Science Insights: An International Journal

Available online at http://www.urpjournals.com Science Insights: An International Journal Universal Research Publications. All rights reserved Original Article CENTRIFUGAL COMPRESSOR FLUID FLOW ANALYSIS USING CFD Vemu.Vara Prasad *1, M. Lava Kumar 2, B. Madhusudhan Reddy 3 1, 2, 3 Department of Mechanical Engineering, G.P.R. Engineering College, Kurnool-518007, India. Email : varaprasad.mtech@gmail.com Abstract Received 14 August 2011; accepted 30 August 2011 This paper describes the procedure of development of centrifugal compressor flow passage using CFD. CFD simulation was used for fluid flow calculation in the flow passage of centrifugal compressor stage. The investigated centrifugal compressor stage was composed of 3D impeller blades, vaneless diffuser and return channel. The CFD simulation was calculated by means of ANSYS CFX software. Analysis is carried out for different Mass flow rates. The off-design cases considered included compressor operations at 70%, 80%, 90%, 100% and 110% of the on-design rotational speed. CFD results were validated with experimental results for certain chosen performance parameters such as polytrophic efficiency, pressure ratio, stage input power. Vector plots, stream line pilots were generated from CFD results and analyzed for better understanding of fluid flow through centrifugal compressor stage. 2011 Universal Research Publications. All rights reserved Keywords: centrifugal compressor, diffuser, return channel, computational fluid dynamics(cfd) 1.INTRODUCTION This paper describes an effort to investigate and better define the capabilities of a modern CFD solver in predicting the off-design performance of centrifugal compressors. The results will validate the use of commercially available CFD software tools for these problems and lead to a better understanding of the flow phenomenon within the centrifugal Compressor Furthermore, better compressor designs that enhance off-design operation and improve performance could result once confidence is gained in applying CFD analysis tools to the centrifugal compressor design process. The compressor geometry was provided in turbo grid format, ANSYS ICEMCFD was used to generate the appropriate mesh, and ANSYS CFX was used to solve the off-design compressor flow problem. The entire range of offdesign conditions were investigated and compared to experimental data. Furthermore the effects of different mesh densities and turbulence models were also examined. Finally the flow characteristics were examined to identify and quantify the sources of losses and inefficiencies. Centrifugal compressors are vital to many mechanical systems. The focus of this thesis is on these types of machines as applied to gas-turbine engines. This particular application places very strict requirements on weight, efficiency and operating flexibility, which has led to arguably the most advanced and high performance compressor designs. As the performance boundaries are pushed ever higher and efficiencies are further increased, the task of designing better and better compressors becomes more difficult. However some additional improvements are still possible through a better understanding of the flow behaviour overt the entire operational range. This can be best accomplished using highfidelity computational fluid dynamics modelling. Currently centrifugal compressors are designed for optimal operations at a single speed - the design speed. The design speed is usually set at the point where the compressor 6

will operate for the longest duration, such as the cruising speed for turbo-fan aircraft engines. However, compressors must also operate at speeds other than the design point for various amounts of time. A low efficiency at these off-design points can lead to problematic engine starts and poor performance during acceleration. Furthermore, as the compressor becomes more optimized for the design speed, it is becomes increasingly likely that this effort can adversely affect its performance at off-design conditions. 2. CFD ANALYSIS The CFD analysis was solved by means of ANSYS CFX software. The fully 3D, Compressible, viscous, turbulent analysis of the fluid (air) flow was solved. The Computational domain was periodically repeating segment of impeller, diffuser and return channel. The solution was so made on the segment around one impeller blade and around two return channel vanes. For rotor-stator interaction we used stage interface. SST k-v turbulent model was used because this model is suitable for such an analysis. The relation between the state values was made by means of state equation of ideal gas. Computational grid was made by TURBO-GRID software. The size (density) of the computational grid was restricted by software and hardware equipment. The boundary condition at the inlet was always set as the total pressure and total temperature. At the outlet static pressure boundary conditions for higher mass flows were set, for lower mass flows the mass flow boundary conditions were set. The speed of revolution was also set. Through the calculation the values of maximum residuals, pressure at the outlet eventually mass flow, imbalances and efficiency were monitored. As the convergence criteria was set for value of maximum residuals below 10-4. At this value all other monitored parameters were constant. Figure 1: Boundary conditions used in the centrifugal compressor stage simulations 3. RESULTS & DISCUSSION Simulation of the off-design performance of the centrifugal compressor stage was considered for five offdesign speeds (rpm): 70%, 80%, 90%, 100% and 110% of design speed and the various performance parameters like pressure, temperature distribution and velocity profiles on the blades, isentropic efficiencies, Power. 2.1 BOUNDARY CONDITIONS As mentioned in table 1, the stage simulation is composed of three sections impeller vane less diffuser and return channel connected by a stage type mixing plane where the flow properties are averaged circumferentially. The exit of the Impeller and the inlet of the diffuser make up this interface. At the inlet, the boundary was defined as a subsonic inlet, with measured total temperature, total pressure and flow direction profiles. The turbulence level was defined to be medium intensity of about 5%. Periodic boundary conditions were applied to the impeller periodic plane at the middle of the impeller flow passage. The blades, hub and shroud were defined as adiabatic walls with the appropriate rotational Figure 2: Temperature across the Centrifugal Compressor velocity. Figure-1 provides a summary of the boundary conditions used in the centrifugal compressor stage simulations 7 Figure-2 shows the temperature difference across the centrifugal compressor it is clearly seen that temperature is increasing. Figure-3 shows the pressure difference across a centrifugal compressor.figure-4 shows the velocity steam

Figure 5: Efficiency of different stages at different flow rates Figure 3: Pressure difference across the centrifugal compressor Figure 6: Mass flow rate Vs Polytropic Efficiency Figure 4: Velocity difference across the centrifugal compressor line difference across the centrifugal compressor. Figure-5 shows the efficiency of different stages at different mass flow rates. 3.1 Comparison of Performance Predictions to Experimental Measurements Figure-6 plots the stage polytrophic efficiency for the analysis. CFD compares the same with experimental values. It is seen that CFD package predict the efficiency at design point by about 3%. The prediction is higher with till higher mass flow. At known mass flow prediction is close to the experimental valves. Figure-7 shows the plot of total pressure ratio Vs mass flow rate with increase in Mass flow the plot of total Figure 7: Mass flow rate Vs Total pressure ratio pressure ratio should give dropping character tic and a negative slope. CFD results are showing a reverse trend which is unlikely to occur in a centrifugal compressor stage. Figure-8 the experimental results and CFD results predicted the similar variation of increase in power input with increase in mass flow rate. 8

Table 1: DESCRITIZATION OF MODEL S.No Component No. of nodes Hexa Aspect ratio Volume elements (Max.) (Min.) 1 Impeller 55726 48320 24.00 156 2 Vaneless diffuser 66000 71735 2.42 101 3 Ubend 4224 5082 4.61 52.9 4 Return channel 96228 84960 20.00 160 Figure 8: Mass flow Rate Vs Axis Power 4. CONCLUSIONS The numerical modelling and simulation of chosen 3Dimpeller with 21 twisted blades, Vane less Diffuser, Return Channel with 18 De swirl vanes was successfully carried out. Analysis of flow in a 3DImpeller is done for different Mass flow rates with and without the Vane less Diffuser attachment. Similar analysis is carried out with the return channel. Finally Return channel is attached then the total centrifugal compressor stage and analysis is carried out for different mass flow rates. An effort was made to model the flow from inlet to the exit of a centrifugal compressor stage consisting of all the components in place using CFD tools. The vector plots, contour plots and Stream line plots are generated for better understanding of fluid flow through centrifugal compressor stage. The results obtained from CFD analysis were validated with the experimental results for performance parameters such as polytrophic efficiency, power input, and total pressure ratio. CFD results on the polytrophic efficiency of a centrifugal compressor predicted the experimental results closely with a variation of 2%. Similarly power input to a centrifugal compressor stage is predicted by CFD which compares closely with experimental results with a variation of 2.9%. Total pressure ratio of a centrifugal compressor stage as estimated by CFD tools almost complies, with negligible variation of 0.03%, with experimental results. References [1] T. J. Barth and D. C. Jespersen. The design and application of upwind schemes on unstructured meshes. Paper 89-0366, AIAA, January 1989. [2] W.P. Jones and B.E. Launder. The prediction of laminarization with a two-equation model of turbulence. International Journal of Heat and Mass Transfer, 15:301{314, 1972. [3] L.M. Larosiliere, J.R. Wood, M.D. Hathaway, A.J. Med, and T.Q. Dang. Aerodynamic design study of advanced multistage axial compressor. Paper TP2002-211568, NASA, December 2002. [4] B.E. Launder, G.J. Reece, and W. Rodi. Progress in the development of a Reynolds-stress turbulence closure. Journal of Fluid Mechanics, 68:537-566, 1975. [5] F. R. Menter. Improved two-equation k - ω turbulence models for aerodynamic flows. Reference Publication 103975, NASA, 1992. [6] F. R. Menter. Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8):1598{1605, August 1994. [7] B. S. PETUKHOV and L.I. ROIZEN. Generalized relationships for heat transfer in a turbulent flow of gas in tubes of annular section. HIGH TEMPERATURE, 2:65-68, 1964. [8] D. A. Roberts and S. C. Kacker. Numerical investigation of tandem-impeller designs for a gas turbine Compressor. Report 2001-GT-324, Pratt and Whitney Canada for ASME, 2001. [9] D. A. Roberts and R. Steed. A comparison of steady-state centrifugal stage CFD analysis to experimental rig data. Submitted to the 2004 ANSYS CFX conference, 2004. [10] D. G. Shepherd. Principles of Turbo machinery. The Macmillan Company, New York, 1956. [11] G.J. Skoch, P.S. Prahst, M.P. Wernet, J.R. Wood, and A.J. Strazisarl. Laser anemometer measurements of the flow field in a 4:1 pressure ratio centrifugal impeller. Report ARLTR-1448, NASA and Army Research Laboratory, March 1997. [12] P. R. Spalart and S. R. Allmaras. A one-equation turbulence model for aerodynamic flows. Paper 92-0439, AIAA, January 1992. [13] C. G. Speziale, R. Abid, and E. C. Anderson. A critical evaluation of two-equation models for near wall turbulence. Technical report, ICASE, June 1990. 9

[14] D. E. van Zante, A. J. Strazisar, J.R. Wood, M.D. Hathaway, and T.D. Okiishi. Recommendations for achieving accurate numerical simulation of tip clearance flows in transonic compressor rotors. Paper TM2000-210347, NASA, September 2000. [15] M.P. Wernet, M.M. Bright, and G.J. Skoch. An investigation of surge in a high-speed centrifugal compressor using digital PIV. Paper TM2002-211832, NASA, December 2002. [16] D. C. Wilcox. Turbulence Modeling for CFD. DCW Industries, Canada, 1993 Source of support: Nil; Conflict of interest: None declared 10