Computational Analysis of Stall and Separation Control in Axial

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Computational Analysis of Stall and Separation
Control in Axial Centrifugal Compressors
Alex Stein Saeid Niazi 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

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Outline

• Research objectives and motivation
• Recap of last presentation

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Motivation and Objectives

• Use CFD to explore and understand stall and surge
• Develop control strategies for centrifugal and
  axial compressors
• Apply CFD to industrial turbomachinery (high
  pressure ratios, multi-stage)
• Investigate both rotating stall surge separately

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Recap of Last Presentation

• 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

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Centrifugal CompressorAllison Engine Impeller

• 15 main 15 splitter blades
• Design Conditions
• 22000 RPM
• Mass Flow 4.54 kg/s
• Tot. Pressure Ratio 4.13
• Adiab. Efficiency 87
• Tip speed 492 m/s
• Inlet Mrel 0.4 (hub)-0.9 (shroud)
• Designed for use in advanced regenerative gas
  turbine engine for truck/bus and power generation

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Centrifugal Compressor - Grid
Computational Grid 101x49x25 (blocks I II)
33x49x81 (block III) 400000 grid points
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Validation Results for 41 Centrifugal Compressor
Circumferentially Averaged Static Pressure Along
Shroud (Design Condition)
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Results for 41 Centrifugal Compressor
Performance Characteristic Map
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Velocity Vectors at MidpassagesOperation near
Choked Flow
Impeller flow well behaved Diffuser flow separated
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Velocity Vectors at MidpassageOperation near
Design Condition

• Possible sources for diffuser stall
• Adverse effect of downstream BC
• Unknown performance of Spalart-Allmaras
  Turbulence model in separated flows
• Compressor geometry (e.g. diffuser) not exactly
  modeled

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Axial CompressorRotor67

• 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

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SIMULATION SETUPAxial Compressor Rotor-67
Computational Grid 86x35x15 (blocks I II) 90300
grid points
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Results for Axial Rotor-67 Performance Map

• Experimentalchoke mass flow rate 34.96 kg/s
• CFD choke mass flow rate 34.76
  kg/s

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Velocity Profile at Pressure Side
(Design)(Colored by Pressure)

• No reversed flow in clearance gap

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Mid-Passage Velocity Profile (Design)

• Flow is well behaved

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Velocity Profile at Pressure Side(Off-Design)

• reversed flow was seen in the clearance gap
• Tip leakage produces vorticity

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CONCLUSIONS

• CFD code has been extended to centrifugal and
  axial compressors with high pressure ratio.
• CFD Performance maps and pressure data show good
  agreement with experiments.
• For centrifugal compressor diffuser separation
  was observed in the simulations not in agreement
  with experiments.
• 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.

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FUTURE WORK

• Continue to Work on Control Issues, e.g. Unsteady
  Injection, Recirculation.

• Improved geometry to validate flow field.
• Multi-flow passage to simulate rotating stall.
• Investigate influence of shock interaction on
  boundary layer.