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Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors. Alex Stein Saeid Niazi Lakshmi N. Sankar School of Aerospace Engineering Georgia Institute of Technology

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Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

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  1. Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors Alex Stein Saeid Niazi Lakshmi N. Sankar School of Aerospace Engineering Georgia Institute of Technology Supported by the U.S. Army Research Office Under the Multidisciplinary University Research Initiative (MURI) on Intelligent Turbine Engines

  2. Outline • Centrifugal compressor work • Axial compressor work • Research objectives and motivation • Recap of last presentation

  3. Motivation and Objectives • Use CFD to explore and understand stall and surge • Develop control strategies for centrifugal and axial compressors • Apply CFD to industrial turbomachinery (high pressure ratios, multi-stage) • Investigate both rotating stall & surge separately

  4. Recap of Last Presentation • Detailed study and simulation of NASA Low Speed Centrifugal Compressor • Simulation and Validation of Air Bleeding & Blowing/Injection as a Means to Control and Stabilize Compressors Near Surge Line • Useful Operating Range of Compressor was Extended to 60% Below Design Conditions

  5. Centrifugal CompressorAllison Engine Impeller 431 mm • 15 main & 15 splitter blades • Design Conditions: 22000 RPM Mass Flow = 4.54 kg/s Tot. Pressure Ratio = 4.13 Adiab. Efficiency = 87% Tip speed = 492 m/s Inlet Mrel= 0.4 (hub)-0.9 (shroud) • Designed for use in advanced regenerative gas turbine engine for truck/bus and power generation

  6. Centrifugal Compressor - Grid diffuser III II I splitter blade main blades Computational Grid101x49x25 (blocks I & II) 33x49x81 (block III) 400000 grid points

  7. Validation Results for 4:1 Centrifugal Compressor Circumferentially Averaged Static Pressure Along Shroud (Design Condition)

  8. Results for 4:1 Centrifugal CompressorPerformance Characteristic Map Choked Flow Design Operation

  9. Velocity Vectors at MidpassagesOperation near Choked Flow A B III II I p/pinf Pressure Passage A-A Suction Passage B-B A B Impeller flow well behaved Diffuser flow separated

  10. Velocity Vectors at MidpassageOperation near Design Condition B III II I Mrel B Suction Passage B-B • Possible sources for diffuser stall: • Adverse effect of downstream BC • Unknown performance of Spalart-Allmaras Turbulence model in separated flows • Compressor geometry (e.g. diffuser) not exactly modeled

  11. Axial CompressorRotor67 514 mm • 22 Full Blades • Inlet Tip Diameter 0.514 m • Exit Tip Diameter 0.485 m • Tip Clearance 0.61 mm • 22 Full Blades • Design Conditions: • Mass Flow Rate 33.25 kg/sec • Rotational Speed 16043 RPM • Rotor Tip Speed 429 m/sec • Inlet Tip Relative Mach Number 1.38 • Total Pressure Ratio 1.63 • Adiabatic Efficiency 0.93

  12. SIMULATION SETUPAxial Compressor Rotor-67 PS I II w SS Computational Grid 86x35x15 (blocks I & II) 90300 grid points

  13. Results for Axial Rotor-67Performance Map Design • Experimentalchoke mass flow rate: 34.96 kg/s • CFD choke mass flow rate: 34.76 kg/s

  14. Velocity Profile at Pressure Side (Design)(Colored by Pressure) Tip Pressure Side • No reversed flow in clearance gap

  15. Mid-Passage Velocity Profile (Design) • Flow is well behaved

  16. Velocity Profile at Pressure Side(Off-Design) Tip Pressure Side • reversed flow was seen in the clearance gap • Tip leakage produces vorticity

  17. CONCLUSIONS • CFD code has been extended to centrifugal and axial compressors with high pressure ratio. • CFD Performance maps and pressure data show good agreement with experiments. • For centrifugal compressor diffuser separation was observed in the simulations; not in agreement with experiments. • For the axial compressor, tip leakage vortex is stronger under off-design conditions compared to design conditions. This may cause the compressor to go into an unstable state.

  18. FUTURE WORK Bleed Air Controller Pressure Sensors Air Inject • Continue to Work on Control Issues, e.g. Unsteady Injection, Recirculation. • Improved geometry to validate flow field. • Multi-flow passage to simulate rotating stall. • Investigate influence of shock interaction on boundary layer.

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