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 Niazi Lakshmi N. Sankar School of Aerospace Engineering Georgia Institute of Technology

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Computational analysis of stall and separation control in axial centrifugal compressors l.jpg

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


Outline l.jpg
Outline Axial & Centrifugal Compressors

  • Centrifugal compressor work

  • Axial compressor work

  • Research objectives and motivation

  • Recap of last presentation


Motivation and objectives l.jpg
Motivation and Objectives Axial & Centrifugal Compressors

  • 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


Recap of last presentation l.jpg
Recap of Last Presentation Axial & Centrifugal Compressors

  • 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


Centrifugal compressor allison engine impeller l.jpg
Centrifugal Compressor Axial & Centrifugal CompressorsAllison Engine Impeller

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


Centrifugal compressor grid l.jpg
Centrifugal Compressor - Grid Axial & Centrifugal Compressors

diffuser

III

II

I

splitter

blade

main

blades

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

400000 grid points


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Validation Results for 4:1 Centrifugal Compressor Axial & Centrifugal Compressors

Circumferentially Averaged Static Pressure Along Shroud (Design Condition)


Results for 4 1 centrifugal compressor performance characteristic map l.jpg
Results for 4:1 Centrifugal Compressor Axial & Centrifugal CompressorsPerformance Characteristic Map

Choked Flow

Design Operation


Velocity vectors at midpassages operation near choked flow l.jpg
Velocity Vectors at Midpassages Axial & Centrifugal CompressorsOperation near Choked Flow

A

B

III

II

I

p/pinf

Pressure

Passage

A-A

Suction

Passage

B-B

A

B

Impeller flow well behaved

Diffuser flow separated


Velocity vectors at midpassage operation near design condition l.jpg
Velocity Vectors at Midpassage Axial & Centrifugal CompressorsOperation near Design Condition

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


Axial compressor rotor67 l.jpg
Axial Compressor Axial & Centrifugal CompressorsRotor67

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


Simulation setup axial compressor rotor 67 l.jpg
SIMULATION SETUP Axial & Centrifugal CompressorsAxial Compressor Rotor-67

PS

I

II

w

SS

Computational Grid 86x35x15 (blocks I & II)

90300 grid points


Results for axial rotor 67 performance map l.jpg
Results for Axial Rotor-67 Axial & Centrifugal CompressorsPerformance Map

Design

  • 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) Axial & Centrifugal Compressors(Colored by Pressure)

Tip Pressure Side

  • No reversed flow in clearance gap


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Mid-Passage Velocity Profile (Design) Axial & Centrifugal Compressors

  • Flow is well behaved


Velocity profile at pressure side off design l.jpg
Velocity Profile at Pressure Side Axial & Centrifugal Compressors(Off-Design)

Tip Pressure Side

  • reversed flow was seen in the clearance gap

  • Tip leakage produces vorticity


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CONCLUSIONS Axial & Centrifugal Compressors

  • 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 Axial & Centrifugal Compressors

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.


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