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