1. School of Aerospace Engineering MITE RECENT PROGRESS IN COMPRESSOR STALL AND SURGE CONTROL L. N. Sankar, J. V. R. Prasad, Y. Neumeier, W. M. Haddad N. Markopoulos, A. Stein, S. Niazi, A. Leonessa 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 Background Modern turbine engines are highly developed, complex systems. There is a continuing trend towards fewer stages, and high pressure ratios per compression stage. Compressor instabilities (rotating stall and surge) develop, that must be controlled at high pressure ratios, especially at low mass flow rates.

3. School of Aerospace Engineering MITE Compressor Performance Map Choke Limit Surge Limit Volumetric Flow Rate Total Pressure Rise Desired Extension of Operating Range Lines of Constant Efficiency Lines of Constant Rotational Speed

4. School of Aerospace Engineering MITE Surge Pressure Rise Flow Rate Peak Performance Mild Surge Deep Surge An “axisymmetric” phenomenon that causes periodic variations in mass flow rate and pressure rise. Deep surge can create a reversed flow in the entire compression system. Pressure Rise Flow Rate Mean Operating Point Limit Cycle Oscillations

5. School of Aerospace Engineering MITE ROTATING STALL Local Separation 1 Rotating Stall is a local separation pattern that rotates at a fraction of the spool RPM

6. School of Aerospace Engineering MITE Different Strategies for Compressor Control Controller Unit Bleed Air Pressure Sensors Air Injection Bleed Valves Movable Plenum Walls Guide Vanes Steady Blowing

7. School of Aerospace Engineering MITE Prior Work An excellent survey by Bram de Jager summarizes worldwide activities on rotating stall and surge control. A number of researchers in U. S. are exploring compressor stall and surge control, using theoretical, computational, and experimental techniques. – MIT, Purdue, Penn State, Cal Tech, Wright Labs, and all major U. S. Industries This presentation will focus on Georgia Tech Activities.

8. School of Aerospace Engineering MITE Georgia Tech Center for Intelligent Turbine Engines –Start Date:November 1, 1995 –Research Team: Eleven faculty members with expertise in controls, compressors, combustion, propulsion, fluid mechanics, diagnostics, MEMS and neural net. –Facilities:Combustion, compressor, micro- electronics and fluid mechanics laboratories –Research Areas: Control of combustor processes, Nonlinear control theory, Control of compressor stall and surge, MEMS

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12. School of Aerospace Engineering MITE Compressor Control- Modeling Efforts Two and three-dimensional compressible flow solvers for modeling compressor stall and surge control Multi-mode models for rotating stall and surge in axial flow compressors Centrifugal compressor model for surge control involving pressure, mass flow rate, and impeller RPM dynamics Model extensions for compressor stall control via fuel modulations

13. School of Aerospace Engineering MITE Compressor Control- Theory Reduced order models based on CFD for modeling compression system transients Optimal nonlinear control framework to address disturbance rejection, control saturation and robustness Adaptive control framework for elimination of rotating stall and surge Nonlinear stabilization framework for interaction between higher order system modes Combined model and fuzzy rule based methodology to address actuator rate and amplitude limits Corrections to rotating stall control theories.

14. School of Aerospace Engineering MITE A Simplified Compressor Model with Heat Addition

15. School of Aerospace Engineering MITE Experimental Studies Experimental Demonstrations –Rotating stall control through Throttling Recirculation of air from plenum to inlet Combustion process modulations Passive means New facility development –A centrifugal compressor facility for the study of flow dynamics, and for the development of active and passive control methods

16. School of Aerospace Engineering MITE Sample Results Experimental Studies Control Theory CFD Modeling

17. School of Aerospace Engineering MITE Schematic of the Axial Compressor Facility (Bleed Control)

18. School of Aerospace Engineering MITE Schematic of the Axial Compressor Facility (Fuel Control) Diffusion flame simulates heat release in a real engine combustor Operating point around 300 0 F

19. School of Aerospace Engineering MITE Fuzzy Logic Control of Rotating Stall Fuzzy Rules were developed using numerical simulations. The numerical simulations utilized the Moore- Greitzer Model, a system of ODEs. Control variable was the amount of opening of a bleed valve placed in the plenum chamber. Following simulations, these rules were implemented in hardware, at our axial compressor facility.

20. School of Aerospace Engineering MITE Fuzzy Logic Controller Compression System Defuzzifier Inference Engine Fuzzifier Measured/Computed Pressure Fluctuations at compressor casing Throttle Opening Output

21. School of Aerospace Engineering MITE Fuzzy Logic Control of Rotating Stall 0 100000 200000 300000 400000 500000 600000 700000 800000 0102030405060708090100 Main Throttle(%) Rotating Stall Amplitude Closed-Loop Fuzzy Logic Control 50% Bleed  bleed  bleed 

22. School of Aerospace Engineering MITE Rotating Stall Control by Flow Separators

23. School of Aerospace Engineering MITE CFD Modeling 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

24. School of Aerospace Engineering MITE Simulation Setup 20 Full Blades with 55° Backsweep Inlet Diameter 0.87 m Exit Diameter 1.52 m Tip Clearance 2.54 mm (1.8% of Blade Height ) Design Conditions: –Mass Flow Rate 30 kg/sec –Rotational Speed 1862 RPM –Total Pressure Ratio 1.14 –Adiabatic Efficiency 0.992 NASA Low Speed Centrifugal Compressor

25. School of Aerospace Engineering MITE Uncontrolled Operation Uncontrolled, Stall Operation Large, Unbounded Fluctuations C

26. School of Aerospace Engineering MITE Off-Design Results (Uncontrolled) Unstable Condition Blades Stall After 3 Cycles (t*) At Beginning After 1 Cycle After 3 Cycles (t*) Velocity Vectors at Midpassage Growing Reversed Flow LE TE

27. School of Aerospace Engineering MITE Compressor Control Setup 0.04R Inlet Impeller Casing  ° R Inlet Rotation Axis Injection Angle,  =5º Yaw Angle,  =0º 5% or 10% Injected Mass Flow Rate

28. School of Aerospace Engineering MITE Controlled Operation Controlled Operation with 10% Air Injection ( Fluctuations are Decreased to 2~3% Extension of Useful Operating Range (60% Below Design) D. m=17.5 kg/sec )

29. School of Aerospace Engineering MITE Air Injection Controlled, Stable Operation Injected Air (10%) Injection Suppresses Stalled Reverse Flow Regions Near LE

30. School of Aerospace Engineering MITE DLR Centrifugal Compressor Control simulations are currently in progress

31. School of Aerospace Engineering MITE NASA ROTOR-67Axial Compressor 0.6 0.8 1 1.2 1.4 1.6 - 125-5025100175250 % Chord M CFD EXP.l Relative Mach No. at 30% Pitch Results for Rotating Stall Simulation considering six flow passages are in progress 51.4 cm

32. School of Aerospace Engineering MITE Concluding Remarks A concerted effort involving control theory, simulations and experimental studies is underway at Georgia Tech to understand and control compressor instabilities. Encouraging results have been obtained in all these areas. A combined CFD-Feedback Control simulation is currently in progress.