1. 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

2. School of Aerospace Engineering MITE Motivation and Objectives Use CFD to explore and understand compressor stall and surge Develop and test flow control strategies (air- injection, bleeding) for compressors Apply CFD to compare low- speed and high-speed configurations Compressor instabilities can cause fatigue and damage to entire engine

3. School of Aerospace Engineering MITE Summary of Earlier Accomplishments 2-D rotating stall was numerically modeled, and the underlying physical phenomena studied A 3-D flow solver capable of modeling unsteady viscous flow through axial and centrifugal compressors was developed and validated The mechanisms behind the onset and growth of surge in NASA Low Speed Centrifugal Compressor was studied Control of Surge through diffuser bleed was simulated

4. School of Aerospace Engineering MITE Diffuser bleed valves Pinsley, Greitzer, Epstein (MIT) Prasad, Neumeier, Haddad (GT) Movable plenum wall Gysling, Greitzer, Epstein (MIT) Guide vanes Dussourd (Ingersoll-Rand Research Inc.) Air-injection Murray (Cal Tech) Fleeter, Lawless (Purdue) Weigl, Paduano, Bright (MIT & NASA Lewis) How to Control Surge (Passive Control) Bleed Valves Movable Plenum Walls Guide Vanes Air-Injection

5. School of Aerospace Engineering MITE Boundary Conditions (GTTURBO3D) Outflow boundary (coupling with plenum) Periodic Boundary at compressor inlet Solid Wall Boundary at compressor casing Periodic Boundary at diffuser Solid Wall Boundary at impeller blades Periodic Boundary at clearance gap Solid Wall Boundary at compressor hub Inflow Boundary

6. 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:

7. School of Aerospace Engineering MITE DLR High-Speed Centrifugal Compressor AGARD Test Case 24 main blades 30  backsweep CFD-grid 141 x 49 x 33 (230,000 grid-points) Design Conditions: 22360 RPM Mass flow = 4.0 kg/s Total pressure ratio = 4.7 Adiab. efficiency = 83% Exit tip speed = 468 m/s Inlet M rel = 0.92

8. School of Aerospace Engineering MITE DLRCC-Results (Off-Design Conditions) Performance Characteristic Map Unsteady fluctuations are denoted by size of circles Fluctuations at 3.1 kg/sec are 30 times larger than at 4.6 kg/sec Total Pressure Ratio

9. School of Aerospace Engineering MITE DLRCC-Results (Surge Conditions) Mild surge develops. Surge amplitude grows to 60% of mean flow rate. Surge frequency = 94 Hz (1/100 of blade passing frequency)

10. School of Aerospace Engineering MITE DLRCC-Results (Surge Conditions) Flow field vectors show two separation zones: near leading edge in the diffuser Mild surge cycle colored by M rel

11. School of Aerospace Engineering MITE DLRCC-Results (Surge Conditions) Stagnation pressure contours Vortex shedding causes reversed flow Origin of separation occurs at leading edge pressure side Direction of rotation

12. School of Aerospace Engineering MITE LSCC-Results (Air-Injection) Injection angle,  = 5º 3 to 10% injected mass flow rate 0.04R Inlet Casing 5° Rotation Axis Impeller R Inlet

13. School of Aerospace Engineering MITE DLRCC-Results (Air-Injection) Different yaw angles, 3% injected mass flow rate Yaw angle directly affects the unsteady leading edge vortex shedding Positive yaw angle is measured in positive direction of impeller rotation

14. School of Aerospace Engineering MITE DLRCC-Results (Air-Injection) Leading edge separation suppressed due to injection Velocity vectors colored by M rel

15. School of Aerospace Engineering MITE DLRCC-Results (Air-Injection) Different yaw angles, 3% injected mass flow rate

16. School of Aerospace Engineering MITE 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 –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

17. School of Aerospace Engineering MITE Performance Map (NASA Rotor 67) measured mass flow rate at choke: 34.96 kg/s CFD choke mass flow rate: 34.76 kg/s

18. School of Aerospace Engineering MITE Mach Contours at Midspan Spatially uniform flow at design conditions

19. School of Aerospace Engineering MITE Summary of Current Year Work The CFD compressor modeling capability was extended to: Higher speed, higher pressure compression systems Turbulence model Shock capturing capability Boundary conditions Development of surge mechanism in centrifugal compressors was studied. Surge Control through upstream injection was optimized In preparation for rotating stall simulations, a multi- blade passage version of the solver was developed and validated

20. 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

21. School of Aerospace Engineering MITE Technology Transition The suite of codes may be used by industry partners for pilot studies of promising concepts: Compact size of the code Optimized for turbomachinery applications Advanced analysis features (fifth order Roe solver, implicit time marching algorithm, Spalart-Allmaras model) Documentation is available Optimized injection control scheme may be implemented in real engines: Injection location Injection rates Injection angles

22. School of Aerospace Engineering MITE DLRCC-Results (Design Conditions) Static Pressure Along Shroud Excellent agreement between CFD and experiment Local Static Pressure, p/p std