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Transcript of Hydraulic Turbines - · PDF fileHydraulic Turbines Theory of operation ... Governing of...
Hydraulic Turbines
Theory of operation
Flowing water is directed on to the blades of a turbine runner,
creating a force on the blades.
Since the runner is spinning, the force acts through a distance
(force acting through a distance is the definition of work). In this
way, energy is transferred from the water flow to the turbine.
Water turbines are divided into two groups; reaction turbines and
impulse turbines.
Reaction Turbines
Reaction Turbines are acted on by water, which changes
pressure as it moves through the turbine and gives up its energy.
They must be encased to contain the water pressure, or they
must be fully submerged in the water flow.
Newton's third law describes the transfer of energy for
reaction turbines.
Most water turbines in use are reaction turbines.
They are used in low and medium head applications.
Impulse turbines change the velocity of a water jet. The jet impinges
on the turbine's curved blades which almost reverse the flow. The
resulting change in momentum (impulse) causes a force on the
turbine blades.
Since the turbine is spinning, the force acts through a distance (work)
and the diverted water flow is left with diminished energy.
Prior to hitting the turbine blades, the water's pressure (potential
energy) is converted to kinetic energy by a nozzle and focused on the
turbine.
No pressure change occurs at the turbine blades, and the turbine
doesn't require a housing for operation.
Newton's second law describes the transfer of energy for impulse
turbines.
Impulse turbines are most often used in very high head applications.
Impulse Turbines
Pelton Turbine Runner
Buckets
Shaft
Runner
The main components of a Pelton turbine
Breaking jet
Jet striking the splitter and getting split in to two parts
Deflectionangle of jet
jet
Vane
Velocity Triangle at inlet and exit
u1 Vr1
V1=Vw1u
Deflection angle
Vr2
u2 Vw2
Vf2
V2
bf
Vf1=0
FRANCIS TURBINE : An Experimental Set up in the Lab
Various types of water turbine runners.
From left to right: Pelton Wheel, two types of Francis
Turbine and Kaplan Turbine
FRANCIS TURBINE RUNNER
Francis turbine runner, rated at nearly one million hp (750 MW),
being installed at the Grand Coulee Dam, United States.
Francis turbine
and generator
cut-away view
Head across a reaction turbine
Francis Turbine Cross-section
Guide vanes
Volute CasingVolute Casing
Guide vanes
Movingvanes
Draft Tube
ShaftRunner
Axial flow reaction turbine
This is a reaction turbine in which 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 larger to form the boss or the
hub.
A number of vanes are fixed to the boss. When the vanes are
composite with the boss the turbine is called propeller turbine.
When the vanes are adjustable the turbine is called a Kaplan
turbine.
An Axial Flow Turbine Runner
The function of the guide vane is
same as in case of Francis
turbine.
Between the guide vanes and the
runner, the fluid in a propeller
turbine turns through a right-
angle into the axial direction and
then passes through the runner.
The runner usually has four or six
blades and closely resembles a
ship's propeller.
schematic diagram of
propeller or Kaplan turbine.
R1R2
O
f b
V2Vr2
u2
Vf2
Vw2FG
H
E
aq
V1 Vr1
u1
Vf1
Vw1
B
D
CA
Wheel
Tangent
Tangent
Velocity Triangle at inlet and exit
Basic Parameters of a Francis Turbine
Speed ratio = where H is the Head on turbine
Flow ratio = where Vf1 is the velocity of flow at inlet
Discharge flowing through the reaction turbine is given by
Q = D1 B1 Vf1 = D2 B2 Vf2
Where D1 and D2 are the diameters of runner at inlet and exit
B1 and B2 are the widths of runner at inlet and exit
Vf1 and Vf2 are the Velocity of flow at inlet and exit
If the thickness (t) of the vane is to be considered, then the area through which flow takes place is given by ( D1 nt) where n is the number of vanes mounted on the runner.
Discharge flowing through the reaction turbine is given by
Q = ( D1 nt) B1 Vf1 = ( D2 nt) B2 Vf2
Hg
u
2
1
Hg
V f
2
1
60
11
NDu
60
22
NDu
secondper striking water ofWeight
secondper doneWork
2211
2211 1uVuV
ggQ
uVuVQww
ww
11
1uV
gw
Work done per second on the runner = a V1 (Vw1u1 Vw2u2)
= Q (Vw1u1 Vw2u2)
Work done per unit weight =
=
If the discharge at the exit is radial, then Vw2 = 0 and hence
Work done per unit weight =
Hydraulic efficiency =
2211
2211 1
..
..uVuV
HgHQg
uVuVQ
PW
PRww
ww
KAPLAN TURBINE - SUMMARY
Peripheral velocities at inlet and outlet are same and given by
where Do is the outer diameter of the runner
Flow velocities at inlet and outlet are same. i.e. Vf1 = Vf2
Area of flow at inlet is same as area of flow at outlet
where Db is the diameter of the boss.
6021
NDuu o
22
4bo DDQ
Governing of Reaction Turbines
Governing of reaction turbines is usually done by altering the
position of the guide vanes and thus controlling the flow rate by
changing the gate openings to the runner.
The guide blades of a reaction turbine are pivoted and connected
by levers and links to the regulating ring.
Two long regulating rods, being attached to the regulating ring at
their one ends, are connected to a regulating lever at their other
ends.
The regulating lever is keyed to a regulating shaft which is turned
by a servomotor piston of the oil
Governing of reaction turbines
Bulb Turbine The bulb turbine is a reaction turbine of Kaplan type which is used for
extremely low heads.
The characteristic feature of this turbine is that the turbine components as
well as the generator are housed inside a bulb, from which the name is
developed.
The main difference from the Kaplan turbine is that the water flows in a
mixed axial-radial direction into the guide vane cascade and not through a
scroll casing.
The giude vane spindles are normally inclined to 600 in relation to the
turbine shaft and thus results in a conical guide vane cascade contrary to
other types of turbines.
The runner of a bulb turbine may have different numbers of blades
depending on the head and water flow.
The bulb turbines have higher full-load efficiency and higher flow capacity
as compared to Kaplan turbine. It has a relatively lower construction cost.
The bulb turbines can be utilized to tap electrical power from the fast
flowing rivers on the hills
Schematic of Bulb Turbine Power Generating Station
Reaction turbine Impulse turbine
1 Only a fraction of the available hydraulic energy is converted into kinetic energy before the fluid enters the runner.
All the available hydraulic energy is converted into kinetic energy by a nozzle and it is the jet so produced which strikes the runner blades.
2.
Both pressure and velocity change as the fluid passes through the runner. Pressure at inlet is much higher than at the outlet.
It is the velocity of jet which changes, the pressure throughout remaining atmospheric.
3 The runner must be enclosed within a watertight casing (scroll casing).
Water-tight casing is not necessary. Casing has no hydraulic function to perform. It only serves to prevent splashing and guide water to the tail race
4.
Water is admitted over the entire circumference of the runner
Water is admitted only in the form of jets. . There may be one or more jets striking equal number of buckets simultaneously.
5.
Water completely fills at the passages between the blades and while flowing between inlet and outlet sections does work on the blades
The turbine does not run full and air has a free access to the buckets
6.
The turbine is connected to the tail race through a draft tube which is a gradually expanding passage. It may be installed above or below the tail race
The turbine is always installed above the tail race and there is no draft tube used
7.
The flow regulation is carried out by means of a guide-vane assembly. Other component parts are scroll casing, stay ring, runner and the draft tube
Flow regulation is done by means of a needle valve fitted into the nozzle.
A kaplan turbine is to be designed to develop 7,350
kW. The net available head is 5.5 m. Assume that the
speed ratio is 2.09 and flow ratio is 0.68 and the overall
efficiency as 60%. The diameter of the boss is ⅓rd of the
diameter of the runner. Find the diameter of the runner,
its speed and its specific speed.
68.02
1
Hg
V fm/s 13.75.510268.01 fV
09.22
1 Hg
um/s 07.235.51022.21 u
5.5101000
1073506.0;
3
0
QHQg
P
72.22213.7344
2
2
1
22
o
ofbo
DDVDDQ
m/s07.2360
69.6
60
NNDu o
(Ans) rpm37.6705.5
735086.65
45
45
H
PNNs
P = 7350 kW, H = 5.5 m
Q = 222.72m3/s
N=65.86 rpm (Ans)
Do = 6.69 m (Ans)