the
initial pressure. When all these four processes are idea, the cycle is an ideal cycle, called
a Ranking cycle. This is a reversible cycle. Figure 2.9.1 shows the flow diagram of the
Rankine cycle, and in Figure 2.9.2, the cycle has been plotted on the p-v, T-s, and h-s
planes. The numbers on the plots correspond to the numbers on the flow diagram. For
any given pressure, the steam approaching the turbine may be dry saturated (state 1)
given pressure, the steam approaching the turbine may be dry saturated (state 1) wet
(state 1’), or superheated (state l1), but the fluid approaching the pump is, in each case,
saturated liquid (state 3). Steam expands reversibly and adiabatically in the turbine from
state 1 to state 2 (or 1 to 2, or 1 to 2), the steam leaving the turbine condenses to water
in the condenser reversibly at constant pressure from state 2 (or 2, or 2) to state 3, the
water at state 3 is then pumped to the boiler at state 4 reversibly and adiabatically, and the
water at state 3 is then pumped to the boiler at state 4 reversibly and adiabatically, and the
water is heated in the boiler to form steam reversibly at constant pressure from state 4 to
state 1 (or 1 or 1).
Figure 2.9.1 A simple steam plant
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For purposes of analysis the Rankine cycle is assumed to be carried out in a steady flow
operation. Applying the steady flow energy equation to each of the processes on the basis
of unit mass of fluid, and neglecting changes in kinetic and potential energy, the work
and heat quantities can be evaluated in terms of the properties of the fluid.
Figure 2.9.2 Rankine cycle on p-v, T-s and h-s diagrams
41
3421
11
)()(
hh
hhhh
Q
WW
Q
W pTnet
The work ratio is defined as the ratio of net work output to positive work output.
Work ratioT
PT
T
net
W
WW
W
W
Usually, the pump work is quite small compared to the turbine work and is sometimes
neglected. Then h4 = h3, and the cycle efficiency approximately becomes
41
21
hh
hh
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The efficiency of the Rankine cycle is presented graphically in the T-s plot in Fig.2.9.3.
Thus Q1 is proportional to area 1564, Q2 is proportional to area 2563, and Wnet (=Q1-Q2)
is proportional to area 1 2 3 4 enclosed by the cycle.
Figure 2.9.3 Q1, Wnet and Q2 are proportional to areas
The capacity of a steam plant is often expressed in terms of steam rate, which is defined
as the rate of steam flow (kg/h) required producing unit shaft output (1 kW). Therefore
kW
skJ
kJ
kg
WWareSteam
PT 1
/1.
1
kWs
kJ
WWkWs
kg
WW PTPT
36001
The cycle efficiency is sometimes expressed alternatively as heat rate which is the rate
output (Q1) required to produce unit work output (1kW)
Heat ratekWs
kJ
WW cyclePT 36003600
From the equation 2
1
,dpWrev it is obvious that the reversible steady-flow work is
closely associated with the specific volume, of fluid flowing through the device. The
larger the specific volume, the larger the reversible work produced or consumed by the
steady-flow device. Therefore, every effort should be made to keep the specific volume
of a fluid as small as possible during a compression process to minimize the work input
and as large as possible, during an expansion process to maximize the work output.
In steam or gas power plants , the pressure rise in the pump or compressor is equal to the
pressure drop in the turbine if we neglect the pressure losses in various other components.
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In steam power plants, the pump handles liquid, which has a very small specific volume,
and the turbine handles vapour, whose specific volume is many times larger. Therefore,
the work output of the turbine is much larger than the work input to the pump. This is one
of the reasons for the overwhelming popularity of steam power plants in electric power
generation.
If we were to compress the steam exiting the turbine back to the turbine inlet pressure
before cooling it first in the condenser in order to “save” the heat rejected, we would have
to supply all the work produced by the turbine back to the compressor. In reality, the
required work input would be still greater than the work output of the turbine because of
the irreversibilities present in both processes.
LOSSES AND EFFICIENCY
Energy Flow diagram for the impulse stage of an axial turbine
GAS TURBINE
A simple gas turbine unit consists of three components, viz., a compressor, a heat
addition device and a turbine. These three components can be arranged either in an open
or a closed form. Accordingly, a gas turbine cycle can be classified into two categories:
i) Open-cycle arrangement, ii) Closed-cycle arrangement
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Of the two, open-cycle arrangements are much more common. In this arrangement fresh
atmospheric air is drawn into the system continuously and energy is added by combustion
of fuel in the working fluid itself. The products of combustion are expanded through the
turbine and exhausted into the atmosphere. In the closed-cycle, the same working fluid,
be it air or some other gas, is repeatedly circulated through the system. It may be noted
that in this type of plant whether the working fluid. is air or some other gas, fuel cannot
be burnt directly in the working fluid and the necessary energy must be added in a heater
or gas boiler.
OPEN-CYCLE ARRANGEMENTS
If a gas turbine is to be operated at a fixed speed and fixed load condition such as peak-
load power generation, a single shaft arrangement as shown in Fig. 3.1.1 may be suitable.
Flexibility of operation, i.e., the rapidity with which the machine can accommodate itself
to changes of load and speed, and efficiency at part load are in this case considered
unimportant. A heat exchanger can be added as shown in Fig. 3.1.2 to improve the
thermal efficiency, although for a given size of the plant, power output may be reduced
by 10% due to pressure losses in the heat exchanger.
Figure 3.1.1 A simple gas turbine
Figure 3.1.2 A simple gas turbine with a heat exchanger
The modified form, as shown in Fig. 3.1.3, is more suitable for fuels whose products of
combustion contain constituents which may corrode or erode the turbine blades. It is
much less efficient than the simple cycle power plant because the heat exchanger,
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inevitably less than perfect, in transferring the energy input because of the effectiveness
of the heat exchanger. Such a cycle may be considered only if inferior fuels are to be
used. When flexibility of operation is of paramount importance, such as in road, rail and
marine applications, a mechanically independent power turbine is used.
In this twin-shaft arrangement (Fig. 3.1.4), the compressor and high- pressure turbine
combination acts as a gas generator for the low pressure turbine. Fuel flow to the
combustion chamber is controlled to achieve van- action of power. It should be noted that
this will cause a decrease in cycle pressure ratio and maximum temperature. At off-
design conditions the power output reduces with the result that the thermal efficiency
deteriorates considerably at part loads.
Alternative arrangements to overcome the above disadvantages are the series flow and
parallel flow gas turbines (Figs. 3.1.5 and 3.1.6). In these arrangements power output is
controlled by the adjustment of fuel supply to the combustion chamber in the power
turbine line.
Fig. 3.1.3 A simple gas turbine with heat exchanger – an alternative arrangement
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Fig. 3.1.4 A simple twin shaft arrangement
Fig. 3.1.5 Series flow twin shaft arrangement
Fig. 3.1.6 Parallel flow twin shaft arrangement
The performance of a gas turbine may be improved substantially by reducing the work of
compression and/or increasing the work of expansion. For any given pressure ratio, the
power required for compression per kg of working fluid is directly proportional to the
inlet temperature. If, therefore, the compression process is carried out in two or more
stages with intervolving, the work of compression will be reduced. This arrangement is
shown in (Fig. 3.1.7).
Similarly, the turbine output can be increased by dividing the expansion into two or more
stages and reheating the gas to the maximum permissible temperature between the stages
(Fig. 3.1.8). By employing a heat exchanger, the cost of additional fuel can be minimized.
Complex cycles offer good part load performance and high flexibility but it is to be noted
that the inherent simplicity and compactness of the power plant are lost.
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To obtain higher thermal efficiencies without a heat exchanger, a high pressure ratio is
essential. Axial compressors are normally preferred, particularly for large units, as its
efficiency is appreciably higher than that of the centrifugal compressor. Unfortunately,
axial compressors are more prone to instability when operating at off-design conditions.
The unstable operation, manifested by violent aerodynamic vibration, is likely when a gas
turbine is started up or operated at low power. The problem is particularly severe if an
attempt is made to obtain a pressure ratio of more than 8:1 in one compressor.
Fig. 3.1.7 Series flow with intercooling
Fig. 3.1.8 Series flow with reheating
One way of overcoming this difficulty is to divide the compressor into two or more
sections. This mechanical separation permits each section to run at different rotational
speed. When the compressors are mechanically independent, each will have its own
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turbine. The LP compressor is driven by the LP turbine and the HP compressor by the
FTP turbine. This arrangement is called straight compounding. Power is normally taken
either from the low pressure turbine shaft or from an additional free power turbine. This
configuration is generally referred to as a twin-spool engine (Fig. 3.1.9). This
arrangement is widely used both for shaft power units and for the turbojet aircraft
engines, employing pressure ratios in the range of 10:1 to 20:1.
Fig. 3.1.9 Straight compounded twin spool arrangement
An arrangement, known as cross compounding wherein the LP compressor is driven by
the HP turbine and the HP compressor by the LP turbine, is claimed to give better
efficiency at part load. Unfortunately, the effect on stability of operation is the opposite
of straight compounding, i.e., it makes the problem worse instead of better.
THE CLOSED-CYCLE
In all the arrangements discussed so far, atmospheric pressure and temperature have been
considered as the datum, and the exhaust gases were discharged at atmospheric pressure.
The average exhaust temperature will be around 700 K for an average maximum
temperature of about 1000 K. It follows that 1 kg of gas will occupy a volume according
to the law PV = mRT which is about of 2.34 m. At low pressure-ratios the constant-
pressure cycle requires large mass flow rate, thus the total volume flow rate for any given
power output will be quite large. In order to accommodate such large flow, a large rotor
diameter and long blades are required. This results in excessive stress at the root of the
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blades due to large centrifugal force. Hence, there is an upper limit on the size and power
output of a turbine or compressor.
It will be shown in the next chapter that the efficiency expression of various constant-
pressure cycles is a function of pressure-ratio and the gas temperatures: i.e., the efficiency
depends not upon the magnitude of the pressures, but only upon their ratio. However, the
volume occupied by a gas depends upon the magnitude of the pressure. When the volume
to be circulated becomes large, the pressure level can be raised so that the volume per kg
of fluid, i.e., the specific volume, is reduced to the level desired. However, the same
pressure ratio (i.e., ratio of maximum to minimum pressures) should be maintained as
before for attaining the same efficiency. It is true that the maximum pressure will
increase, but only in proportion to the change of absolute pressures on the system.
Thus, when the system is closed from the atmosphere, the same fluid will circulate again
and again. It follows that if the fuel is burnt directly in the circulating air, the oxygen will
soon deplete and combustion will fail. It is, thus necessary to supply a certain amount of
fresh air to the closed circuit. The normal overall air-fuel ratio of a gas turbine is between
60:1 to 100:1. It follows that only a small fraction of burnt oxygen is to be supplied each
time through the combustion chamber in order to have continuous operation. Thus,
majority of the exhaust gas leaving the turbine would return to the compressor entrance;,
i.e., the inlet temperature to the compressor would be greatly increased. It may be noted
that the work of compression per kg can be shown to be
11
1
C
TCW p
c
which is seen to depend upon the inlet temperature, T1 and the pressure ratio, . Thus, in a
closed-cycle, the net output would be much reduced unless a gas cooler is added between
the turbine exhaust and the compressor inlet to cool the gas which is being recalculated,
down to approximately normal inlet temperature. The detail of such a system with
internal firing is shown in Fig. 3.2.1.
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Fig. 3.2.1 A closed cycle arrangement with air as working medium
The compressor draws gas from turbine exhaust at point (1) through a regenerator and
gas cooler under pressure p1, which is greater than atmospheric pressure. After
compression, the gas at pressure p2 is delivered to combustion chamber through
regenerator. In the regenerator the corn- pressed gas is heated to higher temperature by
the main turbine exhaust. In the combustion chamber, fuel is added to obtain the desired
maximum gas temperature. From combustion chamber the gases pass on to main turbine,
where they expand from pressure P2 to pressure P1 . The turbine provides net work to the
load after meeting the compression work of the cycle. A small portion of the gases
leaving the combustion chamber is by-passed to auxiliary turbine, where expansion takes
place from p2 to patm.
The auxiliary turbine develops enough work to run a compressor, which draws air from
atmosphere and delivers it at pressure p2 of the combustion chamber. The amount of fresh
air supplied by this compressor must be equal to the amount of gases by-passed after
combustion to atmosphere via auxiliary turbine and also to the amount of air which is
sufficient to burn completely the fuel that is added to maintain the maximum temperature
of the cycle and thereby the maximum output of the main turbine.
Figure 4.11 illustrates another method on the closed-cycle concept, where instead of
supplying the fuel directly to, and burning it with, the air inside the system, the fuel could
be burned in a separate combustion chamber built like a regenerator. The heat of
combustion is transferred through the containing walls of the furnace to the air flow in
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the turbines as shown in Fig. 3.2.2.. Thus, the air or gas contained in the closed system
can be used over and over again without the necessity for make-up air. These results in
that the products of combustion do not come in contact with the moving parts and no
deposit will accumulate on the turbine blades. In such type of closed circuit every kind of
solid fuels like coal, can also be burned in the furnace. Further the working medium other
than air having the desired properties can be used. The details of working of such a
system are shown in Fig. 3.2.2. Advantages and disadvantages of a closed-cycle system
are enumerated in the following sections.
Fig. 3.2.2 Another closed cycle arrangement with working medium other than air
Advantages(i) Use of high pressure (and hence gas density) level throughout the cycle would result
in a reduced size of the plant for a give output. (ii)Wide range of load variation is
possible by varying the pressure levels without altering the maximum cycle temperature.
Hence, there will be almost no variation of overall efficiency. (iii)Erosion of the turbine
blades due to the products of combustion is eliminated. (iv)Filtration of working medium
is not required except charging for the first time. (v) High density of the working medium
improves the effectiveness of the heat exchanger. (vi) Gases other than air having more
desirable thermal properties, such as helium etc., with = 1.66 can be used to increase
the power output and thermal efficiency. (vii) Cheaper fuels can be used.
Disadvantages
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(i)The heating system is quite bulky. (ii)It is quite difficult to make the system
absolutely leak proof. (iii) Large capacity cooler is necessary. (iv) Useful only for
stationary power plants.
BASIC REQUIREMENTS OF THE WORKING MEDIUM
In a closed-cycle arrangement, the operating medium can be other than air and it must
satisfy the following requirements.
(i) Availability as well as cheapness’ of the working medium.
(ii) The circulating working medium must be stable, non-explosive and non-
corrosive.
(iii) It should be non-toxic and non-inflammable.
(iv) It should have high specific heat value Cp, and high specific heat ratio,
(v) It must have a higher thermal conductivity, k.
Applications of Gas Turbine
Gas turbines can he classified into aircraft and industrial gas turbines, the second term
meaning all those gas turbine power plants which are not included in the first category.
The aircraft gas turbines differ from the industrial gas turbines in three main aspects.
(i) The life of the industrial gas turbine is expected to be of the order of 120,000
hours without major overhaul as against 600-1200 hours for aircraft gas
turbines.
(ii) Size and the weight of an aircraft power plant is very crucial compared to
industrial units. .
(iii) The aircraft power plant can make use of kinetic energy of the gases leaving the
exhaust whereas it is wasted in other types and consequently, this energy loss
must be kept as minimum as possible.
These differences in the requirements have considerable effect on design, although
fundamental theory is same for both the categories. Industrial gas turbines are rugged in
construction, with many auxiliary types of equipment. They often employ a single, large
cylindrical combustion chamber. They are also designed for multifuel capability.
Apart from the aircraft market, the widest application of gas turbines has been in pump
sets for oil and gas transmission pipe lines and generation of electricity. So far gas
turbines have made no inroads into the world of merchant shipping but it is extensively
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used in naval operations. A major disadvantage of the gas turbine in naval use is its poor
part load performance and higher specific fuel consumption. To overcome this problem,
combined power plants consisting of gas turbines in conjunction with steam turbines,
diesel engines and other gas turbines have been considered.
To date, little impact has been made in the field of rail transport. Experimental trains
have been operating in some countries. The high speed passenger train with gas turbine
power is an attractive concept for the future. Maybe in the near future, a long haul truck
market will provide a major application for the gas turbine: Major automobile industries
are active in developing engines in the range of 200 — 300 kW. These vehicular engines
employ low pressure ratio, centrifugal compressor, free power turbine and a rotary heat
exchanger. Concern with exhaust pollution will be a critical factor in favour of gas
turbine. The major problem is still with high part-load fuel consumption.
Another concept of potentially great importance is the so-called Total Energy Plant,
where exhaust heat is used to provide building heating in winter and refrigeration/air
conditioning in the summer. Other uses for energy in a gas turbine’s exhaust are found in
process industries. The gas turbine can also be used as a compact air compressor suitable
for supplying large quantities of air at moderate pressures.
The principle of jet propulsion is obtained from the application of New- ton’s laws of
motion. We know that when a fluid is to be accelerated, a force is required to produce
this acceleration in the fluid. At the same time, there is an equal and opposite reaction
force of the fluid on the engine which is known as the thrust. Hence, it may be stated that
the working of jet propulsion is based on the reaction principle. Thus all devices that
move through fluids must follow this basic principle.
In principle, any fluid can be used to achieve the jet propulsion. Thus water, steam or
combustion gases can be used to propel a body in a fluid. But there are limitations in the
choice of the fluid when the bodies are to be propelled in the atmosphere. Experience
shows that only two types of fluids are particularly suitable for jet propulsion.
i. A heated and compressed atmospheric air — admixed with the products of
combustion produced by burning fuel in that air can be used for jet propulsion.
The thermochemical energy of the fuel is utilized for increasing the temperature
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of the air to the desired value. The jet of this character is called a thermal jet and
the jet propulsion engine using atmospheric air is called air breathing engines.
ii. Another class of jet-propulsion engines use a jet of gas produced by the
chemical reactions of fuel and oxidizer. Each of them is carried with the system
itself The fuel-oxidant mixture is called the propellant. No atmospheric air is used
for the formation of the jet. But the oxidant in the ropel1ant is used for generating
the thermal jet. A jet produced in this way is known as rocket jet and the
equipment wherein the chemical reaction takes place is called a rocket motor. The
complete unit including the propellant is, called a rocket engine.
From the above discussion it is clear that jet-propulsion engines may be classified
broadly into two groups. .
(i) air breathing engines and (ii) rocket engines
Air breathing engines can be further classified as follows:
(i) Reciprocating or propeller engines (ii) Gas turbine engines.
GAS TURBINE ENGINES
World War II was the turning point for the development of gas turbine technology. All
modern aircrafts are fitted with gas turbines. Gas turbine engines can be classified into.
(i) ramjet engines, (ii)pulse jet engines, (iii) turbojet engines, (iv)turboprop engines, and
(v)turbofan engines.
Taken in the above order they provide propulsive jets of increasing mass flow and
decreasing jet velocity. Therefore, in that order, it will be seen that ramjet can be used for
highest cruising speed whereas the turboprop engine will be useful for the lower cruising
speed at low altitudes. In practice, the choice of the power plant will depend on the
required cruising speed, desired range of the aircraft and maximum rate of climb.
The details of various gas turbine engines mentioned above are discussed under two
categories: (i) pilotless operation, and (ii) piloted operation. The ramjet and pulse jet
engines come under the category of pilotless operation whereas the turboprop and
turbojet engines are used for piloted operation,
GAS TURBINES FOR SURFACE VEHICLES
The problems and design features of gas turbines employed by surface vehicles are
considerably different from those of aircraft gas turbines.
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Automobiles
Attempts were made by a number of automobile manufacturing companies in various
countries to perfect gas turbine engines for cars. An exhaust heat exchanger was used to
improve the fuel economy. However, inspite of a high degree of technological success,
the gas turbine car engine at present cannot commercially compete with the well-
established piston engine. Some degree of success was achieved in the field of heavy
vehicles with engines of over 200 kW. Many designs employed the combination of an
axial turbine and a low pressure centrifugal compressor along with a rotary heat
exchanger.
The gas turbine automobile engine is mechanically sound and pollutes the atmosphere at
a lesser rate. However it suffers from its inherent high speed, and poor part-load
performance.
Railway locomotives
Long distance passenger trains have employed gas turbine locomotives in many
countries. Gas turbine locomotives (with electrical transmission) can be introduced in
sectors where electric traction is uneconomical.
Exhaust superchargers
Small gas turbines are also used in automobiles in another way. All large truck and
railway diesel locomotive engines are supercharged. They employ exhaust gas driven
turbines (axial or inward flow radial) to drive the centrifugal air compressors (super
chargers).
Hovercrafts
Commercial and naval services are now employing an ever increasing number of air
cushion crafts. They have certain advantages over marine vehicles. Gas turbine provides
all the power-for lift and propulsion in such crafts. In a typical 100 t U.S. design, the air
cushion is generated and maintained by eight lift fans. Three marine gas turbines provide
propulsion through the propellers located astern.
The world’s first commercial hovertrain developed in France employs air screw
propulsion and has a speed of 300 km/h. The train has two sets of six air cushions.
Hydrofoils
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Hydrofoils employ the lift on an aerofoil to lift the craft above the water surface. This
enables them to move at comparatively higher speeds (120 km/h). The application of gas
turbines for powering hydrofoils is on a relatively small scale.
3.6 GAS TURBINES FOR ELECTRIC POWER GENERATION
Gas turbines are used for electric power generation in a number of ways. Some of its
main advantages are ability to start quickly, lower cooling water requirement, and high
temperature and low pressure of the working medium.
Aeroengines
The life of a terrestrial gas turbine between overhauls is 20,000-30,000 h compared to
3000-7000 h for aero engines. Full advantage of the high level of design of aero engines
is taken in designing gas turbine power plants for electric power generation.
Turboprop engines with some modifications of the combustion chamber and bearings can
be used to drive electric generators through reduction gears. A number of derated turbojet
engines (gas generators) can supply the working gas to a separate power turbine. A large
plant uses eight jet engines feeding four power turbines giving a total output of 120-140
MW. Another 100 MW single-stage gas turbine plant employees ten jet engines around
the periphery of a single turbine rotor for supplying gas to its various sectors of nozzles.
Aeroengines in power stations can be brought to full-load operation from cold in a few
minutes. This makes them ideal for peak load operation.
Topping plant
The temperature of the exhaust gases in a gas turbine is high. Therefore the use of gas
turbine plants in electric power stations without any heat recuperating apparatus makes
them uneconomical. Figure 3.6.1 shows a gas turbine as a topping plant. The gas
turbine forms the high temperature loop, whereas the steam plant forms the low
temperature loop. The connecting link between the two loops (or cycles) is the steam
boiler working on the exhaust heat of the gas turbine. The outgoing exhaust gases also
heat the feed water of the steam cycle. The gas turbine, as shown here, woks wholly as a
gas generator for the steam plant, whereas the steam turbine drives the generator. In
another arrangement the gas turbine can also drive a generator, thus contributing to the
output of the combined gas – steam plant.
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Fig. 3.6.1 A topping gas turbine plant (combined gas – steam plant)
Fig. 3.6.2 Gas turbine plant in the total energy system
Total energy plant
Satisfying the demands of heating, cooling and electrical energy from a single source is
the total energy concept. The type of fuel used in a total energy system may be liquid or
gaseous and is chosen on economic considerations, variability, cost and transportation.
Figure 3.6.2 shows a gas turbine plant in the-total energy system. The steam boiler
utilizes the energy in the high temperature turbine exhaust gases. Steam can be used
directly for space heating. For cooling purposes, steam is utilized in producing chilled
water in an absorption chiller. The overall efficiency of the total energy plant is between
60 and 75%.
Nuclear plant
Figure 3.6.3 shows a closed circuit nuclear gas turbine plant. Helium gas is used both as a
coolant in the reactor and the working fluid in the closed circuit gas turbine plant. Helium
after compression is first heated in the heat exchanger and then in the reactor. The high
pressure and temperature (p 25-50 bar, T=1000-1200K) gas drives the helium turbine.
The turbine drives both the compressor and the load (electric generator).
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Fig. 3.6.3 A closed circuit nuclear gas turbine plant
3.7 GAS TURBINES IN PETRO-CHEMICAL INDUSTRIES
Gas turbines have special applications in a variety of industries. Some advantages of the
gas turbines in these applications are:
1. A variety of fuels can be used in gas turbine plants. Some process gases (which
are otherwise lost) can also be used. 2. The energy its exhaust gas can be used in
various processes. 3.They can he used conveniently for industrial utilities, such as
corn- pressed air, hot gases, steam, hot water, mechanical and electrical power. 4.
It is easy to install, cheap, compact and competitive (cost wise); it has ability to
combine with other equipment. 5. Ease in speed regulation in industrial drives.
Figure 3.7.1 shows a gas turbine supplying preheated combustion air to boilers. The
cooling of air after the supercharger reduces the compressor size and its work.
Fig. 3.7.1 Gas turbine supplying preheated combustion air for steam boilers
Figure 3.7.2 shows a gasifier supplying hot gases for an industrial process. Additional
fuel is burnt in a combustion chamber placed after the turbine depending on the heat
requirement in the process. The hot gases, after the process can be further used in steam
boilers. The starting turbine runs of compressed air.
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Fig. 3.7.2 Gasifier supplying hot gases for an industrial process
Figure 3.7.3 shows the application of the gas turbine and compressor in the manufacture
of nitric acid. The gas turbine works on the waste heat of the process it drives the axial
and centrifugal stages of the compressor. Oxygen is removed from the high pressure air
before injecting steam. A steam turbine is employed for starting the plant.
Fig. 3.7.3 Pressurized process used in the manufacture of nitric acid
3.8 GAS TURBINES IN CRYOGENICS
Engineering and scientific aspects of considerably low temperature (- 157°C) form the
subject matter of cryogenics. Low temperatures can be obtained by:
1. Isenthalpic Joule-Thomson expansion and 2.Isentropic expansion.
Isentropic expansion was first obtained by reciprocating expanders which had problems
at very low temperatures. Rotating machines on account of high speeds (up to 6x105 rpm)
are most suitable for this purpose. High speed turbo-expanders employing 8-16 mm
diameter inward flow radial turbines give very low temperatures. Helium and hydrogen
turbo-expanders do not have problems of high Mach number because of the high speed of
sound.
Figure 3.8.1 shows the La-Fleur helium gas turbine cryogenic refrigeration system.
Various processes occurring in this plait are shown in the T-s plane in Fig. 3.8.2.
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Helium is first compressed (AB) to a high pressure in a common compressor at the exit of
the compressor helium s divided between the power and the refrigeration cycles.
In the closed circuit power cycle helium is passed through a regenerator (BC) before
heating it by burning fuel. The heater or combustion chamber (CD) raises its temperature
for doing work in the turbine (DE). The turbine power is used to drive the compressor.
The exhaust from the turbine is sent back to the compressor inlet through the hot side
(EF) of the regenerator and a precooler. Thus the output of the closed circuit helium gas
turbine plant is the high pressure helium available for the refrigeration cycle. This is first
cooled in the after cooler (BM) and
Fig. 3.8.1 The La-Fleur helium gas turbine refrigeration system
Fig. 3.8.2 T-s diagram of the La-Fleur refrigeration system
the regenerator (MP) before expanding it in the high-speed inward flow radial cryogenic
turbine (PQ). The expansion of helium in the turbine reduces it to a very low temperature.
The power output of the cryogenic turbine can also be utilized in driving the helium
compressor. Low temperature helium after having been used for low temperature
refrigeration (QN) goes back to the compressor through the cold side of the regenerator.
MISCELLANEOUS APPLICATIONS OF GAS TURBINES
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Only some major applications of gas turbines have been described in this chapter the aim
of this is to highlight the significance of the gas turbine as a single element in the given
total system.
By virtue of the extreme simplicity and light-weight design, small gas turbines (some of
them working on hot air) have found wide applications in many fields from surgery to
aerospace. Small air turbines are used to operate the drills used by orthopedic and dental
surgeons. The low temperature air after expansion in these turbines is used for cooling
the drilled region. Small gas turbines working on high energy fluids at low flow rates are
employed in the auxiliary power units in under-water and aerospace vehicles. Gas turbine
drives for turbo-pumps are used in rockets and missiles.
Other applications of gas turbines are in steel making, oil and gas pumping, marine
propulsion and helicopter rotor drives.
COMP4RISON OF GAS TURBINES WITH RECIPROCATING ENGINES
Gas turbine is also an internal combustion engine. Its competitor in early stages was the
reciprocating internal combustion engines. Let us compare them.
Advantages of Gas Turbines over Reciprocating Engines
(i) Mechanical efficiency Mechanical efficiency of the gas turbine is considerably
higher than that of the best reciprocating engine. For simple gas turbine design
mechanical efficiency of 90% to 95% has been claimed while for reciprocating
engine it is from 85 to 90% under full load conditions. It is due to more
frictional losses in reciprocating engines.
(ii) Balancing Due to absence of any reciprocating mass in gas turbine engine,
balancing can be near perfect. Torsional vibrations are absent because gas
turbine is a steady flow machine.
(iii)Cost In case of larger output gas turbine units of 2500 kW; it can be built at an
appreciably lower cost and in a shorter time than the corresponding multi
cylinder petrol or diesel engines.
(iv)Weight The fuel consumption per kW hour of best available aircraft gas turbine is
almost twice that of the normal petrol engine. However, it has much lighter
weight per kW so that the total weight of turbine plus fuel does not compare
unfavourably with reciprocating type of engine and its fuel. To give quantitative
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example, the specific weight of (a) steam turbine is about 53 kg/kW, (b) diesel
engine is about 115 kg/kW and (c) gas turbine is about 20 kg/kW.
(v) External shape and size The basic cylindrical shape of turbine and compressor
unit renders the gas turbine more convenient to start, especially in aircraft and
locomotives.
(vi)Fuel The turbine can be designed to operate with cheaper and more readily
available fuels such as benzene, powdered coal, and heavy graded
hydrocarbons. Promising results have been obtained using furnace oil and also
pulverized coal as fuel.
(vii) Lubrication Compared with reciprocating engines the lubrication of gas
turbines is comparatively simpler. The requirement is chiefly to lubricate the
main bearing, compressor shaft and bearings of auxiliaries.
(viii) Maintenance The fact that the gas turbine consists of essentially a single
turbine and compressor unit with a common or coupled shaft running in a
relatively smaller number of main bearings, only minimum maintenance is
necessary as compared to the reciprocating internal combustion engines.
(ix)Low operating pressures The gas turbine generally operates at relatively low
pressures so that the parts exposed to these pressures can be made light although
the effects of thermal expansion and contraction must be taken into account.
The maximum combustion pressure is much lower than that in reciprocating
engines so that the pressure joints and piping do not pose any difficulty.
(x) Silent operation Since the exhaust from a gas turbine occurs under practically
constant-pressure conditions unlike the pulsating nature of reciprocating engine
exhaust, the turbine and compressor, if dynamically balanced, can run very
smoothly. The usual vibrational noises as in the case of reciprocating engine are
almost absent.
(xi)Smokeless exhaust With the present tendency to use relatively large surplus air
for combustion in order to reduce temperature of gases, the exhaust from the
turbine is almost smokeless and generally free from pungent odour associated
with optimum and rich fuel mixture which is characteristic of reciprocating
engines.
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(xii) High operational speed Turbine can be made lighter than the reciprocating
engine of similar output. It can be run at much higher speed than reciprocating
engines. The output of any engine varies directly as the product of the driving
shaft torque and its rpm. Therefore, for a given output and higher speed the
torque will be lower. It may be noted that the torque characteristics of the gas
turbine is much better than that of reciprocating engine, since the former gives a
high initial torque and its variation with speed is comparatively less.
Advantages of Reciprocating Engines over Gas Turbines
1. Efficiency The overall efficiency of the turbine is much less than the
reciprocating engine since 70% of the output of the turbine is to be fed to the
compressor and other accessories and auxiliary parts.
2. Temperature limitation The maximum temperature in gas turbine cannot exceed
1500 K because of the material consideration of the blade while in reciprocating
engines with complete combustion of the fuel the maximum temperature can be
raised to 2000 K. This high temperature is permitted since the piston and cylinder
head are subjected to this high temperature only for a fraction of a second.
3. Cooling We can achieve very good results by efficient cooling in reciprocating
engine by which the heat of the cylinder walls is taken away, which enables to
keep the wall temperature only around 500 K but in gas turbine, cooling is
complicated, and, therefore, much higher temperature cannot be allowed to reach.
4. Starting difficulties It is more difficult to start a gas turbine than a reciprocating
engine as it requires compressed air or some suitable starter mechanism which are
complicated.
***
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INDUSTRIAL PUMPS AND ITS APPLICATIONS
Pumps are used to move fluids (liquids or gases) and slurries from one place to another
by mechanical action. Normally, liquids are moved by gravity through pipes and channels
from elevated tanks. Sometimes, a storage vessel pressurized by an external source of
compressed gas, the liquids are moved somewhere. But by far the most common devices
for the purpose are pumps. The pump is installed in a pipeline to provide the energy
needed to draw liquid from a reservoir and discharge a constant volumetric flow rate at
the exit of the pipeline. Severe erosion and cavity problems may reduce the pump
capacity. So always it is essential to maintain the suction pressure higher than the vapour
pressure of the liquid.
Pumps increase the mechanical energy of the liquid, velocity of the liquid and
pressure of the liquid. Pumps can be classified by their method of displacement
as positive displacement pumps, impulse pumps, velocity pumps, gravity pumps, steam
pumps and valve less and Centrifugal pumps. Positive displacement units apply pressure
directly to the liquid by a reciprocating piston, or by rotating members which form
chambers alternately filled by and emptied of the liquid. Centrifugal pumps generate high
rotational velocities and kinetic energy of the liquid to pressure energy. In pumps, the
liquid density does not change and remains constant.
Many different industries employ different pumps for varied uses. For example,
cryogenics use centrifugal pumps in extreme cold applications; dairy farmers use
centrifugal pumps to keep their product at the proper temperatures, hot and cold; electric
utility companies use centrifugal pumps, or turbines, to produce energy; food service,
construction, distillery, and automotive companies are a few more examples of industries
that employ centrifugal pumps for their many applications. Positive displacement pumps
are used to transport wide range of liquids, slurry and foams to be transported without
product degradation. For example, Bakery – dough, fats, fruit filling, icing, oil, yeast;
Beverages – beer, fruit concentrate, fruit juices, mash; Candy – chocolate, cocoa butter,
corn syrup, gelatin, sugar; Canned Foods – baby food, jams, jellies, mayonnaise, potato
salad, pudding, relish, stews; Cosmetics – creams, emulsions, jellies, lotions, shampoo,
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toothpaste; Dairy – butter, cream, curds, ice cream, margarine, milk, soft cheese, yogurt
etc.
Various types of pumps used in the chemical industry are centrifugal pumps,
reciprocating pumps and helical rotor pumps. Centrifugal pumps operate by applying a
centrifugal force to fluids, many times with the assistance of impellers. These pumps are
typically used in moderate to high flow applications with low-pressure head, and are very
common in chemical process industries. There are three types of centrifugal pumps—
radial, mixed, and axial flow pumps. In the radial pumps, pressure is developed
completely through a centrifugal force, while in axial pumps pressure is developed by lift
generated by the impeller. Mixed flow pumps develop flow through a centrifugal force
and the impeller.
Reciprocating pumps compress liquid in small chambers via pistons or diaphragms.
These pumps are typically used in low-flow and high-head applications. Piston pumps
may have single or multiple stages and are generally not suitable for transferring toxic or
explosive material. Diaphragm pumps are more commonly used for toxic or explosive
materials. Helical rotor pumps use a rotor within a helical cavity to develop pressure.
All pumps use basic forces of nature to move a liquid. As the moving pump part
(impeller, vane, piston diaphragm, etc.) begins to move, air is pushed out of the way. The
movement of air creates a partial vacuum (low pressure) which can be filled up by more
air, or in the case of water pumps.
Mechanical pumps serve in a wide range of applications such as pumping water from
wells, aquarium filtering, pond filtering and aeration. In the car industry for water-
cooling and fuel injection, In the energy industry for pumping oil and natural gas or for
operating cooling towers. In the medical industry, pumps are used for biochemical
processes in developing and manufacturing medicine, and as artificial replacements for
body parts, in particular the artificial heart and penile prosthesis.
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The diagram below provides an overview of pump classification by type
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Rotary positive displacement pumps
The reciprocating pumps, rotary pumps contain no check valves, close tolerances
between the moving and stationary parts minimize leakage from the discharge space back
to the suction space they also limit the operating speed. Rotary pumps operate best on
clean, moderately viscous fluids, such as light lubricating oil.
Gear pumps: - A simple type of rotary pump where the liquid is pushed between two
gears. The gears rotate with close clearance inside the casing.
This is the simplest of rotary positive displacement pumps. It consists of two meshed
gears that rotate in a closely fitted casing. The tooth spaces trap fluid and force it around
the outer periphery. The fluid does not travel back on the meshed part, because the teeth
mesh closely in the centre. Gear pumps see wide use in car engine oil pumps and in
various hydraulic power packs.
Screw pumps - the shape of the internals of this pump is usually two screws turning
against each other to pump the liquid
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Rotary vane pumps
A cylindrical rotor encased in a similarly shaped housing. As the rotor orbits, the vanes
trap fluid between the rotor and the casing, drawing the fluid through the pump
Positive displacement pumps
Positive displacement pumps have an expanding cavity on the suction side and a
decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the
suction side expands and the liquid flows out of the discharge as the cavity collapses. The
volume is constant given each cycle of operation.
Reciprocating pumps are classified as follows:
Plunger pumps
A reciprocating plunger pushes the fluid through one or two open valves, closed by
suction on the way back.
Diaphragm pumps
Similar to plunger pumps, where the plunger pressurizes hydraulic oil which is used to
flex a diaphragm in the pumping cylinder. Diaphragm valves are used to pump hazardous
and toxic fluids.
Piston pumps displacement pumps
Usually simple device for pumping small amount of liquid or gel manually. The common
hand soap dispenser is such a pump.
Air Operating Diaphragm Pump
An air operated double diaphragm pump has two diaphragms. These diaphragms are
connected by a shaft in the center section. The diaphragms are working as separation wall
between the air and the liquid. The air valve is located in the center section of the
diaphragm pump. The air valve directs the compressed air to the back of diaphragm
number one. This way, diaphragm number one moves away from the center section. This
diaphragm causes a press stroke moving liquid out of the pump. At the same time
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diaphragm number two is performing a suction stroke. The air behind diaphragm number
two is being pushed out to the atmosphere. Atmospheric pressure pushes the liquid to the
suction side. The suction ball valve is being pushed away off its seat. This allows the
fluid to flow along the ball valve into the liquid chamber.
When the pressurized diaphragm number one has reached the end of its stroke, the
movement of the air is switched from diaphragm number one to the back of diaphragm
number two by the air valve. The compressed air pushes diaphragm number two away
from the center block. Doing so, diaphragm number one is pulled toward the center
block. In pump chamber number two the discharge ball valve is pushed off its seat. In
pump chamber number one the opposite occurs. Upon completion of the stroke the air
valve leads the air again to the back of diaphragm number one and restarts the cycle as
described above.
Vacuum Pump
A vacuum pump converts the mechanical input energy of a rotating shaft into pneumatic
energy by evacuating the air contained within a system. The internal pressure level thus
becomes lower than that of the outside atmosphere. The amount of energy produced
depends on the volume evacuated and the pressure difference produced. Pumps typically
operate to serve various chemical process support equipments such as chillers, cooling
towers, material transfer, etc., pumping is considered an individual process separate from
the processes of the aforementioned equipment.
Advantages of Centrifugal Pump
As there is no drive seal so there is no leakage in pump, It can pump hazardous liquids,
There are very less frictional losses, There in almost no noise, Pump has almost have 100
efficiency.
Advantages of Reciprocating pump
Reciprocating pumps will deliver fluid at high pressure. They are 'Self-priming' - No
need to fill the cylinders before starting.
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Disadvantages of Piston Pumps
Reciprocating pumps give a pulsating flow. The suction stroke is difficult when pumping
viscous liquids. The cost of producing piston pumps is high. This is due to the very
accurate sizes of the cylinders and pistons. Also, the gearing needed to convert the
rotation of the drive motor into a reciprocating action involves extra equipment and cost.
The close fitting moving parts cause maintenance problems, especially when the pump is
handling fluids containing suspended solids, as the particles can get into the small
clearances and cause severe wear. The piston pump therefore, should not be used for
slurries.
Disadvantages of Centrifugal Pumps
Most centrifugal pumps are not self-priming. In other words, the pump casing must be
filled with liquid before the pump is started, or the pump will not be able to function.
References
http://en.wikipedia.org/wiki/Centrifugal_pump
http://www.renewables-info.com/drawbacks_and_benefits/geothermal_heat_pumps_%25E2%2580%2593
www.italvacuum.it/, www.ksb.com/_Pumps
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Staff Training Programme on “Applied Hydraulics and Fluid Machines”Topic: Dimensional Analysis
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106
DIMENSIONAL ANALYSISINTRODUCTION
Dimensional Analysis is a mathematical technique that makes use of the dimensions as a
tool to the solution of several engineering problems. Each physical phenomenon can be
expressed by an equation composed of physical quantities (or variables). These physical
quantities may be dimensional or non-dimensional quantities. Through dimensional
analysis, the physical quantities or variables can be arranged in a systematic fashion and
the physical quantities can be combined to form non-dimensional parameters.
Uses of dimensional analysis in the study of fluid mechanics:
1. Testing the dimensional homogeneity of any equation in fluid mechanics
2. Deriving equations expressed in terms of non-dimensional parameters to show the
relative significance of each parameter
3. Planning model tests and presenting experimental results in a systematic manner
using non-dimensional parameters; this enables analysis of even complex fluid
flow phenomenon.
DIMENSIONS
Engineers and scientists use various physical quantities to describe a physical
phenomenon. These physical quantities can be described by a set of quantities which are
in a sense independent of each other. These quantities are called fundamental quantities
or primary quantities.
The primary quantities are mass, length, time, and temperature denoted by M, L, T and
respectively.
All other physical quantities such as area, volume, acceleration, force, energy, power, etc.
are termed as derived quantities or secondary quantities. These quantities are called
secondary quantities because they can be expressed in terms of physical quantities.
The expression for a derived quantity in terms of the primary quantities is called the
dimension of the physical quantity. For instance, let us derive the dimension of the
derived quantity namely, force.
As per Newton’s second law of motion, the dynamic force is the product of mass and
acceleration. Acceleration, too, is a derived quantity which is the rate of change of
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velocity. Velocity is yet another derived quantity which represents the rate of change of
displacement. The dimensions of velocity are: LT-1. Hence, the dimensions of
acceleration are: LT-2; so, the dimensions of force are: MLT-2.
Some engineers prefer to use force instead of mass as fundamental quantity because force
is easy to measure. In such a case, the physical phenomenon is represented by variables
expressed in F-L-T system instead of M-L-T system. The advantage with the dimensional
form of any quantity is that it is independent of the system of units and enables us to
convert from one system of units to the other system of units.
DIMENSIONAL HOMOGENEITY
The Fourier’s principle of dimensional homogeneity states that an equation which
expresses a physical phenomenon must be algebraically correct and dimensionally
homogeneous.
An equation is said to dimensionally homogeneous, if the dimensions of the terms on the
left hand side of the equation are same as the dimensions of the terms on the right hand
side of the equation.
Illustration of dimensional homogeneity
Consider the expression for discharge in a rectangular weir,
Q = (2/3)Cd(2g)1/2 LH3/2
Let us list the SI units and dimensions of the various quantities in the above expression
Quantity SI unitsDimensions
(M-L-T system)
Discharge, Q m3/s L3T-1
Coefficint of discharge, Cd No units Dimensionless
(Acceleration due togravity)1/2, g1/2
(m/s2)1/2 (LT-2)1/2 = L1/2T-1
Length of the notch, L M L
(Head over the sill ofnotch)3/2, H3/2
(m)3/2 L3/2
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The dimensions of the left hand side of the equation are: L3T-1. the dimensions of the
right hand side of the equation are: (L1/2T-1).L.L3/2 = L1/2+1+3/2. T-1 = L3T-1 Thus we find
that the dimensions of both the LHS and RHS of the equation are the same. Hence, the
equation is dimensionally homogeneous.
The unique characteristic of a dimensionally homogeneous equation is that it is
independent of the system of units chosen for measurement, i.e., if an equation is
dimensionally homogeneous, it can be used without any modification with either system
of units.
METHODS OF DIMENSIONAL ANALYSIS
There are two methods of dimensional analyses as follows:
(A) Rayleigh Method
(B) Buckingham - Method
(A) Rayleigh Method
This method was proposed by Lord Rayleigh in the year 1989 to determine the
effect of temperature on viscosity of a gas. Let X be a variable which is a function of
different variables namely, X1, X2, ……, Xn. This can be written in the general form as
nXXXfX ,......,, 21 ……
(1)
In the above equation, X is the dependent variable and X1, X2, ……, Xn are the
independent variables. In the Rayleigh method, the functional relationship of the
variables X1, X2, ……, Xn is expressed in the form of an exponential equation which must
be dimensionally homogeneous. Hence, equation (1) can be expressed as
nn
ba XXXCX ......21 ……
(2)
where C is a dimensionless constant; C can be determined either from the physical
characteristics of the problem or from experimental measurements. a, b, ……, n are the
exponents of X1, X2, ……, Xn respectively which can be evaluated on the basis that the
equation is dimensionally homogeneous. By grouping together the variables with like
powers, the dimensionless parameters are formed. The Rayleigh method is illustrated in
the following example.
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Illustration
Let us consider the problem of flow of liquid through a circular orifice
discharging freely into the atmosphere under a constant head. Let Q be the discharge
passing through the orifice of diameter d, under a constant head H. Let be the mass
density and let the dynamic viscosity of the liquid discharged through the orifice. Now,
the discharge Q through the orifice can be assumed to be dependent on the variables
namely, diameter d of the orifice, constant head H, mass density of liquid, dynamic
viscosity of liquid and the acceleration due to gravity g since the flow is freely into the
atmosphere. Hence, the general functional relationship for the dependent variable Q can
be written as
),,,,( gHdfQ ……
(3)
Equation (3) can be expressed by Rayleigh method in the exponential form as
edcba gHdCQ ……
(4)
where C is a dimensionless constant
The following Table shows the SI units and the dimensions of the various quantities
considered in this illustration.
Quantity with symbol SI units Dimensions (in MLT system)
Discharge, Q m3s-1 M0L3T-1
Dynamic viscosity, kg(mass)m-1s-1 ML-1T-1
Mass density, kg(mass)m-3 ML-3T0
Diameter, d m M0LT0
Head, H m M0LT0
Gravitational constant, g ms-2 M0LT-2
Dimensionless constant, C - M0L0T0
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Substituting the dimensions for each variable in equation (4)
M0L3T-1 =(M0L0T0) (ML-1T-1)a (ML-3T0)b (M0LT0)c (M0LT0)d (M0LT-2)e
For dimensional homogeneity of the above equation, the exponents of each of the
dimensions M, L and T on both sides of the equation must be identical. Thus
for M: 0 = a + b
(5a)
for L: 3 = - a – 3b + c + d + e
(5b)
for T: -1 = - a – 2e
(5c)
Now, there are 5 unknowns namely a, b, c, d and e; but there are only 3 equations; hence,
three of the unknowns must be expressed in terms of the other two.
From equation (5a), b = - a ……
(6a)
From equation (5c),22
1 ae ……
(6b)
From equation (5b),22
1)(33
adcaa
21
23
3 dca
da
c 2
325
……
(6c)
Substituting the values of b, c and e from equations (6a), (6c) and (6b) in (4), we have,
22
1
2
3
2
5 a
dd
a
aa gHdCQ
=
ddaa
aa dHgdgdC 22
3
2
1
2
5
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=
da
d
H
gdgddC
2/12/32
1
2
12
=
da
d
H
gdg
ddC
2/12/32
1
2/12 1
=
2/1
2/12/32
12 1
dd
H
gdgdC
da
=
2/1
2/1
2/12/32
12 H
d
H
gdgdC
da
=
2/1
2/12/32
12/12
da
d
H
gdgHdC
=
2/1
2/12/32 2
424
da
d
H
gdgHd
C
=
d
H
gdfgHa ,2
2/12/31
This expression may be written in the usual form as
gHaCQ d 2 ……
(7)
where Cd is the coefficient of discharge of the orifice
dC
d
H
gdf ,
2/12/31 ……
(8)
In the above expression, both the terms
d
H
gd,
2/12/3
are dimensionless and Cd is
also a dimensionless factor.
Buckingham - Method
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Statement of Buckingham’s - Theorem: If a phenomenon is described by n
dimensional variables, and if these n dimensional variables can be completely described
by m fundamental quantities or dimensions (such as mass, length, time, etc.), and are
related by a dimensionally homogeneous equation, then the relationship among the n
quantities (or variables) can always be expressed by (n – m) dimensionless and
independent terms.
Let Y be a variable which depends on the independent variables X1, X2, X3, ……,
Xn. Then, the functional equation can be written as
Y = f(X1, X2, X3,……, Xn) ……
(9)
Equation (9) can be transformed to another functional relationship as
f1(Y, X1, X2, X3,……, Xn) = C ……
(10)
where C is a dimensionless constant. This is as if Y = f(X) = X2 + C; hence, Y – X2 = f1(X,
Y) = C. In accordance with the Buckingham’s - theorem, a non-dimensional equation
can be obtained as
f2(1, 2, 3, ……, n-m) = C1 ……
(11)
How are these - terms formed?
Each dimensionless - term is formed by combining m variables out of the total n
variables with one of the remaining (n – m) variables. These m variables in each of the -
terms are the same. As these m variables appear repeatedly in each of the - terms, these
variables are called repeating variables.
How are these repeating variables chosen?
These repeating variables are chosen from among the n variables such that they
involve all the m fundamental quantities or dimensions and they themselves do not form
any dimensionless number. Thus the different π - terms may be established as below.
132111111 ...... m
mm
cba XXXXX |
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232122222 ...... m
mm
cba XXXXX | ……
(12)
………………………………….. |
nmm
cbamn XXXXX mnmnmnmn ......321 |
In equation (12), each individual equation is dimensionless and the exponents a, b, c, d,
……, m, etc., are determined by considering the dimensional homogeneity for each
equation so that each - term is dimensionless.
The final general equation for the phenomenon may be obtained by expressing one
- term as a function of other - terms. That is,
mnf ,......,, 43211 |
mnf ,......,, 43122 |
……………………………… | (13)
13211 ,......,, mnmn f |
Illustration of Buckingham’s - method
Let us consider the same problem of flow through a small orifice as considered
under the Rayleigh’s method.
Step 1. The discharge of an orifice depends upon the diameter d of orifice, constant
supply head H, acceleration due to gravity g, dynamic viscosity of liquid and mass
density of liquid. The functional equation for discharge Q can be written as
),,,,( gHdfQ ……
(14)
Equation (14) can be expressed in its most general form as
CgHdQf ),,,,,(1 ……
(15)
The total number of variables (including both the dependent variable Q and all the
independent variables) n = 6. All these variables can be expressed by the three
fundamental dimensions of either the M-L-T or F-L-T system. Hence, the number of
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fundamental quantities m = 3. Therefore, the number of dimensionless - terms to be
formed are (n – m) = (6 – 3) = 3, so that
13212 ),,( Cf ……
(16)
Step 2. Selection of Repeating Variables.
In order to form these - terms, we have to choose m = 3 repeating variables. The
criteria for choosing these m repeating variables is that these variables among themselves
contain all the three fundamental dimensions and they themselves do not form any
dimensionless parameter. Thus let us choose the dynamic viscosity with dimensions
ML-1T-1, constant supply head H with dimension L and acceleration due to gravity g with
dimensions LT-2 as the repeating variables.
Step 3. Formulation of the different - terms.
QgH cba 1111
2222
cba gH ……
(17)
dgH cba 3333
Step 4. Determination of the - terms
Let us express the 1 – term in the dimensional form using the M-L-T system.
132110001
111 TLLTLTMLTLMcba
Equating the exponents of M, L and T, we obtain
for M: 0 = a1 ……(18a)
for L: 0 = - a1 + b1 + c1 + 3 ……(18b)
for T: 0 = - a1 – 2c1 – 1 ……(18c)
From (18a), a1 = 0; from (18c), c1 = - ½; from (18b), b1 = - 5/2
Hence,2/12/5
2/12/501 gH
QQgH
Now, Let us express the 1 – term in the dimensional form using the M-L-T system.
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32110002
222 MLLTLTMLTLMcba
Equating the exponents of M, L and T, we obtain
for M: 0 = a2 + 1 ……(19a)
for L: 0 = - a2 + b2 + c2 - 3 ……(19b)
for T: 0 = - a2 – 2c2 ……(19c)
From (19a), a2 = - 1; from (19c), c2 = ½; from (19b), b2 = 3/2
Hence, 2 2/12/31 gH =
2/32/1 Hg
Now, Let us express the 3 – term in the dimensional form using the M-L-T system.
LLTLTMLTLMcba 333 211000
3
Equating the exponents of M, L and T, we obtain
for M: 0 = a3 ……(20a)
for L: 0 = - a3+ b3+ c3 + 1 ……(20b)
for T: 0 = - a3 – 2c3 ……(20c)
From (20a), a3 = 0; from (20c), c3 = 0; from (20b), b3 = - 1
Hence, 3 dgH 010 =H
d
Step 5.
As per equation (16), we have,
13212 ),,( Cf
H
dHg
gH
Qf ,,
2/32/1
2/12/52
= C1
or
2/12/5 gH
Q=
H
dHgfC ,
2/32/1
32
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Modeling and Similitude
Modeling and Similitude is to develop the procedures for designing models so that the
model and prototype will behave in a similar fashion.
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Model vs. Prototype
A model is a representation of a physical system that may be used to predict the behavior
of the system in some desired respect. Mathematical or computer models may also
conform to this definition, our interest will be in physical model.The physical system for
which the prediction are to be made.
Models that resemble the prototype but are generally of a different size, may involve
different fluid, and often operate under different conditions. Usually a model is smaller
than the prototype. Occasionally, if the prototype is very small, it may be advantageous to
have a model that is larger than the prototype so that it can be more easily studied. For
example, large models have been used to study the motion of red blood cells.
With the successful development of a valid model, it is possible to predict the behavior of
the prototype under a certain set of conditions. There is an inherent danger in the use of
models in that the predictions can be made that are in error and the error not detected
until the prototype is found not to perform as predicted. It is imperative that the model be
properly designed and tested and that the results be interpreted correctly.
Similarity of Model and Prototype
Hydraulic models may be either true or distorted models. True models reproduce features
of the prototype but at a scale - that is they are geometrically similar.
Geometric similarity
Geometric similarity exists between model and prototype if the ratio of all corresponding
dimensions in the model and prototype are equal.
Lprototype
el
Lp
Lm
L
Lmod
whereL is the scale factor for length.
For area
L
prototype
el
pL
mL
A
A 22
2mod
All corresponding angles are the same.
Kinematic similarity
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Kinematic similarity is the similarity of time as well as geometry. It exists between model
and prototype
i). If the paths of moving particles are geometrically similar
ii). If the rations of the velocities of particles are similar
Some useful ratios are:
Velocity vLmLp
TmLm
Vp
Vm
T
L
/
/
Acceleration aT
L
pp
mm
p
m
LL
TL
a
a
22
2
/
/
Discharge QT
L
PP
mm
p
m
TL
TL
Q
Q
3
3
3
/
/
This has the consequence that streamline patterns are the same.
Dynamic similarity
Dynamic similarity exists between geometrically and kinematically similar systems if the
ratios of all forces in the model and prototype are the same.
Force ratio 22
2
223
3
ULT
LL
T
L
P
M
MP
MM
p
m
L
L
aM
aM
F
F
This occurs when the controlling dimensionless group on the right hand side of the
defining equation is the same for model and prototype.
Validation of Models Design
The purpose of model design is to predict the effects of certain proposed changes in a
given prototype, and in this instance some actual prototype data may be available. The
model can be designed, constructed, and tested, and the model prediction can be
compared with these data. If the agreement is satisfactory, then the model can be changed
in the desired manner, and the corresponding effect on the prototype can be predicted
with increased confidence.
Distorted Models
In many model studies, to achieve dynamic similarity requires duplication of
several dimensionless groups. In some cases, complete dynamic similarity between
model and prototype may not be attainable. If one or more of the similarity requirements
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are not met, for example, if π2 = π2m , then it follows that the prediction π1= π1m
equation is not true; that is, π1≠ π1m. Models for which one or more of the similarity
requirements are not satisfied are called distorted models.
Example
Determine the drag force on a surface ship, complete dynamic similarity requires thatboth Reynolds and Froude numbers be duplicated between model and prototype.
21
21
)()( P
PP
m
mm
gl
VFr
gl
VFr Froude numbers
P
PPP
m
mmm U
lV
U
lV ReRe Reynolds numbers
To match Froude numbers between model and prototype2
1
P
m
P
m
l
l
V
V
To match Reynolds numbers between model and prototype
2
1
P
m
P
m
P
m
P
m
P
m
l
l
U
U
l
l
V
V
U
U 2
3
P
m
P
m
l
l
l
l
If lm/ lp equals 1/100 (a typical length scale for ship model tests) , then υm/υp must be1/1000. Thus, the kinematic viscosity ratio required to duplicate Reynolds numberscannot be attained.Reference1. http://www.freestudy.co.uk/fluid%20mechanics/t6203.pdf2. http://www.efm.leeds.ac.uk/CIVE/CIVE1400/PDF/Notes/section5.pdf3. www.iust.ac.ir/files/mech/mazidi_9920c/fluid_i/Lecture9.ppt4. http://www.docstoc.com/docs/168425252/5. http://www.fkm.utm.my/~syahruls/3-teaching/1-fluid-I/2note/9%20buckingham%
20note % 201.pdf6. http://www.daniel-huilier.fr/Enseignement/Notes_Cours/AnalyseDimensionnelle/
Taiwan Shieh fluid07.pdf
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PNEUMATICS – An Overview
Pneumatics is the study of air and gases and the relationship between volume, pressure
and temperature of the air or gases. Initially used for carrying out simplest mechanical
tasks but is playing an important role in the development in the development of
pneumatic technology for automation.
Compressed air used for:
The use of sensors to determine the status of processes
Information processing
Switching of actuators by means of final control elements
Carrying out work
The pneumatic cylinder has a significant role as a linear drive unit due to its,
Relatively low cost
Ease of installation
Simple and robust
Ready availability in various sizes and lengths
Pneumatic components can perform the following types of motion:
Linear
Swivel
Rotary
Some industrial applications of pneumatics:
General methods of
material handling:
General applications: Machining and
working operations:
Clamping
Shifting
Positioning
Orienting
Packaging
Feeding
Metering
Door control
Transfer of materials
Turning or inverting parts
Sorting of parts
Drilling
Turning
Milling
Sawing
Forming
Finishing
Quality control
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Stocking of components
Stamping or embossing of
components
Advantages and distinguishing characteristics
Availability - Air is available in unlimited quantity
Transport - Can be easily transported in pipelines even over long distances
Storage - Compressor need not be in continuous operation. Even reservoir is
transportable
Temperature - Compressed air is relatively insensitive to temperature fluctuations
Explosion proof- Compressed air offers minimum risk of explosion
Cleanliness- Unlubricated exhaust air is clean
Components- Components are relatively inexpensive
Speed- Compressed air is a very fast working medium
Adjustable- Speeds& forces of pneumatic components are infinitely variable
Overload safe- Pneumatic tools and operating components can be loaded to the
point of stopping
Disadvantages
Preparation - Compressed air requires good preparation. Dirt and condensate should
not be present
Compressible- Not always possible to achieve uniform and constant piston speeds
with compressed air
Force requirement- Compressed air is economical only up to a certain force
requirement
Noise level- Exhaust air is loud
Costs- Compressed air is a relatively expensive of conveying power
A comparison with other forms of energy is essential
Factors to be considered if pneumatics is to be used as a control or working medium:
Work output requirements
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Preferred control methods
Resources & expertise available to support project
Systems currently installed which are to be integrated with the new project
Choice of working media:
Electrics
Hydraulics
Pneumatics
A combination of the above
Selection criteria for the working section:
Force, Stroke, Type of motion, Speed, Size
Service life, Safety, Reliability
Sensitivity, Controllability
Energy Costs
Handling, Storage
Choice of control media:
Mechanical
Electrical / Electronic
Pneumatic (normal pressure or high pressure)
Hydraulic
Selection criteria for the control section:
Reliability of components
Sensitivity to environmental influences
Ease of maintenance and repair
Switching time of components
Signal speed
Space requirements
Service life
Training requirements of operators and maintenance personnel
Modification of the control system
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A pneumatic system can be broken down into a number of levels, representing hardware
and signal flow, as shown in Fig. 1.
Figure 1 Pneumatic system structure and signal flow
The primary levels in a pneumatic system are:
Energy supply
Input elements (Sensors)
Processing elements (Processors)
Actuating devices (Actuators)
ENERGY SUPPLY
Pneumatic Gases
Qualities
The ideal fluid medium for a pneumatic system is a readily available gas that is
nonpoisonous (nontoxic), chemically stable, free from any acids that cause corrosion of
system components, and nonflammable. It also will not support combustion of other
elements.
Gases that have these desired qualities may not have the required lubricating power.
Therefore, lubrication of the components of some pneumatic systems must be arranged
by other means. For example, some air compressors are provided with a lubricating
system, some components are lubricated upon installation or, in some cases, lubrication is
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introduced into the air supply line. Two gases meeting these qualities and most
commonly used in pneumatic systems are compressed air and nitrogen.
Compressed Air
Compressed air has most of the desired properties and characteristics of a gas for
pneumatic systems. The unlimited supply of air and the ease of compression make
compressed air the most widely used fluid for pneumatic systems. Compressed air is a
mixture of all gases contained in the atmosphere. It is nonpoisonous and nonflammable
but does contain oxygen, which supports combustion.
One of the most undesirable qualities of compressed air as a fluid medium for pneumatic
systems is moisture content. The atmosphere contains varying amounts of moisture in
vapor form. Changes in the temperature of compressed air will cause condensation of
moisture in the pneumatic system. This condensed moisture can be very harmful to the
system, as it increases corrosion, dilutes lubricants, and may freeze in lines and
components during cold weather. Although moisture and solid particles must be removed
from the air, it does not require the extensive distillation or separation process required in
the production of other gases.
The supply of compressed air at the required volume and pressure is provided by an air
compressor.
Compressed air systems are categorized by their operating pressures as follows: high-
pressure (HP) air, medium-pressure (MP) air, and low-pressure (LP) air.
Nitrogen
For all practical purposes, nitrogen is considered to be an inert gas. It is nonflammable,
does not form explosive mixtures with air or oxygen, and does not cause rust or decay.
Due to these qualities, its use is preferred over compressed air in many pneumatic
systems, especially aircraft and missile systems, and wherever an inert gas blanket is
required.
Contamination Control
As in hydraulic systems, fluid contamination is also a leading cause of malfunctions in
pneumatic systems. In addition to the solid particles of foreign matter which find a way to
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enter the system, there is also the problem of moisture. Most systems are equipped with
one or more devices to remove this contamination. These include filters, water separators,
air dehydrators, and chemical driers. In addition, most systems contain drain valves at
critical low points in the system. These valves are opened and closed periodically (Either
automatically or manually) to allow the escaping gas to purge a large percentage of the
contaminants, both solids and moisture, from the system.
The air service unit consists of the following:
Compressed air filter
Compresses air regulator
Compressed air lubricator
INPUT ELEMENTS (SENSORS)
Valves
Primary function of the valves is to alter, generate or cancel signals for the purpose of
sensing, processing and controlling. Additionally, the valve is used as a power valve for
the supply of working air to the environment.
They can be divided into a number of groups according to their function in relation to,
Signal type
Actuation method
Construction
Therefore the following categories are relevant
Directional Control Valves (DCV)
Signal elements
Processing elements
Power elements
As a signal element the DCV is operated by roller lever to detect the piston rod position
of a cylinder. The signal element can be small in size and create a small air pulse. A
signal pulse created will be at full operating pressure but have a small flow rate.
As a processing element the DCV redirects, generates or cancels signals depending on the
signal inputs received.
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It can be supplemented with additional elements such as the AND function and the OR
function valves to create desired control conditions.
As a power element the DCV must deliver the required quantity of air to match the
actuator requirements and hence there is a need for larger volume rates and therefore
larger sizes.
DCV can be of,
The poppet type used for small flow rates and for generation of input and process
signals, Or The slide type used for larger flow rates and hence lends itself to the power
and actuator control valve.
Non-return valves and its derivatives:
The non-return valve allows a signal to flow through the device in one direction and in
the other blocks the flow.
There many variations in construction and size derived from the basic non return valve.
Other derived valves utilize features of the non-return valve by the incorporation of non-
return elements.
Flow control valves
A flow control valve restricts or throttles the air in a particular direction to reduce the
flow rate of the air and hence control the signal flow.
If flow control valve is wide open then the flow should be almost the same as if restrictor
not fitted can be fitted with a non-return valve then flow is uni-directional. Flow control
valve as close as possible to working element
Pressure control valves
Pressure regulating valves controls the pressure in a control circuit and keeps the
pressure constant irrespective of any pressure fluctuations in the system.
Pressure limiting valves are utilized on the up-stream side of the compressor to ensure the
receiver pressure is limited, for safety, and that the supply pressure to the system is set to
the correct pressure.
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Pressure sequence valve senses the pressure of any external line and compares the
pressure of the line against a pre-set adjustable value, creating a signal when the pre-set
limit is reached.
Combinational valves
The combined functions of various elements can produce a new function. The new
component can be constructed by the combination of individual elements or
manufactured in a combined configuration to reduce size and complexity.
Valves are described by:
No. of ports or openings (ways)⇒2 way, 3 way, 4 way, etc.
No. of positions⇒2 positions, 3 positions, etc.
Method of actuation of the valve⇒Manual, air-pilot, solenoid, etc.
Methods of return actuation⇒ Spring-return, air-return, etc.
Special features of operation⇒Manual overrides, etc.
PROCESSING ELEMENTS (PROCESSORS)
To support the DCV at the processing level, there are various elements which condition
the signal for the task, viz.
Two pressure valve (AND function)
Shuttle valve (OR function)
They have logic based role and are fitted at the junction of three lines. They have three
connections, 2 in and 1 out.
Modular processing unit consisting of DCV functions and logic elements to perform a
combined processing task have been designed to reduce cost, size and complexity of
system.
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ACTUATING DEVICES (ACTUATORS)
Actuator group includes various types of linear and rotary actuators of varying size and
construction. The actuators are complemented by the final control element, which
transfers the required quantity of air to drive the actuator.
Normally this valve is fitted close to actuator to minimize losses and is connected directly
to the air supply.
Linear actuators
Rotary Actuators
ELECTRO-PNUEMATICS
Electro pneumatics is now commonly used in many areas of industrial low cost
automation. They are also used extensively in production, assembly, pharmaceutical,
chemical and packaging systems. There is a significant change in controls systems. In
recent developments, relays have increasingly been replaced by the programmable logic
controllers in order to meet the growing demand for more flexible automation.
Electro-pneumatic control consists of electrical control systems operating pneumatic
power systems. In this solenoid valves are used as interface between the electrical and
pneumatic systems. Devices like limit switches and proximity sensors are used as
feedback elements.
Electro-pneumatic control integrates pneumatic and electrical technologies, is more
widely used for large applications. In Electro-pneumatics, the signal medium is the
electrical signal either AC or DC source is used. Working medium is compressed air.
Operating voltages from around 12 V to 220 Volts are often used. The final control valve
is activated by solenoid actuation
The resetting of the valve is either by spring [single Solenoid]or using another solenoid
[Double solenoid Valve]. More often the valve actuation/reset is achieved by pilot-
assisted solenoid actuation to reduce the size and cost of the valve
Control of electro-pneumatic system is carried out either using combination of
relays and contactors or with the help of Programmable Logic Controllers [PLC]. A
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Relay is often is used to convert signal input from sensors and switches to number of
output signals [either normally closed or normally open].Signal processing can be easily
achieved using relay and contactor combinations
A PLC can be conveniently used to obtain the out puts as per the required logic, time
delay and sequential operation. Finally the output signals are supplied to the solenoids
activating the final control valves which control the movement of various cylinders. The
greatest advantage of electro pneumatics is the integration of various types of proximity
sensors [electrical] and PLC for very effective control. As the signal speed with electrical
signal, can be much higher, cycle time can be reduced and signal can be conveyed over
long distances.
In Electro pneumatic controls, mainly three important steps are involved:
Signal input devices -Signal generation such as switches and contactor, Various
types of contact and proximity sensors
Signal Processing - Use of combination of Contactors of Relay or using
Programmable Logic Controllers
Signal Outputs -Outputs obtained after processing are used for activation of
solenoids, indicators or audible alarms
ELECTRICAL DEVICES
Seven basic electrical devices commonly used in the control of fluid power systems are:
1. Manually actuated push button switches
2. Limit switches
3. Pressure switches
4. Solenoids
5. Relays
6. Timers
7. Temperature switches
Other devices used in electro pneumatics are:
1. Proximity sensors
2. Electric counters
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Programmable Logic Controllers [PLC]
A PLC can be defined as the digitally operating electronic apparatus which uses a
programmable memory for the internal storage of instructions for implementing specific
functions such as logic, sequencing, timing, counting, and arithmetic to control, through
digital or analog input/output modules, various types of machines or processes.
In essence, the programmable logic controller consists of computer hardware, which is
programmed to simulate the operation of the individual logic and sequence elements that
might be contained in a bank of relays, timers, counters, and other hard-wired
components.
The PLC was introduced around 1969 largely as a result of specifications written by the
General Motors Corporation. The automotive industry had traditionally been a large
buyer and user of electro-mechanical relays to control transfer lines, mechanized
production lines, and other automated systems.
There are significant advantages in using a programmable logic controller rather than
conventional relays, timers, counters, and other hardware elements. These advantages
include:
Programming the PLC is easier than wiring the relay control panel.
The PLC can be reprogrammed. Conventional controls must be rewired and are
often scrapped instead.
PLCs take less floor space then relay control panels.
Maintenance is easier, and reliability is greater.
The PLC can be connected to the plant computer systems more easily than relays
can.
Components of the PLC:
A schematic diagram of a programmable logic controller is presented in fig. 2. The basic
components of the OPLC are the following:
Input module
Output module
Processor
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Memory
Power supply
Programming device
Figure 2 Schematic sketch of the Programmable Logic Controller
The components are housed in a suitable cabinet designed for the industrial environment.
The input module and output module are the connections to the industrial process that is
to be controlled. The inputs to the controller are signals from limit switches, pushbuttons,
sensors, and other ON-OFF devices. In addition, as we will describe later, larger PLCs
are capable of accepting signals from analog devices of the type modeled. The outputs
from the controller are ON-OFF signals to operate motors, valves, and other devices
required to actuate the process.
The processor is the central processing unit (CPU) of the programmable controller. It
executes the various logic and sequencing functions by operating on the PLC inputs to
determine the appropriate output signals. The processor is microprocessor very similar in
its construction to those used in personal computers and other data-processing equipment.
Tied to the CPU is the PLC memory, which contains the program of logic, sequencing,
and other input/output operations. The memory for a programmable logic controller is
specified in the same way as of storage capacity for a computer.
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The PLC is programmed by means of a programming device. The programming device
(sometimes referred to as a programmer) is usually detachable from the PLC cabinet so
that it can be shared between different controllers. Different PLC manufactures provide
different devices, ranging from simple teach pendant-type devices, similar to those used
in robotics, to special PLC programming keyboards and CRT displays.
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