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Computational Analysis of Stall and Separation Control in Compressors

Computational Analysis of Stall and Separation Control in Compressors. Lakshmi Sankar Saeid Niazi, Alexander Stein School of Aerospace Engineering Georgia Institute of Technology

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Computational Analysis of Stall and Separation Control in Compressors

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  1. 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

  2. Overview • Recap of Last Presentation • NASA Axial Rotor 67 Results • Design Conditions • Off-Design Conditions • DLR Centrifugal Results • Conclusions • Future Work

  3. 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

  4. Axial Compressor (NASA Rotor 67) • 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 (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

  5. % 30 Pitch % 50 Pitch LE TE LE TE Relative Mach Number at %10 Span (Design Conditions)

  6. I II III IV  TE LE Relative Mach Contours at Mid-Span (Design Conditions) Spatially uniform flow at design conditions

  7. Shock TE LE Near Suction Side Shock-Boundary Layer Interaction (Design Conditions)

  8. TE LE Near Pressure Side Shock-Boundary Layer Interaction (Design Conditions)

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

  10. LE Clearance Gap TE 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

  11. Stall Unstable Onset of Stall Design Point Performance Map (NASA Rotor 67) • measured mass flow rate at choke: 34.96 kg/s • CFD choke mass flow rate: 34.76 kg/s

  12. Design Point Onset of Stall Mild Surge 76.4Hz Transient of Massflow Rate Fluctuations

  13. NASA Rotor 67 Results (surge Conditions) f=76.4 Hz = 1/3.5 of Rotor’s frequency

  14. I II III IV TE LE I III II 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

  15. Time (Rotor Revolution) Onset of the Stall (Clean Inlet) Probes show same fluctuations and flow is symmetric

  16. 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

  17. Casing 0.04RInlet 5° Rotation Axis Impeller RInlet DLR Centrifugal Compressor • 24 main blades • CFD-grid 141 x49 x 33 (230,000 grid-points) • 22360 RPM • Mass flow = 4.0 kg/s • Total pressure ratio = 4.7

  18. Surge Phenomenon Animation of stagnation pressure contours shows unsteady leading edge vortex shedding just before boundary layer separation

  19. Air-Injection ResultsAngle of Attack No Injection Yaw angle directly affects local angle of attack. 3.2% Injection

  20. Optimum: Surge amplitude/main flow = 8 % Injected flow/main flow = 3.2 % Yaw angle = 7.5 degrees Parametric Air-Injection Study

  21. 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.

  22. 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

  23. Plenum Chamber • u(x,y,z) = 0 • pp(x,y,z) = const. • isentropic . mt ap, Vp Outflow Boundary . mc Outflow BC (GTTURBO3D) Conservation of mass:

  24. Massflow Rate at the Onset of the Stall Iterations

  25. Bleed Area  Bleed Control

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