School of Aerospace Engineering MITE Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors Alex Stein Saeid NiaziLakshmi 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

School of Aerospace Engineering MITE Outline Research objectives and motivation Recap of last presentation Centrifugal compressor work Axial compressor work

School of Aerospace Engineering MITE 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

School of Aerospace Engineering MITE 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

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

School of Aerospace Engineering MITE Computational Grid 101x49x25 (blocks I & II) 33x49x81 (block III) grid points main blades splitter blade I II III diffuser Centrifugal Compressor - Grid

School of Aerospace Engineering MITE Validation Results for 4:1 Centrifugal Compressor Circumferentially Averaged Static Pressure Along Shroud (Design Condition)

School of Aerospace Engineering MITE Design Operation Choked Flow Results for 4:1 Centrifugal Compressor Performance Characteristic Map

School of Aerospace Engineering MITE p/p inf Pressure Passage A-A Suction Passage B-B I II III A A B B Impeller flow well behaved Diffuser flow separated Velocity Vectors at Midpassages Velocity Vectors at Midpassages Operation near Choked Flow

School of Aerospace Engineering MITE Velocity Vectors at Midpassage Velocity Vectors at Midpassage Operation near Design Condition I II III B B Suction Passage B-B M rel 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

School of Aerospace Engineering MITE Axial Compressor Rotor mm 22 Full Blades Inlet Tip Diameter m Exit Tip Diameter m Tip Clearance 0.61 mm 22 Full Blades Design Conditions: –Mass Flow Rate kg/sec –Rotational Speed RPM –Rotor Tip Speed 429 m/sec –Inlet Tip Relative Mach Number 1.38 –Total Pressure Ratio 1.63 –Adiabatic Efficiency 0.93

School of Aerospace Engineering MITE Computational Grid 86x35x15 (blocks I & II) grid points II SS I PS  SIMULATION SETUP Axial Compressor Rotor-67

School of Aerospace Engineering MITE Design Results for Axial Rotor-67 Results for Axial Rotor-67 Performance Map Experimental choke mass flow rate: kg/s CFD choke mass flow rate: kg/s

School of Aerospace Engineering MITE Tip Pressure Side No reversed flow in clearance gap Velocity Profile at Pressure Side (Design) Velocity Profile at Pressure Side (Design) (Colored by Pressure)

School of Aerospace Engineering MITE Mid-Passage Velocity Profile (Design) Flow is well behaved

School of Aerospace Engineering MITE Velocity Profile at Pressure Side Velocity Profile at Pressure Side (Off-Design) reversed flow was seen in the clearance gap Tip leakage produces vorticity Tip Pressure Side

School of Aerospace Engineering MITE 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. CONCLUSIONS

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