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
5 Axial Pump Configuration
Axial Pump Flowpath Geometry Pipe Propeller Inlet 6
7 Flowpath Mesh in STAR
8 Mesh Details
Mesh Statistics 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 9
Axial Pump Model Setup Realizable k-ε turbulence model Segregated flow solver 2 nd -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/m 3 ) Spalart-Allmaras turbulence model Finer mesh (remeshed with all mesher sizing values halved) 11
12 Flowfield and Pressure Contours
13 Streamlines
Vapor Fraction Contours Inlet Total Pressure 137.9 kpa Inlet Total Pressure 82.74 kpa Inlet Total Pressure 55.16 kpa Inlet Total Pressure 37.92 kpa 14
Cavitation Breakdown Results Main Model 15 N ss 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%
Cavitation Breakdown Results Alternative Models: SST Model SSTmodel double seed density Spalart-Allmaras model rke model finer mesh NPSHa [ft] TDH [ft] NPSHa [ft] TDH [ft] NPSHa [ft] TDH [ft] NPSHa [ft] TDH [ft] 16 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
17 Turbulence Model Results
18 Seed Density Results
19 Mesh Refinement Results
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
20 Double-Suction Pump Drawing
21 Double-Suction Pump Test Data
22 Double-Suction Pump Mesh
Mesh Statistics 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 23
Double-Suction Pump Setup SST k-ω turbulence model Segregated flow solver 2 nd -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 m 3 /hr) Transient timestep of 1.675e-4 s (360 per rev) 20 iterations per step 24
25 Velocity Flowfield
26 Pressure Contours
27 Streamlines
Vapor Fraction Contours Inlet Total Pressure 175 kpa Inlet Total Pressure 80 kpa Inlet Total Pressure 40 kpa Inlet Total Pressure 27 kpa 28
Cavitation Breakdown Results N ss 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
30 NPSH Curve
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