A Ph.D. Proposal Saeid Niazi Advisor:Lakshmi N. Sankar School of Aerospace Engineering

1. Numerical Monitoring of Rotating Stall and Separation Control in Axial Compressors A Ph.D. Proposal Saeid Niazi Advisor:Lakshmi 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

2. Overview • Objectives and Motivation • Surge and Rotating Stall • Mathematical and Numerical Formulation • NASA Axial Rotor 67 Results • Background • Peak Efficiency Conditions • Off-design Conditions • Bleed Valve Control • Conclusions • Proposed Work

3. Objectives and Motivation Safety Margin Lines of Constant Efficiency Desired Extension of Operating Range • Use CFD to explore and understand compressor stall and surge • Develop and test control strategies (bleed valve) for axial compressors Lines of Constant Rotational Speed Total Pressure Rise Surge Limit Choke Limit Flow Rate

4. What is Rotating Stall? • Rotating stall is a 2-D unsteady local phenomenon • Types of rotating stall: • Part-span • Full-span

5. Mean Operating Point Pressure Rise Limit Cycle Oscillations Flow Rate Pressure Rise Pressure Rise Flow Rate Flow Rate What is Surge? Modified Surge Mild Surge Flow is not symmetric • Surge is a global 1-D instability that can affect the whole compression system. • In contrast to rotating stall, the average flow through the compressor is unsteady. Deep Surge

6. Computational Background on Rotating Stall • Most research activities were on 2-D bases. • Jonnavithula, Sisto, (Stevens Institute of Technology) 1990 • Elder (Cranfield Institute of Technology) 1993 • Rivera (Georgia Tech) 1997 • A few research activities were on 3-D Study, such as, He (university of Durham) 1998.

7. Air Injection Bleed Valves Movable Plenum Walls Guide Vanes How to Control Stall • Air-injection • Murray (CalTech) • Fleeter, Lawless (Purdue) • Weigl, Paduano, Bright (MIT & NASA Glenn ) • Movable plenum wall • Gysling, Greitzer, Epstein (MIT) • Guide vanes • Dussourd (Ingersoll-Rand Research Inc.) • Diffuser bleed valves • Pinsley, Greitzer, Epstein (MIT) • Parsad, Numeier, Haddad (GT)

8.        ˆ ˆ ˆ ˆ ˆ ˆ    q dV  E i  F j  G k  n dS  R i  S j  T k  n dS  t  MATHEMATICAL FORMULATION Reynolds Averaged Navier-Stokes Equations in Finite Volume Representation: where, q is the state vector. E, F, and G are the inviscid fluxes, and R, S, and T are the viscous fluxes. A cell-vertex finite volume formulation using Roe’s scheme is used for the present simulations.

9. MATHEMATICAL FORMULATION • The viscous fluxes are computed to second order spatial accuracy. • A three-factor ADI scheme with second-order artificial damping on the LHS is used to advance the solution in time. • The Spalart-Allmaras turbulence model is used in the present simulations.

10. Periodic Boundaries: Properties are averaged on either side of the boundary Exit: . mt specified; all other quantities extrapolated from Interior Inlet: p0,T0,v,w specified; Riemann-Invariant extrapolated from Interior Zonal Boundaries: Properties are averaged on either side of the boundary Solid Walls: no-slip velocity conditions; dp/dn=dr/dn = 0 Boundary Conditions

11. . mt Actual mass flow rate Desired mass flow rate . mc Outflow Boundary Conditions Conservation of mass: • Plenum • Chamber • u(x,y,z) = 0 • pp(x,y,z) = CT. • isentropic ap, Vp Outflow Boundary All other quantities extrapolated from interior

12. 514 mm 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 • 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

13. Literature Survey of NASA Rotor 67 • Computation of the stable part of the design speed operating line: • NASA Glenn Research Center (Chima, Wood, Adamczyk, Reid, and Hah) • MIT (Greitzer, and Tan) • U.S. Army Propulsion Laboratory (Pierzga) • Alison Gas Turbine Division (Crook) • University of Florence, Italy (Arnone ) • Honda R&D Co., Japan (Arima) • Effects of tip clearance gap: • NASA Glenn Research Center (Chima and Adamczyk) • MIT (Greitzer) • Shock boundary layer interaction and wake development: • NASA Glenn Research Center (Hah and Reid). • End-wall and casing treatment: • NASA Glenn Research Center (Adamczyk) • MIT (Greitzer)

14. Axial Compressor (NASA Rotor 67) Meridional Plane TE LE 4 Blocks 73X32X21 Total of 196224 cells Hub Plane Normal to Streamwise

15. I II III IV  TE LE Relative Mach Contours at Mid-Span(Peak Efficiency) Spatially uniform flow at design conditions

16. % 30 Pitch % 50 Pitch LE TE LE TE Relative Mach Number at %90 Radius (Peak Efficiency)

17. Shock TE LE Near Suction Side Shock-Boundary Layer Interaction (Peak Efficiency)

18. % Pressure Fluctuations % Mass Flow rate Fluctuations TE LE Velocity Profile at Mid-Passage (Peak efficiency) Shock Fluctuations are very small (2%) • Flow is well aligned. • Very small regions of separation observed in the tip clearance gap(Enlarged view)

19. LE Clearance Gap TE Enlarged View of Velocity Profile in the Clearance Gap (Peak efficiency) • The reversed flow in the gap and the leading edge vorticity grow in size and magnitude as the compressor operates at off-design conditions

20. Peak Efficiency Near Stall Adiabatic Efficiency (NASA Rotor 67)

21. C B Unstable Conditions A Near Stall Controlled Peak Efficiency Performance Map (NASA Rotor 67) • measured mass flow rate at choke: 34.96 kg/s • CFD choke mass flow rate: 34.76 kg/s D

22. (A) Peak Efficiency (B) Mild Surge (C) Modified Surge Transient of Massflow Rate Fluctuations Rotor Revolutions (Wt/2p)

23. I II III IV TE LE I III II IV  Location of the Probes for Observing the Pressure and Velocity Fluctuations The probes are located at 30% chord upstream of the rotor and 90% span. They are fixed in space.

24.  I II III IV Time (Rotor Revolution) III II I IV Onset of the Stall (Clean Inlet) • Probes show • identical • fluctuations. • Flow while • unsteady, is still • symmetric from • blade to blade.

25. NASA Rotor 67 Results (surge Conditions) f= 1/80 of blade passing frequency

26. NASA Rotor 67 Results (Rotating Stall) 

27. NASA Rotor 67 Results (Rotating Stall)

28.  Onset of the Stall (Disturbed Inlet) • Inlet distortion • simulated by dropping • the stagnation pressure • in one block by 20% • Flow is no longer symmetric from blade to blade. • Frequency of rotating stall • is NW/3.6, where • NW : blade passing frequency

29. Bleed Area Shroud Hub  Bleed Valve Control • Pressure, density • and tangential • velocities are • extrapolated from • interior. • . • Un = mb/(rAb)

30. Bleed Valve Control 3% Bleeding nearly eliminates reversed flow near LE

31. Bleed Valve Control Without Control With Bleed Valve % Total Pressure Fluctuations 3% bleed air reduces the total pressure fluctuations by 75% % Mass Flow Rate Fluctuations

32. After 1.5 Rev. % From Hub After 0.5 Rev. Bleed Valve. Bleed Valve ControlAxial Velocity Near LE

33. Conclusions • The CFD compressor modeling was applied to the NASA Rotor 67 axial compressor. • The calculated shock strength and location at the peak efficiency are in good agreement with experimental results. • For the axial compressor, tip leakage vortex is stronger under off-design conditions compared to peak efficiency conditions.

34. Conclusions (Continued…) • Results revealed that instabilities during the onset of stall in NASA Rotor67 is of mild surge type. The mild surge was followed by a modified surge. (Surge and rotating stall interaction) • When flow in the inlet at the onset of the stall was disturbed, flow-field became asymmetric and rotating stall was triggered. • Stall and surge can be eliminated by the use of small amounts of bleeding from the diffuser.

35. Proposed Work • Two additional types of bleed control will be studied. Bleed A : Rotating stall amplitude W1 : Rotating stall frequency n : 1 (linear control) 2 (quadratic control) • Should recent Rotor 37 rotating stall data become publicly available (Contact: Dr. Michelle Bright, NASA Glenn), rotating stall control of Rotor 37 will be attempted.