Unsteady flow in centrifugal compressor stage

7/27/2019 Unsteady flow in centrifugal compressor stage

http://slidepdf.com/reader/full/unsteady-flow-in-centrifugal-compressor-stage 1/61

S.Ramamurthy, NCAD, National Aerospace Laboratories, Bangalore-560 017, CSIR

Unsteady Phenomena in Centrifugal compressors-Impeller and Diffuser Interaction 21 July, 2012

Unsteady Phenomena in Centrifugal Compressor

Impeller and Diffuser InteractionBy

S . Ramamurthy





BASIC CONCEPTS

FLOW, SIGNALS, AVERAGING, SIGNAL-NOISE, CORRELATIONS

COMPRESSOR EVALUATION

TIME AVAERAGE MEASUREMENTS

UNSTADY VELOCITY & PRESSURE MEASUREMENTS

CHARACTERIZATION OF COMPRESSOR EXIT FLOW

JET-WAKE

SLIP FACTORS

EFFICIENCY

SUPERSONIC COMPRESSOR-DIFFUSER

FLOW BEHAVIOUR

FLOW VIDEO

CHARACTERIZATION OF ROTATING STALL & SURGE

STALL FREQUENCY AND STALL CELLS

CHARACTERIZATION OF ROTATING STALL INCEPTION

EARLY WARNING

FLOW VIDEO

CHARCATERIZATION OF SPECTRAL FLCTUATIONS IN CFC

FLOW UNSTAEDYNESS AT IMPELLER EXIT

FREQUENCY OF SPETRAL FLUCTUATIONS



CLASSIF ICATION OF FLOW

Uniform flow

I f the flow velocity is the same magni tude and direction at every poin t in the fl uid i t is said to be

uniform.

Non-uniform flow

I f at a given instant, the velocity is not the same at every point the flow is non-un iform.

Steady fl ow

A steady flow is one in which the conditions (veloci ty, pressur e and cross-section) may dif fer from

point to point but DO NOT change with time.

Unsteady flow I f at any point in the fluid, the conditi ons change with time, the flow is descri bed as unsteady.





Combining the above we can classify any flow in to one of four type:

1. Steady uniform flow

Conditions do not change with position in the stream or with time. Example-flow of water in a pipe of constant diameter at constant velocity

2. Steady non-uniform flow

Conditions change from point to point in the stream but do not change withtime.

Example-flow in a tapering pipe with constant velocity at the inlet - velocity will change

as you move along the length of the pipe toward the exit.

3. Unsteady uniform flow.

At a given instant in time the conditions at every point are the same, but will change with time .

Example-Pipe of constant diameter connected to a pump pumping at a constant rate

which is then switched off.

4. Unsteady non-uniform flow Every condition of the flow may change from point to point and with time at every point.

Example-waves in a channel.





Ensemble Average

In ensemble average successive sets of data are collected and summed point by point.

A prerequisite for the application of this method is the ability to reproduce the signal asmany times as possible starting always from the same data point.

Repetitive additions of noisy signals tend to emphasize their systematic characteristics and to

cancel out any zero-mean random noise.

If (SNR)o

is the original signal-to-noise ratio of the signal, the final (SNR)f

after N

repetitions (scans) is given by the following equation:

Therefore, by averaging 100 (or 1000) data sets a 10-fold (or a 100-fold) reduction of noiselevel is achieved.

N SNRSNR o f )()(





x M

s M

z

s z

avg i

i

M

i

i

M

avg avg

1 1

1 1





The signal to noise ratio: (S/N)

Every measurement is made up of two components.

One component, the signal, carries information about the fluid that is of interest to us.

The second, called noise, is made up of extraneous information that is unwanted because it degrades the

accuracy and precision of an analysis.

Noise free data can never be realized in the laboratory because some types of noise arise from

thermodynamic, quantum and electric effects that are impossible to avoid in measurement.

The signal to noise ratio is a representative marker-is used in describing the quality of an analytical method or

the performance of an instrument.

For a signal, S/N = mean / standard deviation

The average strength of the noise, N, is constant and independent of the magnitude of the signal, S.

The effect of noise on the relative error of a measurement becomes greater and greater as the quantity being

measured decreases in magnitude.





SOURCES OF NOISE Thermal Noise: Noise that originates from the thermally induced motions in charge carriers is known as

thermal noise.

Chemical : This noise arises from uncontrollable variables in the chemistry of the system such as variationin temperature, pressure, humidity, light and chemical fumes present in the room.

Instrumental : Noise that arises due to the instrumentation itself. It could come from any of the following

components- source, input transducer all signal processing elements, and the output transducer.

This noise has many types and can arise from several sources. There are four main categories of

instrumental noise: Thermal , Shot, Fl icker and Environment al .

Electromagnetic radiation in the environment including ac power lines, radio and TV stations, gasoline

engine ignition systems, arcing switches, brushes in electrical motors, lightening, and ionospheric

disturbances.





FOURIER TRANSFORMATION

The transformation from the time domain to the

frequency domain is based on forward Fouri er

Transform





Correlation addresses the question: “to what degree is signal A similar to signal B.”

0 5 10 15 20 250

0.5

1

1.5

Signal A

0 5 10 15 20 25

-2

0

2

Signal B1

0 5 10 15 20 250

0.5

1

1.5Signal B2

Sample

By inspection, A is “correlated” with B2, but B1 is “uncorrelated” with both A and

B2. This is an intuitive and visual definition of “correlation.”

CORRELATION





0 5 10 15 20 250

0.5

1Original Signal A

0 5 10 15 20 250

0.5

1Sample-Shifted Signal In this case, the simple cross-correlation

would be zero despite the fact the two

signals are obviously “correlated.”

1

12 1 2

0

1( ) [ ] [ ]

N

n

r k x n x n k N

The cross-correlation of a signal with itself is called the auto-correlation

1

0

1111 ][][1

)( N

n

k n xn x

N

k r

The “zero-lag” auto-correlation is the same as the mean-square signal

power 1 1

2

11 1 1

0 0

1 1(0) [ ] [ ] [ ]

N N

n n

r x n x n x n N N

CROSS CORRELATION:

AUTO CORRELATION:





EXPERIMENTAL EVALUATION OF CF COMPRESSOR

(PERFORMANCE)





Moderate tip speed

High head back to back

High tip speed

High flow, double flow

Compressor classification

Vane diffuser and volute

Hub wall

shroud wall





c R

W

dn

dW 2 + Forward Blade

-Backward Blade





CLOSED CIRCUIT CENTRIFUGAL COMPRESSOR TEST FACILITY INSTRUMENTATION PC based data acquisition system, Electronic torque meter, Hot wire anemometer / FFT analyzer

Accelerometers, Pressure transducers/Thermocouples, Sonic analyzer

Conventional pressure and temperature probes for time averaged measurements- Steady

High response probes for pressure and velocity measurement s- Unsteady







3-HOLE YAW PROBE



P

T m

10132501 P

P

15.28801T

T

P

T m

1

1

01

02

1

01

02

T

T

P

P

i

df b

swer measuredPo

P

P

1

1

01

05

65.0)1000/( N b

2.0

2

2

3

20201356.0

en

df R

DU

Line of Zero Incidence

(kg/s) (kg/s)

Line of Zero Incidence





HEAD-FLOW CHARACTERISTICS

2

2

201

4

U D

m

2

1

2

2

1

01

201

U

P

P T C

S

P

ts



POINTS A,B,C ARE CONSIDERED

FOR FURTHER DETAILED

STUDY



Axial Width (b/b2) Axial Width (b/b2)

R a d i a l V e l o c

i t y ( m / s )

T a n g e n t i a l V

e l o c i t y ( m / s )

HUB HUB SHROUDSHROUD

Time averaged radial velocity

distribution at impeller outlet

Time averaged tangential velocity




b2b



Axial Width (b/b2) Axial Width (b/b2)

A b s o l u t e f l o w

a n g l e ( d e g . )

R e l a t i v e f l o w a n g l e ( d e g . )

HUB HUBSHROUD SHROUD

Time averaged absolute flow angle


Time averaged relative flow angle






OBSERVATIONS ON PERFORMANCE CHARACTERISTICS

Maximum impeller efficiency 90% - Positive incidenceMaximum stage efficiency 75% - Negative incidence

Optimum incidence to impeller may not be the optimum to diffuser

Matching of Impeller with diffuser important

Head flow Characteristics – No Hystericis







KULITE PRESSURE SENSOR MOUNTING

Total pressure Static Pressure

KULITE PRESSURE SENSOR

HOT-WIRE SENSOR







Location of pressure and suction surface trailing edges

q’



1. HW senses the absolute flow at 8mm away

from impeller outlet

2. There exists a time diff for the flow to reachthe HW and be sensed, from the point where

it leaves the impeller

3. The time lag is function of location of HW,

magnitude & direction of velocity vector



HUB MID SHROUD HUB MID SHROUD

128.0d

094.0d

078.0d

Blade wake

Space averaged(Hot-Wire)

Yaw probe

hubshroud

mid

U2

U2

U2

wake

FlowCoeff.

Near Hub Wall Mid Channel Near Shroud Wall

Yaw

Probe

Hot-

wire

Yaw

Probe

Hot-

wire

Yaw

Probe

Hot-

Wire

0.128 63 65 56 60 49 50

0.094 38 40 36 37 40 50

0.078 29 30 32 33 39 50

FlowCoeff.

Near Hub Wall Mid Channel Near Shroud Wall

Yaw

Probe

Hot-

wire

Yaw

Probe

Hot-

wire

Yaw

Probe

Hot-

wire

0.128 62 60 77 63 77 90

0.094 77 130 83 105 81 110

0.078 67 130 86 113 86 120



Absolute Vel

Relative Vel

Blade Speed

Radial Vel



Jet flow behaves like potential flow

Relative flow angle don't fit conventional jet-wake model nor 1-D slip theory

Jet-Wake parameters depends on flow coeff





Blade Wake

Constant

Pressure is higher at SS

Due to higher tangential vel

Pressure not effected by

end wall BL

Higher Energy transfer in wake

Tot. Pr. in jet is constant

Tot. Pr. in wake varies linearlyfrom high vale to low value from

SS

Unstaedy flowLarge variation in angle

Rel. Total Pr. Low

Section F P02

(Kulite)

P02

(Yaw Probe)

% Diff

Hub 0.128

0.094

0.078

1.0558

1.1034

1.1177

1.0726

1.1118

1.1257

1.60

0.80

0.70

Mean 0.128

0.094

0.078

1.0529

1.1119

1.1299

1.0692

1.1117

1.1303

1.50

0.02

0.04

Shroud 0.128

0.094

0.078

1.0619

1.1169

1.1319

1.0757

1.1176

1.1308

1.30

0.70

0.10





C2

P02

C2

P02

A

B

Static pressure at end walls affected by BL

Shroud side more than hub side

At low flow coeff. Static pr distortion

adjacent to SS

In model Ps decrease from PS to SS

Av. Ps in jet – Isentropic

Present- Ps constant in jet

Cannot be assessed in wake

Approx. increase linearly from SS to

Jet Ps





SECTION FLOW

COEFF

STATIC

PR.(CAL)

STATIC

PR.

(KULITE)

STATIC

PR.(WALL)

%

DIFF

HUB 0.128

0.078

1.0352

1.0432

1.0674

1.0855

1.0627

1.0895

3.0

3.8

MEAN 0.128

0.078

1.0559

1.0622

1.0704

1.0669

1.3

0.4

SHROUD 0.128

0.078

1.0399

1.0485

1.0725

1.0789

3.0

2.8

NORMALIZED STATIC PRESSURE VALUE

AT IMPELLER OUTLET

Total and static pressure in the jet remains constant

Whereas in the wake they vary





c2ww2j W2w

U2

U2

c2j

0.85 0.89 0.96 Mass averaged Slip Factor

0.91 1.16 1.18 Wake Slip Factor (Hot wire)

0.88 0.9 0.92 Overall Slip factor (Wislineous)

0.91 0.91 0.91 Slip factor overall (Wisner)

0.92 0.93 0.95 Jet Slip factor (Stanitz)

0.83 0.84 0.86 Jet slip factor (hot wire)

0.8 0.82 0.88 From yaw probe

0.128 0.094 0.078 Flow coefficient

jw )1(0

w

)1(8

1

22

0 j

7.0

2 )cos(1

b

b

N

2/63.01 jq

q

b

cr2

U2

cq2

c2 w2 2b

2

cs

Cs=Slip Velocity

)tan( 2

2

2

2

2b

r

U C

U C q Slip Factor (1 -

CS

U2



S R h NCAD N i l A L b i B l 560 017 CSIR



)1(cos

17.0

2

j

b

b

j N

q

)1(26.11 3

80

2

j j q



S R th NCAD N ti l A L b t i B l 560 017 CSIR



Slip Factor depends on flow coefficient

Slip factor increases with decrease in flow coefficient

Jet slip factor is always < 1

Wake slip factor depends on wake width

For large wake width wake slip factor>1 indicating flow angle is less than the blade angle and

specific work is less than Euler work Yaw probe cannot respond to high frequency unsteady flows can measure only average

velocity of jet flow

Variation of slip factor in tangential direction

New correlations – Evaluate wake and jet flow angles






FLOW COEFF.= 0.128

FLOW COEFF.= 0.094

FLOW COEFF.= 0.078

Full Line – From hot-wire anemometry measurements

Chain Line- From conceptual jet-wake model

RELATIVE VELOCITY

AT IMPELLER EXIT

jetwake

2b

JET-WAKE MODEL COMPARISION






ds

dW

RW

dS

WdW R C

i

/2

Stability of rotating

shear layer

Stability of shear

Layer past curvedwall

C

S R

W R

Richardson Number

Rossby Number

Pressure gradient

Positive away from the surface (SS or

Convex wall) Turbulence intensity

stabilized, Turbulence shear stress

Decreases-BL less resistance to

separation



S Ramamurthy NCAD National Aerospace Laboratories Bangalore 560 017 CSIR



m

mw

j

w

W W

2

2

2

2

q

q w

Wake mass flow rate

Total mass flow rate

Relative velocity of the wake flow

Relative velocity of the jet flow

Circumferential space occupied by wake

Blade to blade spacing






----- Present model----- Conceptual model

Relative

velocity

Relative

flow

angle

Static

pressure

Total

pressure

Jet-Wake Parameters

94.686.0 m

mw

q q

q

13.913.1 w j

j

84.44.02

2 j

w

w

w

Conceptual Model Slip

j j q 2

63.0

10.1w

Present model

)1(cos

1 7.0

2

j

b

b

j N q

j

w

)1(0

Where )1(26.11 3

80

2

j j q Relative flow angle in wake

m jw bb 222 3)31( for 3/1b

2/)13()1(3 222 jmw bb for 3/1b

Where32

2 969623131936 m

Jet wake

interfaceJetWake

ε

/3

2m






Flow coefficient 0.128 0.094 0.07

8

Three hole yawprobe

4.50 3.26 2.78

Hot-wire

anemometer

4.51 3.35 2.86

Calibrated wall

static pressure

4.60 3.27 2.83

2

0

222 .2

b

r S dbC r m

q

2

0 022

2b

r S dbd C m

ESTIMATED MASS FLOW RATE ESTIMATED POWER (KW)

Flow

Coefficient

0.128 0..094 0.078

Ideal Power 53.1 45.5 41.3

Measured

Power

39.0 37.0 33.0

Estimated from

yaw probe

measurements

36.2 34.7 32.3

Estimated from

hot-wire

measurements

38.0 42.3 40.3

Jet flow power 35.1 33.1 26.7

Wake flow

power

2.9 9.2 13.6

2

0

222222

b

r S dbC C U r P q

q q 2

0 0

2222

2

.. d dbC C U P

b

r S



S Ramamurthy NCAD National Aerospace Laboratories Bangalore-560 017 CSIR



q

q

2

0

2

0

2

0

2

0

))((

))((

b

r

b

r h

h

dbd C

dbd C

22

1

01

02 1

q

C U

P

P C p

h

and Velocity form hot-wire

Wake flow efficiency is high. At the interface between

Jet – wake the efficiency is low due to large shear



S Ramamurthy NCAD National Aerospace Laboratories Bangalore-560 017 CSIR



Wake mass flow and wake width depends on flow coefficient

Increases with increase in flow coefficient

Considerable amount of wake mass flow and wake width exists and cannot be

neglected in the off design performance prediction

Wake flow absorbs part of the power from shaft input power and part from jet

power which has undergone slip

Excess power available in the wake is considered as apparent power

Power input within the channel is non-uniform






Kulite Transducer Terminal

Box 1

Terminal

Box 2

16 Channel

Programmable Gain

Amplifier card

Data Acquisition

Processor Card

Plotter

Differential signal

Differential signal Amplified Single endedsignal

Eddy Current ProbeDriver

Analog to TTL

( transistor transistor logic)

Converter

(-15V to – 10V) (0 to 5V TTL Pulse)

Pentium

Layout of high speed data acquisition processor system.

KULITE PRESSURE TRANSDUCER

S.Ramamurthy, NCAD, National Aerospace Laboratories, Bangalore 560 017, CSIR


Supersonic Centrifugal Compressor Stage




DIFFUSER – INSTRUMENTED BLADE

Channel static pressure Static Pressure Close to Diffuser

Suction Surface Pressure taps

Pressure Surface Pressure taps

KULITE LOCATIONS



5 /7S.Ramamurthy, NCAD, National Aer

ospace Laboratories, Bangalore-560 017, CSIR



ms

0.0 2.5 5.0 7.5 10.0 12.5 15.

5rc/a7

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

ms

0.0 2.5 5.0 7.5 10.0 12.5 15.

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

ms

0.0 2.5 5.0 7.5 10.0 12.5 15.

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

150

100

50

Impeller outlet

Diffuser leading edge

Diffuser throat

Diffuser channel

Diffuser channel

Diffuser outlet

F.C = 0.038

F.C = 0.031 F.C = 0.023

Circumferential variation of static pressure at different locations along vane diffuser






ms

0.0 2.5 5.0 7.5 10.0 12.5 15.

ms

0.0 2.5 5.0 7.5 10.0 12.5 15.0

F.C = 0.027

F.C = 0.021

F.C = 0.022

F.C = 0.023

F.C = 0.024

F.C = 0.020

F.C = 0.029

F.C = 0.030

F.C =0.031

F.C = 0.032

F.C = 0.033

F.C = 0.034

F.C = 0.035

F.C = 0.038

F.C = 0.053

Pressure surface Center Suction surface

50

150

ms

0.0 2.5 5.0 7.5 10.0 12.5 15.0

50

150

500

-500

-1000

1000

-7000

7000

1000

-1000

1000

-10003000

-3000

Diffuser throat static pressure variation at different flow coefficients

y, , p , g ,





ms

0 5 10 15 20 25 30 35 40 45 50 55 60 65

200

150

100

50

0

200

150

100

50

0

200

150

100

50

0

200

150

100

50

0

Circumfrential variation of static pressure at impeller outlet - stalled condition

(Flow coefficient = 0.02)

K1

K2

K3

K4

y, , p , g ,







SOME OBSERVATIONS

* Diffuser plays an important role in the stage performance

* Optimum stage performance is achieved close to the stall

* The flow from leading edge to throat of the diffuser depends

on the incidence

* The channel from throat to diffuser exit provides good

guidance to the flow

* Good correlation exists between the newly evolved stabilityparameter and flow coefficient

* This parameter is able to demarcate the stable, un-stable and

the stall regions

* The demarcation of stability is through static pressure variation

within the blade channel which are not in repetitive coherence

with impeller rotation.

* Since this phenomenon occurs well ahead of stalling of the

impeller flow, detection of this through the stability parameter is

advantageous.

y p g





STALL AND SURGE





Stall- Flow Separation due to high incidence with reduction CL

Rotating Stall: Non-axisymmetric flow with circumferentially non-uniform pattern rotating around

the annulus

D pD p

m. m m

D p

.

Rotating

stall

Average mass

flow rate

Notaxisymetric

Not axisymetric

Progressiv

Stall

Abrupt

Stall

Surge

Flow

Through flow

Velocity(axial)

Circumferential

velocity

Cell rotation

Instability of Axi-symetric

Very small Very small

Very large Very large

50% 20-40%

Conserved Conserved Varies with time

Effect of stall/surge: Vibrational excitation resulting in Mechanical failure.

Characterization: Stall propagating speed/No. of cells

Detect: Change in noise/high frequency instrumentation.

Time scale: Stall Surge

50-100Hz 3-10Hz

.A

B`

C





Instantaneous hot-wire signals taken at impeller outlet


This if for 90 deg.- angular displacement of two sensors 90 Deg.

Time for One revolution of impeller =12 msTo travel 90 deg , stall cell takes DT=20*12/57 =4.21ms

To travel 360 deg. (1 revolution) , stall cell takes=4*4.21 =16.84ms

Hence absolute speed of stall cell Us=1000/16.84 =60 c/s

Rotor speed = 1000/12=83 c/s

Relative speed of stall cell =60-83= -23 c/s

Ratio of stall speed to impeller speed=23/83=1/4 (approx.)

Frequency of stall cell Fs =Ns*Us

At flow coeff=0.057 , Fs=60 c/s, hence Ns=1

At Flow Coeff==0.034, Fs=120 c/s, Hence Ns=2

60Hz

120 Hz

FFT of hot wire signals

57

20




SOME OBSERVATIONS

Flow coefficient 0.057

Rotating Stall initiation – Single rotating stall cell covering 7 blade passages rotating at

¼ Impeller speed

Flow Coefficient 0.034

Well inside stall region- two rotating stall cells covering three blade passages located

opposite to each other and Rotating at ¼ impeller speed

At both flow coeff. the disturbances propagates upstream of the impeller

Further reduction in Flow Coeff. Lead to Surge – Unstable behavior





STALL INCEPTION













ensembleavr

ensemblet

rawavr

rawt

C

C

C

C IP )(

)(

)(

)(

,,

D

D

min,max, t t t C C C D







OBSERVATIONS ON STALL INCEPTION

1.The characteristics clearly demarcates regions of stable operation,

instability and rotating stall

2. Instability is identified through bursts of velocity differentials within blade

channel which are not coherence with impeller rotation.

3. Corrective action can be implemented ahead of the stall





SPECTRAL FLUCTUATIONS





ENSEMBLE AVERAGED RADIAL AND WHIRL VELOCITY AT IMPELLER OUTLET

TWO CHANNEL VELOCITIES


Tangential component

(Whirl component)

Radial component




Temporal Variation Spatial Variation

Characteristics of absolute velocity





Fourier Transformation-Absolute velocity





1. The disturbing vortices in the flow had their origin near the suction surface of the

blades well inside the channel and they are found to have travelled half the blade

channel across the flow before exiting and coming out of the impeller.

2. Characterization of vortex shedding is similar to Strouhal Number. Strouhal number for this

particular case works out to be 0.25, which compares well with the universal value of 0.2

3. Unsteady flow observed at impeller outlet is characterized as coherent structure

due to periodic formation of vortex type of disturbances near the blade suction

surfaces and travelling out of the blade passage.4. This vortex formation has not lead to a total separation of flow on the blade suction surface

unlike the consequence of the boundary layer growth.

5. This phenomenon is responsible for the reduced relative velocity as well as

angle and increased work on the flow in the suction side half of the passage

and also for the increasing relative velocity and angle of the flow near the

pressure side, leading to what is known as slip.

6. The periodic formation and cross channel movement of such vertices could

also be the cause of initiation of rotating Stall in impellers.

OBSERVATIONS

Documents

Unsteady flow in centrifugal compressor stage