Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

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Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

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Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

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Computational Analysis of Stall and Separation Control in Axial & Centrifugal Compressors

Alex Stein Saeid NiaziLakshmi 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

- Centrifugal compressor work

- Axial compressor work

- Research objectives and motivation
- Recap of last presentation

- 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

- 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

431 mm

- 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

diffuser

III

II

I

splitter

blade

main

blades

Computational Grid101x49x25 (blocks I & II) 33x49x81 (block III)

400000 grid points

Circumferentially Averaged Static Pressure Along Shroud (Design Condition)

Choked Flow

Design Operation

A

B

III

II

I

p/pinf

Pressure

Passage

A-A

Suction

Passage

B-B

A

B

Impeller flow well behaved

Diffuser flow separated

B

III

II

I

Mrel

B

Suction

Passage

B-B

- 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

514 mm

- 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

PS

I

II

w

SS

Computational Grid 86x35x15 (blocks I & II)

90300 grid points

Design

- Experimentalchoke mass flow rate: 34.96 kg/s
- CFD choke mass flow rate: 34.76 kg/s

Tip Pressure Side

- No reversed flow in clearance gap

- Flow is well behaved

Tip Pressure Side

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

- 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.

Bleed Air

Controller

Pressure

Sensors

Air

Inject

- 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.