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


PEMP RMD 2501Centrifugal Compressor


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,


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


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


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


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


PEMP RMD 2501Multistage Centrifugal Compressor


Dresser-Rand Company Multistage Centrifugal Compressor

PEMP RMD 2501Return Channels in Multistage

C t if l CCentrifugal Compressor


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


r Radial componentw, θ, u Whirl component

PEMP RMD 2501Single Stage Centrifugal Compressor

(Splitter blade) (Vaned diffuser)

Impeller eyeInducer


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


PEMP RMD 2501Types of Impellers

• One sided• Two sided• Shrouded• Unshrouded• Radial vanes• Backward swept vanes

d• Forward swept vanes


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


outlet blade angles

PEMP RMD 2501Velocity Triangles

W2

β

U2

α2β2

Outlet velocity triangle

W1

U1 α1= 0

Cw1= 0W1β1

w1


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.


PEMP RMD 2501Pre-Swirl Guide Vanes


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.


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 −=== &&


PEMP RMD 2501Compression Process on T-s Diagram


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


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


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


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


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.


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


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


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


PEMP RMD 2501Straight Wedge-Shaped Diffuser


PEMP RMD 2501Pipe Diffuser


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


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


and

PEMP RMD 2501Volute System

V lVaneless space


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


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


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


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.


PEMP RMD 2501

Thank you


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