Session delivered bycourseware.cutm.ac.in/wp-content/uploads/2020/06/Inducer... · 2020. 6. 19. ·...
Transcript of Session delivered bycourseware.cutm.ac.in/wp-content/uploads/2020/06/Inducer... · 2020. 6. 19. ·...
PEMP RMD 2501
Centrifugal Compressors
Session delivered by:Session delivered by:
Prof Q H NagpurwalaProf Q H NagpurwalaProf. Q.H. NagpurwalaProf. Q.H. Nagpurwala
© M.S. Ramaiah School of Advanced Studies07 1
PEMP RMD 2501
Session Objectives
This session is intended to introduce the following:
• Basic aerothermodynamics of centrifugal compressors
• Types of impellersTypes of impellers
• Euler turbine equation and velocity triangles
• Centrifugal compressor performance• Centrifugal compressor performance
• Stall and surge in centrifugal compressors
• Vaneless and vaned diffusers• Vaneless and vaned diffusers
© M.S. Ramaiah School of Advanced Studies07 2
PEMP RMD 2501Centrifugal Compressor
© M.S. Ramaiah School of Advanced Studies07 3
PEMP RMD 2501Introduction
T b hi l i t if l ff t f i i fl idTurbomachines employing centrifugal effects for increasing fluid pressure have been in use for more than a century.
The earliest machines using this method were hydraulic pumps followed later by ventilating fans and blowers.
A centrifugal compressor was incorporated in the Whittle turbojet engine.
A i l fl it bl f l i i t fAxial flow compressors are more suitable for larger engines in terms of smaller frontal area (and drag) and 3-4% higher efficiency for the same duty than centrifugal compressors.
But for very small compressors with low flow rates, the efficiency of axial compressors drops sharply, blading is small and difficult to make accurately, and the centrifugal compressor is again preferable .
Many applications are found in small gas turbines for road vehicles and commercial helicopters as well as bigger applications, e.g., diesel engine turbochargers, chemical plant processes, factory workshop air supplies,
© M.S. Ramaiah School of Advanced Studies07 4
g , p p , y p pp ,large-scale air-conditioning plant, etc.
PEMP RMD 2501Applications of Centrifugal Compressors
Axial compressorAxial compressor
Centrifugal compressor used by Si F k Whi l i
The Turbomeca Centrifugal Compressor
fitt d t th RTM322
© M.S. Ramaiah School of Advanced Studies07 5
Sir Frank Whittle in his gas turbine
fitted to the RTM322 engine
PEMP RMD 2501Applications of Centrifugal Compressors
Rolls Royce Goblin II engine using centrifugal compressor
© M.S. Ramaiah School of Advanced Studies07 6
Use of centrifugal compressor in turbocharger
PEMP RMD 2501Axial-Centrifugal Compressor
Honeywell TPE331-10U / -11U turboprop engine employing axial andemploying axial and
centrifugal compressor stages
© M.S. Ramaiah School of Advanced Studies07 7
PEMP RMD 2501Automotive Gas Turbine Engine
(A) Accessory drive, (B) Compressor, (C) Right regenerator rotor, (D) Variable nozzle unit, (E) Power turbine, (F) Reduction gear, (G) Left regenerator rotor (H) Gas(G) Left regenerator rotor, (H) Gas generator turbine, (I) Burner, (J) Fuel Nozzle, (K) Igniter, (L) Starter-generator, (M) Regenerator drive shaft, (N) Ignition unitunit
Gas turbine engine of a Chrysler car
© M.S. Ramaiah School of Advanced Studies07 8
PEMP RMD 2501Multistage Centrifugal Compressor
© M.S. Ramaiah School of Advanced Studies07 9
Dresser-Rand Company Multistage Centrifugal Compressor
PEMP RMD 2501Return Channels in Multistage
C t if l CCentrifugal Compressor
© M.S. Ramaiah School of Advanced Studies07 10
PEMP RMD 2501Nomenclature
C Absolute velocityn Number of vanesN Rotational speedr RadiusU Impeller speed at tipUe Impeller speed at mean radius of eyee p p yW, V Relative velocityα Absolute flow angleβ Relative flow angleβ gσ Slip factorψ Power input factorω Angular velocityω Angular velocitysuffixesa Axial component, ambientr Radial component
© M.S. Ramaiah School of Advanced Studies07 11
r Radial componentw, θ, u Whirl component
PEMP RMD 2501Single Stage Centrifugal Compressor
(Splitter blade) (Vaned diffuser)
Impeller eyeInducer
© M.S. Ramaiah School of Advanced Studies07 12
PEMP RMD 2501Centrifugal Compressor
Vaneless space
© M.S. Ramaiah School of Advanced Studies07 13Centrifugal compressor stage and velocity diagrams at impeller entry and exit
PEMP RMD 2501Radial Impeller with Diffuser Vanes
Splitter Blade
Main Blade
Diffuser Vane
© M.S. Ramaiah School of Advanced Studies07 14
PEMP RMD 2501Types of Impellers
• One sided• Two sided• Shrouded• Unshrouded• Radial vanes• Backward swept vanes
d• Forward swept vanes
© M.S. Ramaiah School of Advanced Studies07 15
PEMP RMD 2501Types of Impellers
Radial exit impellerBack swept impeller
Forward swept impeller02 90=<β
02 90=>β 0
2 90=β
Forward sweep Vθ < U2p θ 2
Radial exit Vθ = U2
Backward sweep Vθ > U2
© M.S. Ramaiah School of Advanced Studies07 16Impeller with splitter blades Shrouded impeller
PEMP RMD 2501Ideal Performance of Impellers
Head – flow characteristics for various
© M.S. Ramaiah School of Advanced Studies07 17
outlet blade angles
PEMP RMD 2501Velocity Triangles
W2
β
U2
α2β2
Outlet velocity triangle
W1
U1 α1= 0
Cw1= 0W1β1
w1
© M.S. Ramaiah School of Advanced Studies07 18
Inlet velocity triangle
PEMP RMD 2501Inlet Prewhirl
Prewhirl can be achieved by fixing inlet guide vanes to the
icompressor casing.
This changes the inlet velocity triangletriangle.
The work capacity of the compressor decreases.compressor decreases.
Wsp = U2Cw2 - U1Cw1
It i i d t l d iIt is required to properly design the inlet guide vanes in order to minimise pressure loss.
© M.S. Ramaiah School of Advanced Studies07 19
PEMP RMD 2501Pre-Swirl Guide Vanes
© M.S. Ramaiah School of Advanced Studies07 20
PEMP RMD 2501Impeller Intake Mach Numbers
• Air enters axially towards the inducer and then flows through the• Air enters axially towards the inducer and then flows through the radial channel of the impeller.
• Flow separation might occur on the suction surface (convex face) f th d t f th i llof the curved part of the impeller vane.
• There is possibility of shock waves in the inducer tip region for high pressure ratio compressors.
1
1
RTVMγ
=• At higher altitudes M will increase
as there is a decrease in Tas there is a decrease in T.
• To reduce M, V must reduce.
Thi b d b i t d i• This can be done by introducing prewhirl at inducer inlet.
© M.S. Ramaiah School of Advanced Studies07 21
PEMP RMD 2501Specific Work
Specific work is given by Euler turbine equation
( ) ( ))( hhCUCUWW &&
If the flow enters the impeller axially then α = 0 and C = 0 and
( ) ( )01021122 )( hhCUCUmWW wwcc −=−==
If the flow enters the impeller axially, then α1= 0 and Cw1= 0, and the specific work is given by
( ) ( ))( hhCUWW && ( ) ( )010222 )( hhCUmWW wcc −===
For impeller with radial exit, β2= 90° and Cw2= U2, hence specific work
( ) ( )010222 hhUmWW cc −=== &&
© M.S. Ramaiah School of Advanced Studies07 22
PEMP RMD 2501Compression Process on T-s Diagram
© M.S. Ramaiah School of Advanced Studies07 23
PEMP RMD 2501
Absolute velocity is given by
Specific WorkAbsolute velocity is given by,
Rothalpy,
Adding and subtracting ½ U 2 this becomesAdding and subtracting ½ U 2, this becomes
From the velocity triangle
© M.S. Ramaiah School of Advanced Studies07 24
PEMP RMD 2501
Across the impeller I = I hence
Specific WorkAcross the impeller, I1 = I2, hence
This expression provides the reason why the static enthalpy rise in a centrifugal compressor is so large compared with a single-stage axial compressor.
The term is the contribution from the diffusion of relative velocity and the term is the contribution from the centrifugal action caused by th h i dithe change in radius.
For axial entry to the compressor, , and specific work
© M.S. Ramaiah School of Advanced Studies07 25
PEMP RMD 2501Power Input Factor
Due to friction between the casing and the air carried around by the vanes, and other losses which have a braking effect such as di f i ti i d th li d t d th f thdisc friction or windage, the applied torque and therefore the actual work input is greater than the theoretical value. To account for this, a power input factor ψ can be introduced, and the specific work is given by
( ) ( )& ( ) ( )01021122 )( hhCUCUmWW wwcc −=−== ψψ&
ψ > 1
© M.S. Ramaiah School of Advanced Studies07 26
PEMP RMD 2501Slip
Even under ideal (frictionless) conditions the relative flow leaving the impeller does not receive perfect guidance from the vanes and thereceive perfect guidance from the vanes and the flow is said to slip. Hence, β2 > β2’.
vane angleaverage relative vane anglegflow angle
The slip velocity is defined as wCC ≡θVelocity triangle at impeller exit
2'
2 wwws CCC −=
12 <wCd Sli F t 1'
2
2 <=w
w
Cσand Slip Factor,
C
© M.S. Ramaiah School of Advanced Studies07 27
For radial impeller,2
2
UCw=σ
PEMP RMD 2501Slip Factor Correlations
Stanitz correlation
7.02cos
1Zs
βσ −=Wiesner correlation
Stodola correlationStodola correlation
and β’2 is measured from radial direction.
© M.S. Ramaiah School of Advanced Studies07 28
PEMP RMD 2501Slip and Specific Work
Taking into account slip factor and power input factor,
( ) ( )01021122 )( hhCUCUmWW wwcc −=−== ψσψ&&
and for radial impeller with axial inlet
Temperature rise,
22 UW σψ=&
UTT22
0103 σψ
=−Temperature rise,pc
TT 0103
( ) ( ) ( )1
0103
1'0303 1
−−
⎥⎤
⎢⎡ −+=⎟⎟
⎞⎜⎜⎛
=γγγγ
η TTTp c
010101
1 ⎥⎦
⎢⎣+⎟⎟
⎠⎜⎜⎝ TTp
( )12
−⎤⎡
γγσψη U
© M.S. Ramaiah School of Advanced Studies07 2901
21⎥⎥⎦
⎤
⎢⎢⎣
⎡+=
σψηTc
U
p
c
PEMP RMD 2501Diffuser
In a centrifugal compressor, the flow leaving the impeller, passes through diffuser.In a centrifugal compressor, the flow leaving the impeller, passes through diffuser.
The diffuser can be vaneless space, vaned or a combination of both.
The function of the diffuser is to convert the exit kinetic energy into pressure.gy pDiffuser being a static part,
the total conditions (pressure and temperature) do not p )change across it. But the static pressure and temperature increase with
Vaneless diffuser
Vaned diffuser
consequent decrease in absolute velocity.
In high stage pressure ratio g g pcompressors, the diffuser leading edge region is critical because of high Mach
© M.S. Ramaiah School of Advanced Studies07 30
numbers giving rise to shocks and shock losses.
PEMP RMD 2501Types of Diffusers
Uniform thickness curved vanes
Aerofoil shape vanes
Uniform thickness Wedge shape vanes
straight vanes
© M.S. Ramaiah School of Advanced Studies07 31
PEMP RMD 2501Straight Wedge-Shaped Diffuser
© M.S. Ramaiah School of Advanced Studies07 32
PEMP RMD 2501Pipe Diffuser
© M.S. Ramaiah School of Advanced Studies07 33
PEMP RMD 2501
enthalpystaticinchangeIsentropic
Diffuser Performance
enthalpy staticin change Actualenthalpystaticin changeIsentropicefficiencyDiffuser =
( )( )2
22
1
22
21
12
12
2/1
2/1
CCCC
hhhh ss
D −−
=−−
=η
112
112
TTTTTT s
−−
=112
For the isentropic process 1-2s
© M.S. Ramaiah School of Advanced Studies07 34
PEMP RMD 2501
F t t t t 01 02
Diffuser PerformanceFor constant temperature process 01-02
From the gas law
U i t ti f th h l tUpon integrating for the whole process we get
For the constant pressure process, 2s - 2,
Therefore
Equating these expressions for the entropy increase and using
Therefore
and
© M.S. Ramaiah School of Advanced Studies07 35
and
PEMP RMD 2501Volute System
V lVaneless space
© M.S. Ramaiah School of Advanced Studies07 36
PEMP RMD 2501
P f h t i ti l tt d i ti f l i d
Compressor PerformancePerformance characteristics are plotted as variation of total pressure ratio and isentropic efficiency versus corrected mass flow rate, for various corrected speeds .
0101 pTm&
01TN
© M.S. Ramaiah School of Advanced Studies07 37
PEMP RMD 2501Stall and Surge
Stall / surge line
Stable operating point
Any stable operating point lies at the intersection of the compressor and
throttle characteristics
Instability (stall) occurs when the slope of the throttle characteristic
becomes larger than that of the h t i ti
© M.S. Ramaiah School of Advanced Studies07 38
compressor characteristic
PEMP RMD 2501Stall, Surge and Choke
ω
Pressure
Stall Design point
+ i
Deflected flow
Pressure surface
Suction surface
ChokeSurge
At low flow rates at a given speed, the reduction in axial velocity
Stall and Surge
At hi h fl t t i d thChoking
the reduction in axial velocity causes the flow to enter the inducer at large positive incipience resulting in flow separation on the suction
At high flow rates at a given speed, the pressure and density reduce, causing an increase in radial velocity (continuity equation). The relative velocity also become high with negative
surface leading to the phenomenon of rotating stall.
Stall can also initiate at the diffuser d l i i i id
y g gincidence at inducer and diffuser leading ends.
Finally, choking may occur owing to large flow blockage due to separation on the pressure
© M.S. Ramaiah School of Advanced Studies07 39
due to large positive incidence at reduced flow rates.
surface or due to formation of shocks in the inducer / diffuser passages.
PEMP RMD 2501Session Summary
• The construction and components of centrifugal compressors l i dare explained.
• Types of impellers and diffusers are introduced.• Concept of slip and prewhirl and their effects on the specific• Concept of slip and prewhirl, and their effects on the specific
work output are discussed.• Performance characteristics, including the phenomenon of
choking and stall / surge, are discussed.• Performance analysis of diffusers is explained in detail.
© M.S. Ramaiah School of Advanced Studies07 40
PEMP RMD 2501
Thank you
© M.S. Ramaiah School of Advanced Studies07 41