School of Aerospace Engineering MITE Computational Analysis of Stall and Separation Control in Compressors Lakshmi Sankar Saeid Niazi, Alexander Stein 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 Overview l Recap of Last Presentation l NASA Axial Rotor 67 Results Design Conditions Off-Design Conditions l DLR Centrifugal Results l Conclusions l Future Work

School of Aerospace Engineering MITE Recap of Last Presentation The CFD compressor modeling was applied to higher speed, higher pressure compression systems Development of surge mechanism in centrifugal compressors was studied. Surge Control through upstream injection was optimized (Advisory Board) 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

School of Aerospace Engineering MITE Axial Compressor (NASA Rotor 67) 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 (267.4 Hz) –Rotor Tip Speed 429 m/sec –Inlet Tip Relative Mach Number 1.38 –Total Pressure Ratio 1.63 –Adiabatic Efficiency 0.93 Multi-flow- passage-grid for rotating stall modeling

School of Aerospace Engineering MITE % 30 Pitch Relative Mach Number at %10 Span (Design Conditions) TELE % 50 Pitch TELE

School of Aerospace Engineering MITE Relative Mach Contours at Mid-Span (Design Conditions) Spatially uniform flow at design conditions  IV III II I LE TE

School of Aerospace Engineering MITE Shock-Boundary Layer Interaction (Design Conditions) LE TE Shock Near Suction Side

School of Aerospace Engineering MITE Shock-Boundary Layer Interaction (Design Conditions) Near Pressure Side LE TE

School of Aerospace Engineering MITE LE TE Shock Velocity Profile at Mid-Passage (Design Conditions) Flow is well aligned. Very small regions of separation observed in the tip clearance gap(Enlarged view)

School of Aerospace Engineering MITE LE TE Clearance Gap Enlarged View of Velocity Profile in the Clearance Gap (Design Conditions) The reverse flow in the gap and the leading edge vorticity are growing as the compressor goes to the off-design conditions

School of Aerospace Engineering MITE Performance Map (NASA Rotor 67) measured mass flow rate at choke: kg/s CFD choke mass flow rate: kg/s Design Point Stall Unstable Onset of Stall

School of Aerospace Engineering MITE Transient of Massflow Rate Fluctuations Design Point Onset of Stall Mild Surge 76.4Hz

School of Aerospace Engineering MITE NASA Rotor 67 Results (surge Conditions) f=76.4 Hz = 1/3.5 of Rotor’s frequency

School of Aerospace Engineering MITE I IIIII IV LE TE  I II III IV Location of the Probes to Calculate the Pressure and Velocity Fluctuations The probes are located at 30% chord upstream of the rotor and 90% span and are fixed

School of Aerospace Engineering MITE Onset of the Stall (Clean Inlet) Time (Rotor Revolution ) Probes show same fluctuations and flow is symmetric

School of Aerospace Engineering MITE Onset of the Stall (Disturbed Inlet) Inlet stagnation pressure in Block II is Reduced by 20% Flow is asymmetric and the frequency of rotating stall is 1337 Hz

School of Aerospace Engineering MITE DLR Centrifugal Compressor 5° 0.04R Inlet Casing Rotation Axis Impeller R Inlet 24 main blades CFD-grid 141 x 49 x 33 (230,000 grid-points) RPM Mass flow = 4.0 kg/s Total pressure ratio = 4.7

School of Aerospace Engineering MITE Surge Phenomenon Animation of stagnation pressure contours shows unsteady leading edge vortex shedding just before boundary layer separation

School of Aerospace Engineering MITE Air-Injection Results Angle of Attack Yaw angle directly affects local angle of attack. No Injection 3.2% Injection

School of Aerospace Engineering MITE Parametric Air-Injection Study Optimum: Surge amplitude/main flow = 8 % Injected flow/main flow = 3.2 % Yaw angle = 7.5 degrees

School of Aerospace Engineering MITE Conclusion The CFD compressor modeling was applied to multi- blade passage axial NASA Rotor 67 compressor. The calculated shock strength and location showed good agreement with the experimental results When the inlet flow at off-design was disturbed, a circumferentially non-uniform flow pattern evolved. Parametric study revealed optimum air injection configuration for DLR centrifugal compressor.

School of Aerospace Engineering MITE Future and Planned Activities 3-D rotating stall phenomenon and efficient stall control in axial compressors (bleeding, vortex generators) will be modeled Develop a criterion for efficient injection control of centrifugal compressors Examine the effectiveness of control laws developed by Drs. Haddad, Prasad and Neumeier through CFD- simulations

School of Aerospace Engineering MITE Outflow BC (GTTURBO3D) Plenum Chamber u(x,y,z) = 0 p p (x,y,z) = const. isentropic a p, V p mcmc. mtmt. Outflow Boundary Conservation of mass:

School of Aerospace Engineering MITE Massflow Rate at the Onset of the Stall Iterations

School of Aerospace Engineering MITE Bleed Area  Bleed Control