Cavitation CFD using STAR-CCM+ of an Axial Flow Pump with

Cavitation CFD using STAR-CCM+ of

an Axial Flow Pump with Comparison

to Experimental Data

Edward M. Bennett, Ph.D.

Vice President of Fluids Engineering

March 17, 2014

The Project

• Mechanical Solutions, Inc. (MSI), an engineering consultancy, was

approached by a major pump manufacturer to undertake a

redesign of a line of axial pumps

• A class of axial pumps requires redesign to achieve reduced Net

Positive Suction Head Required (NPSHr)

• The customer wishes to validate STAR-CCM+ against the existing

configuration before proceeding with redesign

• An Internal Research and Development Effort was undertaken to

examine the efficacy of the cavitation model in STAR-CCM+

• The axial pump configuration was analyzed using several

turbulence models and associated cavitation parameters

• A second complex test configuration for which exact test

conditions were known was also analyzed

• Conclusions were made regarding STAR-CCM+ capability to

resolve cavitation problems in complex pump configurations

2

Definitions

• The phenomenon of cavitation occurs when the net pressure in

the fluid decreases below the vapor pressure, e.g. 3170 Pa for

water at 25°C

• The net pressure in the fluid is a function inlet pressure, which

is commonly stated in terms of head, i.e. Net Positive Suction

Head (NPSH)

• Net Positive Suction Head Available (NPSHa) is the actual fluid

energy at the inlet, defined as the difference between the inlet

total head and vapor pressure expressed in terms of head

• Net Positive Suction Head Required (NPSHr) is the NPSH point

at which the pump performance drops below some acceptable

level, often defined as a point at which the total dynamic head

(TDH) produced by the pump drops by 3%

• Cavitation can thus be reduced by increasing NPSHa via inlet

conditions or decreasing NPSHr via geometry modifications

3

Breakdown Curve

• The occurrence of cavitation is visually presented via the

breakdown curve, where NPSHa is plotted against TDH

• As NPSHa is lowered, the onset of cavitation is marked by a drop

in TDH – the value of NPSHa at the 3% drop point defines NPSHr

4

Axial Pump Configuration

5

Axial Pump – Flowpath Geometry

6

Inlet

PropellerPipe

Flowpath Mesh in STAR

7

Mesh Details

8

Mesh Statistics

9

Domain Vertex Count Cell Count

Inlet 107,154 26,848

Propeller 2,095,119 796,104

Pipe 624,174 230,902

TOTAL 2,826,447 1,053,854

Axial Pump – Model Setup

• Realizable k-ε turbulence model

• Segregated flow solver

• 2nd-order convection scheme

• Multi-phase Volume of Fluid (VOF) model

• Rayleigh-Plesset cavitation model

• Boundary conditions:

- Variable inlet total pressure via pressure reference point

- 785 rpm rotating speed

- 755.236 kg/s inlet and exit mass flow (12000 gpm)

• Transient timestep of 2.123e-4 s (360 per rev)

• 15-20 iterations per step

IMPORTANT: MSI did not receive any details regarding test

conditions, such as temperature or experimental setup

10

Alternative Setups

• SST k-ω turbulence model

• SST model with doubled cavitation seed density (2e12/m3)

• Spalart-Allmaras turbulence model

• Finer mesh (remeshed with all mesher sizing values halved)

11

Flowfield and Pressure Contours

12

Streamlines

13

Vapor Fraction Contours

14

Inlet Total Pressure – 137.9 kPa Inlet Total Pressure – 82.74 kPa

Inlet Total Pressure – 55.16 kPa Inlet Total Pressure – 37.92 kPa

Cavitation Breakdown Results

Nss

NPSHa

[ft]

Inlet

Total

Pressure

[psi]

Outlet

Total

Pressure

[psi]

Total

Pressure

Rise

[psi]

TDH

[ft]

TDH

Drop

[%]

4856 46.2 20.474 27.206 6.732 15.5 94.1%

5248 41.6 18.502 25.654 7.152 16.5 100.0%

5739 36.9 16.475 23.303 6.828 15.7 95.5%

6337 32.4 14.494 21.735 7.242 16.7 101.3%

7121 27.7 12.471 19.623 7.152 16.5 100.0%

8164 23.1 10.470 17.558 7.087 16.3 99.1%

8834 20.8 9.471 16.524 7.053 16.3 98.6%

9649 18.5 8.471 15.625 7.154 16.5 100.0%

10665 16.2 7.470 14.597 7.127 16.4 99.7%

11270 15.0 6.972 13.774 6.802 15.7 95.1%

11962 13.9 6.475 13.053 6.578 15.2 92.0%

12754 12.7 5.982 12.175 6.193 14.3 86.6%

15

Main Model

Cavitation Breakdown Results

NPSHa

[ft]

TDH

[ft]

NPSHa

[ft]

TDH

[ft]

NPSHa

[ft]

TDH

[ft]

NPSHa

[ft]

TDH

[ft]

46.2 15.7 46.2 15.5 46.2 15.7 46.2 15.6

34.7 14.8 34.6 15.3 34.6 15.1 34.6 16.8

23.1 15.6 23.1 15.6 30.0 15.7 23.1 15.9

18.5 15.5 18.5 15.3 23.1 16.0 18.5 16.1

16.2 16.0 16.2 15.4 18.5 15.2 17.3 15.4

15.1 14.7 15.1 14.9 16.2 15.4 16.2 15.3

12.8 12.3 13.9 13.7 15.0 14.9 15.0 15.0

12.8 11.9 13.9 14.1 12.7 12.3

12.8 13.5

16

Alternative Models:

SST ModelSST model –

double seed density

Spalart-Allmaras

model

rKE model –

finer mesh

Turbulence Model Results

17

Seed Density Results

18

Mesh Refinement Results

19

Axial Pump CFD Conclusions

• STAR-CCM+ performed well in predicting the trend

of the cavitation breakdown

• Further mesh refinement may bring results even

closer to data

• Turbulence model did not greatly impact the

results

• Bubble seed density did not have a major impact

• MSI did not have access to the experimental rig

setup and this could have additional effect on

results

Additional Test Case

• An additional test case became available to MSI

• A complex double suction pump was made

available with complete data regarding the

cavitation data

• The data included fluid temperature, so precise

representations of the liquid and vapor density

could be applied in the CFD model

Double-Suction Pump – Drawing

20

Double-Suction Pump – Test Data

21

Double-Suction Pump – Mesh

22

Mesh Statistics

23

Domain Vertex Count Cell Count

Suction 4,898,638 1,459,107

Impeller 6,945,706 2,639,844

Volute 2,929,837 911,935

TOTAL 14,774,181 5,010,886

Double-Suction Pump – Setup

• SST k-ω turbulence model

• Segregated flow solver

• 2nd-order convection scheme

• Multi-phase Volume of Fluid (VOF) model

• Rayleigh-Plesset cavitation model

• Boundary conditions:

- Variable inlet total pressure via pressure reference point

- 996 rpm rotating speed

- 150.477 kg/s inlet and exit mass flow (1088.5 m3/hr)

• Transient timestep of 1.675e-4 s (360 per rev)

• 20 iterations per step

24

Velocity Flowfield

25

Pressure Contours

26

Streamlines

27

Vapor Fraction Contours

28

Inlet Total Pressure – 175 kPa Inlet Total Pressure – 80 kPa

Inlet Total Pressure – 40 kPa Inlet Total Pressure – 27 kPa

Cavitation Breakdown Results

Nss

NPSHa

[m]

Inlet

Total

Pressure

[kPa]

Outlet

Total

Pressure

[kPa]

Total

Pressure

Rise

[kPa]

TDH

[m]

TDH

Drop

[%]

2191 17.9 178.88 708.04 529.16 54.0 100.0%

3328 10.3 103.85 627.38 523.53 53.4 98.9%

3929 8.2 83.85 604.71 520.86 53.1 98.4%

4865 6.2 63.85 588.33 524.48 53.5 99.1%

6565 4.1 43.85 570.07 526.22 53.7 99.4%

8112 3.1 33.86 551.61 517.75 52.8 97.8%

8763 2.8 30.86 535.98 505.12 51.5 95.5%

29

NPSH Curve

30

Conclusions

• STAR-CCM+ proves to be an accurate tool for

cavitation analysis

• Turbulence model selection does not appear to have

major effect on the results

• Bubble seed density does not appear to have major

effect on the results

• Matching the fluid temperature and experimental

setup is critical to good results

31

Acknowledgements

• MSI is acknowledged for funding this effort

• MSI gratefully acknowledges the Technical

Support Group of CD-adapco for their continued

guidance and support