Design Verification of Hydraulic Performance for a Centrifugal Pump using Computational Fluid Dynamics (CFD) Anil S. Akole (Asst. Manager) Center for Design & Research Pentair Water India Private Limited New Delhi (India).
Abstract: This paper discusses the use of Computational Fluid Dynamics (CFD) in verification of hydraulic performance for the Centrifugal Pump at Design Stage. The need for delivering efficient new products within short span of time and at competitive price has stimulated use of CFD simulation software in Pump Industry. The CFD software provides virtual test rig for verification of the hydraulic performance of the Pump to the fairly good accuracy. Thus CFD software helps in the decision process at the early design stage of new product development ensuring optimum performance of the product. The concept will be further elaborated with the help of two new Centrifugal Pump design projects which were recently undertaken by Pentair Water using CFdesign software. 2
Introduction: The acute competition in terms of efficient, cost effective products & short time available to bring new products into the market has forced engineering companies to change their design & development process radically. One of these efforts is adaptation of various simulation techniques during the design stage to deliver first time right, efficient new products. Computational Fluid Dynamics (CFD) tool was originally developed for academic research and aerospace is now being used by other industries handling products dealing with flow. Centrifugal Pump manufacturers are also not exception to CFD usage while designing new products. CFD is the simulation of flows, such as the flow of water through a pump or blood through a valve. The use of CFD in Centrifugal Pump design gives following benefits. Provides virtual test rig to predict the pump performance. Identifies failure modes at early stage of design. Flow visualization through the pump. Overhead & time reduction in new product development. Eco friendly as against traditional development process. Surety of achieving successful design with optimum performance within short span of time. This paper will be discussing two of new product hydraulic design projects undertaken recently by Pentair Water using CFdesign software for performance verification. Scope: The scope of this paper is to highlight how design verification of hydraulic performance is carried out using CFD software for two stage Split Case & Inline Centrifugal Pumps. Before taking decision of using CFD, the software was validated by comparing actual performance data with the CFD analysis output. To validate the CFD software, before starting of these new projects, the CFD analysis was carried out for one of the existing pump with known performance to establish the performance curve. The results achieved were within 10 % to the actual tested curves of that pump. The results are shown in Fig. 1 (a) & (b). Fig.1 (a) Existing End Suction Centrifugal Pump Performance Establishing with CFD CFD Analysis at five different duty Conditions 3
Fig.1 (b) Existing End Suction Centrifugal Pump Performance comparison CFD Vs Tested Curve H-Q Curve Comparison Head (Meter) 40 35 30 25 20 15 10 5 0 0 100 200 300 400 500 600 700 800 900 Flow (GPM) CFDesign Curve Tested Curve Methodology: Following are hydraulic flow volumes of the two pumps [Figures 2 (a) & 2 (b)] as launched in CFD software which were considered for these new design projects. Fig.2 (a) -Two Stage Split Case Pump Fig.2 (b) - Inline Pump Then the analysis was carried out for full open flow and for the other flow conditions towards shut off side to establish Performance Curve. The number of alternate designs were created in SolidWorks for each pump and tried in CFdesign software to finalize the best combination of Impeller & Casing to achieve desired performance. 4
Comparison CFD vis-a-vis conventional Methodology: Sr.No. Parameter Conventional CFD 1 Performance Prediction Based on experience, assumptions & manual calculations hence may not be accurate. 2 Performance verification at design stage Only duty point & shut off head are considered during calculations. 3 Flow visualization Not possible. Need some special transparent prototypes for visualization. 4 Pressure / velocity distribution across the pump Visualization not possible. 5 Failure Mode Depends on experience, skill. identification at design stage 6 Investment Huge, as number of physical prototypes are needed to be built & tested before finalizing the design. 7 Time Time required in finalizing new design is more since more alternate design prototypes are to be made & tested. 8 Success rate For high success rates need more skill, experience & time. 9 Eco friendly No, as more number of prototypes required. 10 Decision making Difficult, skill & experience process 11 Presentations to top management for approvals based. Difficult to present visualization of the new design. Automatic, virtual test rig for pump, around 90% guarantee to achieve desired performance. Entire Performance Curve can be verified. Possible. The problematic areas such as turbulence, recirculation, and flow separation can be visualized. Can be visualized. Easily possible. No physical prototype needed for freezing the design, only no. of 3D Models needs to be created for analysis. Less time required as design is finalized before going for actual prototype. High, around 90%. Yes. Easy. Effective presentation by flow visualization & output results. 5
CFD - Simulation Assumptions: Various assumptions were made for the simulation of these Centrifugal Pumps as listed below. Transient - state conditions. Incompressible water flow. Thermal effects considered negligible. Smooth walls. Steps followed in CFD analysis: Achieving 3D model adequacy for CFD software - Fig. 2 (a) & 2 (b) shows the 3D models launched into CFD software. Certain changes were made in the model to achieve this adequacy. Defining boundary conditions - Depending upon the requirement of analysis, inlet & outlet boundary conditions were defined either flow or pressure. Suppressing unwanted solid bodies - The solid bodies are not required for meshing; only the flow volume is required. Hence these solid bodies were suppressed. Meshing The meshing was done using the manual meshing option. Some care was taken to refine the mesh in critical areas. Meshing on the pumps is shown in Fig. 3 (a) & 3 (b). Fig.3 (a) -Two Stage Split Case Pump Meshing Fig.3 (b) - Inline Pump Meshing Specifying material - Specifying the material to fluid domain, solid parts & Impeller. The roughness values can also be specified as per requirement. For Impeller, rotating region need to be created to envelop the Impeller and rotation is given to this rotating region. Conducting analysis Ran the analysis till converged results were obtained for different flow conditions. View results. Results were compiled and compared with the expected performance. 6
Analysis Results: 1) Convergence After each analysis convergence was ensured as can be seen in the convergence monitor in Fig. 4 (a) & 4 (b). Fig.4 (a) -Two Stage Split Case Pump - Result Convergence Fig.4 (b) -Inline Pump Result Convergence 7
2) Flow Vectors Flow vectors were checked for any abnormality as shown in Fig. 5 (a) & 5 (b). The flow vectors gave very good visualization of the flow through the pump. Wherever any major abnormality was observed, the same was taken care off. Some minor problems may still remain which based on experience can be neglected. But risk of carrying those problems will be very meager as all majors are tackled already. Fig.5 (a) -Two Stage Split Case Pump Flow Vectors Fig.5 (b) -Inline Pump Flow Vectors 3) Pressure / Velocity Distribution Fig. 6 (a) to 6 (e) shows various Pressure Distribution plots for both the pumps under consideration. The advantage is the visualization of the Pressure Distribution across the pump from Suction to Delivery. No abnormality in pressure distribution was found in both the pumps. Fig. 6 (e) shows low pressure zones in Suction which is within acceptable limit. Fig. 7 shows the Velocity Distribution Plot through Inline Pump. 8
Fig.6 (a) Two Stage Split Case Pump Pressure distribution. Fig.6 (b) Two Stage Split Case Pump Pressure distribution Fig.6 (c) Inline Pump Pressure distribution Fig.6 (d) Inline Pump Pressure distribution 9
Fig.6 (e) Inline Pump Pressure distribution Fig.7 Inline Pump Velocity Distribution 10
4) Particle Trace Fig. 8 shows the particle traces from Suction to Delivery. This shows how the liquid particles would travel through the pump. Actual animation was also observed. In both the pumps the particle traces were found normal. Fig.8 Two Stage Split Case Pump Particle Trace 5) Results reading The results at outlet boundary plane were noted. The output was either flow or pressure. The comparison was done for CFD results & expected curve. The numbers of modifications were done till the expected curve as shown in Fig. 8 (a) & 8 (b) was achieved. Typically in two stage Split Case pump, five designs were analyzed & for Inline pump, two designs were analyzed to freeze final hydraulic design. Fig.8 (a) -Two Stage Split Case Pump Performance Comparison 11
Fig.8 (b) -Inline Pump Performance Comparison 12
Conclusion: The concept of usage of CFD for design verification of hydraulic performance for a Centrifugal Pump has proved in achieving Engineering Breakthroughs in early stages of the product design process. It has tremendous advantage over the conventional method with almost 90 % surety to achieve desired performance right at first time. The initial time required to freeze the design may be slightly more as analyzing more alternate designs before making final decision. However much more time will be saved as compared to the total time taken for conventional method for new product development. There will be no need to do multiple prototypes, back to design modification, again building prototype, testing & so on. Also the cost incurred in making more prototypes will be reduced substantially. Acknowledgement: I take this opportunity to declare that without the active support of Center for Design & Research of Pentair Water, this paper would have been a dream. Also the work was supported by CFdesign team, Pricol Technologies and their principals Blue Ridge Numerics. References: Various CFD analysis reports from Center for Design & Research of Pentair Water. CFdesign Users Guide. Papers published on Internet. 13