Design of Centrifugal CompressorDesign of Centrifugal ...

PEMP RMD510

Design of Centrifugal Compressor-1Design of Centrifugal Compressor-1

Session delivered by:Session delivered by:

Prof Q H NagpurwalaProf Q H NagpurwalaProf. Q. H. NagpurwalaProf. Q. H. Nagpurwala

09 @ M S Ramaiah School of Advanced Studies, Bengaluru 1

PEMP RMD510Session Objectives

To introduce the delegates tog

• the procedure for aerodynamic design of centrifugal compressors

• the methods for obtaining impeller geometry

• the procedure for the design of vaned and vaneless diffusersthe procedure for the design of vaned and vaneless diffusers

• Effect of geometric parameters on centrifugal compressor performance p


PEMP RMD510Nomenclature

C Absol te elocitC Absolute velocityn Number of vanesp pressureN Rotational speedN Rotational speedr RadiusT TemperatureU Impeller speed at tipp p pUe Impeller speed at mean radius of eyeV, W Relative velocity Relative flow angle Slip factor Power input factor Angular velocity

Suffixesa, x Axial componenta Ambient


r Radial componentw, Whirl component

PEMP RMD510Centrifugal Compressor

C1 = Ca1

Cw2Ww2


Centrifugal compressor stage and velocity diagrams at impeller entry and exit

PEMP RMD510Centrifugal Impeller with Vaned Diffuser

Without Splitter Blades With Splitter Blades


Without Splitter Blades With Splitter Blades

PEMP RMD510Types of Impellers

C C C

Head – flow characteristics f ifor various outlet blade angles


PEMP RMD510Components of Fluid Forces on Blades / Vanes

Fluid particles flowing through a rotor experience forces in axial,radial and tangential directionsradial and tangential directions.

T i lA i l Tangential component

Radial

Axial component

Radial component

Shaft


Shaft

PEMP RMD510Velocity Triangles

Approximate blade shapes can be obtained from velocity triangles.

C2 W2Cw2

U

2 Cr2W2

Outlet velocity triangle

Ue

Ca1=C1W1


Inlet Velocity Triangle

PEMP RMD510Design Formulae

From Euler turbine equation of energy exchange,

hhCUCUWSpecific work 0103122 hhCUCUW wew Specific work,

0103 TTcp Diffuserp

Impeller

Vaneless space

If the flow at inlet to the impeller is axial, then Cw1 = 0, and

p

22 wCUW 0103 TTcp


PEMP RMD510

CDesign Formulae

C , '2

2

w

w

CC

Slip Factor

2Then UW C’w2

impeller radialfor 2

UCw

Then, UW

Introducing power input factor, 2UW

Cw2Cws

and temperature rise UW

UTT2

Velocity triangle at impeller exit

CC

pcTT 0103

1

0103

1'0303 1

TTTp c

wCC

01

0103

01

03

01

03 1

TTpp c

12

U


1

Tc

U

p

c

PEMP RMD510Compression Process on h-s Diagram

The fluid is accelerated fromThe fluid is accelerated from velocity C0 to velocity C1 and the static pressure falls from p0 to p1. 0 1

Since the stagnation enthalpy is constant in steady, adiabatic flow without shaft work thenflow without shaft work then h00 = h01

22 11 ChChor 211

200 22

ChCh


PEMP RMD510Design Procedure

• Assume rotational speed, tip speed and air entry velocity

• Determine the pressure ratio of the compressor and the power i d d i i i h h l i f h i i l irequired to drive it, assuming that the velocity of the air at inlet is

axial,

• Calculate the inlet angle of the impeller vanes at the root and tipCalculate the inlet angle of the impeller vanes at the root and tipradii of the eye assuming that the axial inlet velocity is constantacross the eye

• Estimate the number of vanes

• Estimate the axial depth of the impeller channels at the periphery ofthe impellerthe impeller

• Estimate the inlet angle of the diffuser vanes, and

• the throat width of the diffuser passages which are assumed to be of


• the throat width of the diffuser passages, which are assumed to be ofconstant depth

PEMP RMD510Design Specifications

The following data are suggested as a basis for the design of a single-sided centrifugal compressor:

P i t f t 1 04Power input factor, 1.04

Slip factor, 0.9

Rotational speed N 290 rev/sRotational speed, N 290 rev/s

Overall diameter of impeller 0.5 m

Eye tip diameter 0.3 mEye tip diameter 0.3 m

Eye root diameter 0.15 m

Air mass flow, m 9 kg/s

Inlet stagnation temperature, T01 295 K

Inlet stagnation pressure, p01 1.1 bar


Isentropic efficiency, c 0.78

PEMP RMD510Design Requirements

Requirements are

(a) To determine the pressure ratio of the compressor(a) To determine the pressure ratio of the compressor and the power required to drive it assuming that the velocity of the air at inlet is axial y

(b)To calculate the inlet angle of the impeller vanes at the root and tip radii of the eye, assuming thatat the root and tip radii of the eye, assuming that the axial inlet velocity is constant across the eye

(c) To estimate the axial depth of the impeller(c) To estimate the axial depth of the impeller channels at the periphery of the impeller


PEMP RMD510Pressure Ratio and Power

(a) Impeller tip speed U = *0.5*290 = 455.5 m/s

Temperature equivalent of the work done on unit mass flow of air isp q

1935.455*9.0*04.1 22 KUTT

193*780

19310*005.1

5.31

30103

TT

Kc

TTp

23.4295

193*78.0111

01

0103

01

03

T

TTpp c

Power required = m cp( T03-T01) = 9*1.005*193 = 1746 kW


PEMP RMD510Static Density at Inlet

(b) To find the inlet angle of the vanes it is necessary to determine the inlet velocity, which in this case is axial, i.e. Ca1= C1.

Ca1 must satisfy the continuity equation m = 1 A1Ca1, where A1 is the flowCa1 must satisfy the continuity equation m 1 A1Ca1, where A1 is the flow area at inlet.

Since the density 1 depends upon C1 and both are unknown, a trial and error process is requiredprocess is required.


PEMP RMD510Design Procedure

• The iterative procedure is not critically dependent on the initial value assumed for the axial velocity, but clearly it is desirable to have some rational basis for obtaining an estimated value for starting the iteration.

• The simplest way of obtaining a reasonable estimate of the axial velocity is to calculate the density on the basis of the known stagnation temperature and pressure; in practice this will give a density that is too high and a velocity that is too low.

• Having obtained an initial estimate of the axial velocity. The density can be recalculated and hence the actual velocity from the continuity equation; ifrecalculated and hence the actual velocity from the continuity equation; if the assumed and calculated velocities do not agree it is necessary to iterate until agreement is reached.

• Note that it is normal to design for an axial velocity of about 150 m/s thus• Note that it is normal to design for an axial velocity of about 150 m/s, thus providing a suitable compromise between high flow per unit frontal area and low frictional losses in the intake.


PEMP RMD510

22 )15030(

Iteration on Ca1

mkgRTp /30.1

295*2870100*1.1 301

1 Based on stagnation diti

Annulus area of impeller eye, A1 =2

22053.0

4)15.03.0( m

smA

mC

RT

a /131053.0*30.1

9295287.0

111

01

conditions

C

Kc

C

p5.8

201.031.1

10*005.1*2131

2

12

2

3

21

2

Since C1= Ca1, the equivalent dynamic temperature is

bar

TTpp

Kc

CTTp

992.05.286295

1.1

5.2865.82952

5.31101

011

1011

mkgRTp /21.1

5.286*287.0100*992.0 3

1

11

101


smA

mCa /140053.0*21.1

9

111

Check on Ca1

PEMP RMD510

Fi l t i l

Iteration on Ca1 (… contd.)

Final trial:Try Ca1= C1=145 m/s

E i l t d i t t i

K 5.10201.045.1

10*005.1*2145

2

2

3

221

cC

p

Equivalent dynamic temperature is

b96801.1

K 5.2845.102952

01

21

011

p

cCTT

p

kg/m 185.15.284*287.0

100*968.0

bar968.05.284295

3

11

5.31101

011

RTm

TTpp

m/s 1430530*1851

9

5.284287.0

1

1

A

mC

RT

a Check on Ca1:


053.0185.111 A

PEMP RMD510Flow Angles at Inducer

This is a good agreement and a further trial using Ca1= 143 m/s is unnecessary because a small change in Ca1 has little effect upon

F hi i i h fi l l 143. For this reason it is more accurate to use the final value 143 m/s, rather than the mean of 145 m/s (the trial value) and 143 m/s. The vane angles can now be calculated as follows:

Peripheral speed at the impeller eye tip radius

= * 0 3 * 290 = 273 m/s 0.3 290 273 m/s

and at eye root radius = 136.5 m/s

143 33465136143tan 1 ..

6527273143tan 1 .

at root =

at tip =


6527273tan . at tip

PEMP RMD510Radial Velocity and Static Density at Exit

(c) To calculate the required depth of the impeller channel at the periphery we must make some assumptions regarding both h di l f l i h i d h di i i fthe radial component of velocity at the tip and the division of

losses between the impeller and the diffuser so that the density can be evaluated. The radial component of velocity y p ywill be relatively small and can be chosen by the designer; a suitable value is obtained by making it approximately equal to the axial velocity at inlet to the eyethe axial velocity at inlet to the eye.

To estimate the density at the impeller tip, the static pressure and temperature are found by calculating the absolute velocity p y g yat this point and using it in conjunction with the stagnation pressure which is calculated from the assumed loss up to this point


point.

PEMP RMD510Absolute Velocity at Exit

Making the choice Cr2 = Ca1, we have Cr2 = 143 m/s

Cw2 = U = 0.9*455.5 = 410 m/s

K 8.93201.0

10.443.122

2222

22

22

p

wr

p cCC

cC


PEMP RMD510Distribution of Total Loss

Assuming that half the total loss, i.e. 0.5(1- c) = 0.11, occurs in the impeller, the effective efficiency of compression from p01 to p02

ill b 0 89 th t

5.35.3

02 582.1193*89.01

p

will be 0.89 so that,

01 295

p

Now, (p2 /p02) = (T2 /T02)3.5

K239489348822CTT

2 02 2 02

and T02 = T03 = 193+295 = 488 K

th t

5.32 2.394

p

K2.3948.934882

2022

pcTTso that


02

2488

p

PEMP RMD510Impeller Width at Exit

Since (p2 /p01) = (p2 /p02) (p02 /p01)5.3

01

2 35.2488

2.394*582.1

pp

322

2

01

kg/m 28.22394*2870

100*58.258.21.1*35.2

488

RTp

barpp

2

2 g2.394*287.0RT

The required flow area normal to the radial direction at the impeller tip is

29m 2

22

m 0276.0143*28.2

9

rCmA

Hence the depth of impeller channel at exit

5.0*0276.0

b = 0.0176 m or 1.76 cm


(This result will be used when discussing the design of the diffuser in the next section)

PEMP RMD510Estimation of Number of Blades

The number of impeller vanes can be obtained from the correlations for slip factor

Stanitz correlation Wiesner correlation

7.02cos

1Zs

Stodola correlation

and ’2 is measured from radial direction. 2

In the present case, ’2 = 0

Hence, Stanitz correlation reduces to Zs 63.01


ZFor s = 0.9, number of blades, Z = 19.79 ~ 20

PEMP RMD510Generation of Blade Profile

Impeller vane profiles can be generated• by using softwares, like BLADEGEN

Profile A

• by trial and error process, using CAD and CFD tools

Profile A

Profile BFinal Profile D


Profile C

PEMP RMD510Types of Diffusers

• Vaneless diffuser

C i l diff

Angular momentum is conserved in the vaneless space; i.e. r.Cw = constant. Cr also varies in the vaneless space because of the change in radius. H ti ti f C d C ill i ld th• Conical diffuser Hence, estimation of Cw and Cr will yield the absolute velocity and flow angle at inlet to the vaned /conical diffuser.



Uniform thickness curved vanes

Aerofoil shape vanes

Uniform thickness Wedge shape vanes

straight vanes



• Straight wedge type• Aerofoil shaped vanes• Aerofoil shaped vanes

(a) Straight wedge type (b) Aerofoil shaped vanes


PEMP RMD510Types of Impellers and Diffusers

Impellers• Radially ending• Backswept Diffusers• Backswept• Pre-swirl• Above w/splitter blades

• Vaneless• VanedAbove w/splitter blades

– Radial– Wedge

Di t• Discrete-passage


NASA Glenn Research Center

PEMP RMD510Typical Diffuser Geometry

Throat


Flow regions of vaned diffuser

PEMP RMD510Diffuser Design

Radial width of vane less space = 5 cm

Approximate mean radius of diffuser throat = 0.33 m

Depth of diffuser passages = 1.76 cm

Number of diffuser vanes = 12

I i i d d iIt is required to determine:

(a) the inlet angle of the diffuser vanes, and

(b) the throat width of the diffuser passages which are assumed to be of(b) the throat width of the diffuser passages, which are assumed to be of constant depth.

For simplicity, it will be assumed that the additional friction loss in the short distance between impeller tip and diffuser throat is small and therefore the 50 % of the overall loss can be considered to have occurred up to the diffuser throat. For convenience, suffix 2 will be used to denote any plane in the flow after the impeller tip the conte t making it clear hich plane is nder consideration


impeller tip, the context making it clear which plane is under consideration.

PEMP RMD510

( ) C id diti t th di f th diff l di d i t

Iteration on Cr2 at Diffuser Inlet(a) Consider conditions at the radius of the diffuser vane leading edge, i.e. at

r2 = 0.25+0.05 = 0.3m. Since in the vaneless space Cwr = constant for constant angular momentum,

25.0

The radial component of velocity can be found by trial and error. The iteration may be started by assuming that the temperature equivalent of the

smCw /34230.025.0*4102

y y g p qresultant velocity is that corresponding to the whirl velocity, but only the final trial is given here.

Try C = 97 m/sTry Cr2 = 97 m/s

K 9.62201.0

97.042.32

2222

pcC

Ignoring any additional loss between the impeller tip and the diffuser vane leading edges at 0.3m radius, the stagnation pressure will be that calculated for the impeller tip, i.e.

p


5.3

01

02 582.1pp

PEMP RMD510Static Density at Diffuser Inlet

Proceeding as before we have

K1425962488T5.3

2

2

4881.425

K1.4259.62488

pp

T

5.32

02

07.3488

1.425*582.1

488

pp

p

2

01

100*383bar 38.31.1*07.3

488

p

p

32 kg/m 77.2

1.425*287.0100*38.3


PEMP RMD510Diffuser Inlet Angle

Area of cross-section of flow in radial direction

2

9

= 2 *0.3*0.0176=0.0332 m2

Check on Cr2: W2

Taking C as 97 9 m/s the angle of the diffuser vane leading

sCr m/9.970332.0*77.2

92

Taking Cr2 as 97.9 m/s, the angle of the diffuser vane leading edge for zero incidence should be

169.97tt 121 C 16342

tantan 1

2

21

w

rC

C


PEMP RMD510Static Density at Diffuser Throat

(b) The throat width of the diffuser channels may be found by a similar calculation for the flow at the assumed throat radius of 0.33 m

/31125.0*410C

2222

2

m/s 83

m/s31133.0

*410

C

C

r

w

Try22

22

K5436551488

K 5.51201.0

83.011.32

T

CC

p

5.3

01

2

2

37.3488

5.436*582.1

K5.4365.51488

pp

T

32

2

01

kg/m96.2100*71.3bar 71.31.1*37.3

pp


2 kg/m96.25.436*287.0

PEMP RMD510Diffuser Angle at Throat

As a first approximation, we may neglect the thickness of the diffuser vanesso that the area of flow in the radial direction

= 2* *0.33*0.0176 = 0.0365 m2

m/s 3.8303650*962

92 rC

Check on Cr2 :

0365.0*96.2

3.831 15311

3.83tan 1 Direction of flow =


PEMP RMD510Diffuser Throat Width

,or ,2

222222222 C

CAACACA rrrr Now, m =

Hence, flow area in the direction of resultant velocity, i.e. total throat area of the diffuser passages is

0.0365 sin 15° = 0.00945 m2

Therefore, with 12 diffuser vanes, the width of the throat in each passage of depth 0.0176 m is,

cm4 40orm0440000945.0 cm4.40or m0440.0

0176.0*12


PEMP RMD510Inducer Relative Mach Number on Ground

Consider inlet Mach number at the tip radius of the impeller eye

Inlet velocity = 143 m/s (axial)

Eye tip speed = 273 m/s

Relative velocity at tip =

Velocity of sound =

m/s 308273143 22

m/s33810*5284*2870*41 3Velocity of sound =

Maximum Mach number at inlet = 308/338 = 0.91

m/s33810*5.284*287.0*4.1 3

This would not be considered satisfactory even if it were actually the maximum value likely to be reached. But, if the compressor is part of an aircraft engine required to operate at an altitude of 11000 m, where the atmospheric temperature


q p , p pis only about 217 K, we must calculate the Mach number under these conditions.

PEMP RMD510

Si th ill b t t i d t th ff t i th i t k h th

Inducer Relative Mach Number in FlightSince there will be a temperature rise due to the ram effect in the intake when the aircraft has a forward speed, the effect of drop in atmospheric temperature will not be quite so great as might be expected. We will take 90 m/s (324 km/hr) as being the minimum speed likely to be reached at high altitudebeing the minimum speed likely to be reached at high altitude.

Temperature equivalent of forward speed = 4K

Inlet stagnation temperature = 217 + 4 = 221 KInlet stagnation temperature 217 4 221 K

Temperature equivalent of axial inletvelocity from the first example = 10.5 K

Inlet static temperature at altitude = 210.5 K

5284 21Inlet Mach number at altitude = 06.1

5.2105.28491.0 2


This is clearly too high and we will find the Mach number when an inlet prewhirl of 30° is introduced.

PEMP RMD510Effect of Inlet Prewhirl

I thi th b l t i l t l it ill b li htl hi h th b f th t th i l t

Try C =150 m/s

In this case the absolute inlet velocity will be slightly higher than before, so that the inlet static temperature will be slightly lower. A new value for the axial velocity must be found by the usual trial and error process. Reverting to the original sea-level static case:

Try Ca1 =150 m/s

C1 = 150/cos 30 = 173.2 m/s

Temperature equivalent of C1 = 14.9 K

T1 = 295-14.9 = 280.1K

p1 = 0.918 bar and

9

31 /14.1 mkg

Check on Ca1 =

Whirl velocity at inlet, Cw1 = 149 tan 30

= 86 m/s

sm /149053.0*14.1

9

30º= 86 m/s

Maximum relative velocity =

= 239 m/s239

22 86273149


Hence maximum inlet Mach number at T01= 295 K is 71.010*1.280*287.0*4.1

2393

PEMP RMD510

U d ltit d diti thi ld i t littl th 0 8 H

Effect of Prewhirl on Pressure RatioUnder altitude conditions this would rise to little more than 0.8. Hence, a prewhirl of 30° can be regarded as adequate.

To show the effect of 30° prewhirl on the pressure ratio, we will take the worst case and assume that the prewhirl is constant over the eye of the impeller.

Speed of impeller eye at mean radius, m/s8.2042

5.136237

eU

2e

1

2 ewp

UCUc

Actual temperature rise

K 17510*005.1

8.204*865.455*9.004.13

2

175*780 5.303 p 79.3

29517578.01

01

03

pp

This pressure ratio may be compared with the original value of 4.23 obtained ith no pre hirl It is sometimes ad antageo s to se adj stable inlet g ide


with no prewhirl. It is sometimes advantageous to use adjustable inlet guide vanes to improve the performance under off-design conditions.

PEMP RMD510Mach Number at Impeller Exit

We next consider the relevant Mach numbers in the diffuser. The maximum value will occur at the entry to the diffuser, that is, at the impeller tip. Once again the values calculated in the previous example will be used.

The temperature equivalent of the resultant velocity of the air leaving the impeller was found to be

2

KC

C

p

8.932

22

and hence

C2 = 434m/s

T f d b 394 2 K d h h M h b h i ll i lT2 was found to be 394.2 K and thus the Mach number at the impeller tip equals

09.110*2394*2870*41

4343


10*2.394*287.0*4.1

PEMP RMD510Mach Number at Diffuser Inlet

Now consider the leading edge of the diffuser vanes. The whirl velocity was found to be 342 m/s and the radial component 97.9 m/s. The resultant velocity at this radius is therefore 356 m/s. The static temperatures was 425.1 K at this radius so that the Mach number is

86.010*1.425*287.0*4.1

3563

0.587.0.

In the particular design under consideration, the Mach number is 1.09 at the impeller tip and 0.86 at the leading edges of the diffuser vanes. It has been f d h l h di l l i i b i h bfound that as long as the radial velocity component is subsonic, Mach numbers greater than unity can be used at the impeller tip without loss of efficiency. It appears that supersonic diffusion can occur without the formation of shock waves if it is carried out at constant angular momentum with vortex motion inwaves if it is carried out at constant angular momentum with vortex motion in the vaneless space. But the Mach number at the leading edge of the diffuser vanes is rather high and it would probably be advisable to increase the radial width of the vaneless space or the depth of the diffuser to reduce the velocity at


width of the vaneless space or the depth of the diffuser to reduce the velocity at this radius.

PEMP RMD510Comments

High Mach numbers at the leading edges of the diffuser vanes are undesirable, not only because of the danger of the shock losses, but because they imply high air speeds and relatively large pressures at the stagnation points where the air is brought to rest locally at the leading edges of the vanes. This causes a circumferential variation in static pressure, which is transmitted upstream in a radial direction through the vaneless space to excite the impeller tip. Although the variation would considerably reduce by the time it reaches the impeller it may well be large enough to excite thereduce by the time it reaches the impeller, it may well be large enough to excite the impeller vanes and cause mechanical failure due to vibrational fatigue cracks in the vanes. This will occur when the exciting frequency, which depends on the rotational speed and relative number of impeller and diffuser vanes, is of the same order as one of the natural frequencies of the impeller vanes. To reduce the likelihood of this, care is taken to see that the number of vanes in the impeller is not an exact multiple of the number in the diffuser; it is common practice to use a prime number for the impeller vanes and an even number for the diffuser vanes.vanes and an even number for the diffuser vanes.

The reason for the vaneless space will now be apparent: the dangers of both, shock losses and excessive circumferential variation in static pressure, would be considerably increased if the leading edges of the diffuser vanes were too near the impeller tip where


g g p pthe Mach numbers are very high.

PEMP RMD510

Wi h h di i

Further WorkWith these dimensions

1. Create a geometric model of the Centrifugal compressor using a CAD toolusing a CAD tool.

2. Export the model to a CFD tool and carry out the fluid flow analysisflow analysis

3. Using an FEM tool, carry out the structural dynamic analysis choosing appropriate material properties.analysis choosing appropriate material properties.

4. Create a shaft and choose bearings and carryout rotor dynamic analysisy y

5. Generate manufacturing drawings

6 Generate compressor operating characteristics using a


6. Generate compressor operating characteristics using a suitable post processor.

PEMP RMD510Design Guidelines

Relative velocity ratio, W2/Wsh1 > 0.75 de Haller number

Exit absolute flow angle, 2 = 60º - 65º

Sweep angle for backward swept impeller = 30º Ratio of inducer tip diameter to

impeller outer diameter, dsh1/ d2 = 0.4 - 0.55 cos p , sh1 2 Number of impeller blades by Wiesner’s correlation,

Inducer hub-tip diameter ratio = 0.5 – 0.6

7.02cos

1Zs


PEMP RMD510Effect of Geometric Parameters

Performance of the centrifugal compressor is affected by the following geometric parameters:

• Impeller vane thickness

• Splitter bladesSplitter blades

• Trailing end skew

• Exit trim• Exit trim

• Inducer leading edge sweep

• Vaned shroud at inducer

• Impeller with integral shroud


• Inlet guide vanes

PEMP RMD510Impeller Vane Thickness

Because of manufacturing problems and physical necessity, impeller vanes have definite thickness. When fluid leaves the impeller, the vanes no longer contain the flow, and the meridional velocity decreases. Hence, both the relative andthe flow, and the meridional velocity decreases. Hence, both the relative and absolute velocities also decrease with consequent change in exit flow angle.

C


PEMP RMD510Splitter Blades

Compressor specifications

(a) (b)Inducer hub dia. 22 mmInducer tip dia. 65 mmImpeller tip dia 87 mm

Compressor specifications

Impeller tip dia. 87 mmNumber of vanes 18Rotational speed 81000 rpmInlet pressure 0.98 bar

( )

Inlet temperature 303 K

(a) LR- 0.81 (b) LR-0.65 (c) LR-0.50 (d) LR-0.35

(c) (d)


Geometric models of impellers with different splitter to main blade length ratios (LR=0.81, 0.65, 0.50 and 0.35)

PEMP RMD510Splitter Blades

The optimum length of splitter blade is half the length p gof the main blade.

Maximum efficiency and pressure ratio at 0.50 splitter length.

i ifi i No significant improvement in performance when splitter length is reduced further.


g

PEMP RMD510Trailing End Skew

Hub Hub Hub45° 90°

45°45 - 45°


45° Skew 0° Skew -45° Skew

PEMP RMD510Trailing End Skew

O ti S d 30 0000.95

Operating Speed 30,000 rpm

3.50

4.00

4.50

Pre

ssur

e R

atio

2/

P01

)

Operating Speed 30,000 rpm

0.85

0.90

ic E

ffici

ency 1.4%

2.00

2.50

3.00

Tota

l to

Tota

l (P

0 2

45° 30°

0° -30°

-45°

0.70

0.75

0.80

Isoe

ntro

p

45° 30°0° -30° -45°

000.50 1.00 1.50 2.00 2.50

Corrected Mass flow rate (Kg/s)

0.701.00 1.50 2.00 2.50

Corrected Mass flow rate (Kg/s)

Effect of exit skew on impeller performance


PEMP RMD510

5

Leading End Skew

3 5

4

4.5

5

sure

ratio Baseline

Back sweep 10˚Back sweep 15˚

Stall margin has increased with 20º forward sweep

2

2.5

3

3.5

Tota

l pre

ss Back sweep 20˚Back sweep 25˚Forward sweep 10˚Forward sweep 20˚

23 3.5 4 4.5 5 5.5 6 6.5 7

Corrected mass flow rate (kg/s)

5

4

re ra

tio Baseline(0.5 mm tip clc.)

Forward sweep 20˚(0.5 mm tip clc.)

3

Tota

l pre

ssur

Forward sweep 20˚(0.75 mm tip clc.)Definition of leading end sweep

A higher tip clearance with forward d f b t th


23 4 5 6 7

Corrected mass flow rate (kg/s)

sweep may reduce performance but the compressor production cost will be low

PEMP RMD510Vaned Shroud at Inducer

Inducer leading


Inducer leading end sweep Barton M.T. et al: ASME J. of Turbomachinery,

Vol. 128, October 2006, pp 627-631

PEMP RMD510Vaned Shroud at Inducer


Barton M.T. et al: ASME J. of Turbomachinery, Vol. 128, October 2006, pp 627-631

PEMP RMD510Exit Trim

Radial exitAltering the axial width of the i ll t it k

Exit width trimimpeller vanes at exit, known as Exit Trim, with respect to the baseline design, helps the manu-facturer in producing design variants to meet different customer requirements without

Axial entry

customer requirements without spending effort in carrying out an ab initio design for a new product

ithi t i f Axial entrywithin a certain range of specifications.


PEMP RMD510Impeller with Integral Shroud


PEMP RMD510Inlet Guide Vanes

• Inlet Guide Vanes (IGV) provide prewhirl to the flow entering the impellerimpeller.

• The inlet velocity triangle is altered.

• Th k it f th• The work capacity of the compressor also changes due to introduction of whirl component of velocity.

W = U2Cw2 + U1Cw1

• Introduction of IGVs is desired to i h ff d i f fimprove the off-design performance of the compressor and to limit the inducer tip Mach number; but the friction losses


will tend to increase.

PEMP RMD510Session Summary

This session has covered:This session has covered:• Basic design methodology of centrifugal compressors.• Calculation of aerodynamic and geometric parameters of the y g p

impeller and generation of velocity triangles.• Calculation of diffuser parameters, especially the diffuser inlet

angle and the throat width of diffuser passageangle and the throat width of diffuser passage.• Effect of geometric parameters on compressor performance.


Design of Centrifugal CompressorDesign of Centrifugal ...

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