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Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

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## Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

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### Computational Analysis of Centrifugal Compressor Surge Control Using Air Injection

Alexander Stein, Saeid Niazi and Lakshmi 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

High Performance Computer Time was Provided by the Major Shared Resource Center of the U.S. Army Engineer Research and Development Center (ERDC MSRC).

Outline of Presentation

- Objectives and motivation
- Background of compressor control
- Introduction of numerical tools
- Configuration and validation results
- DLR high-speed centrifugal compressor (DLRCC)
- Off-design results without control
- Surge analysis
- Off-design results with air injection control
- Steady jets
- Pulsed jets
- Conclusions and recommendations

Motivation and Objectives

Lines of Constant

Efficiency

Desired Extension of Operating Range

Lines of Constant

Rotational Speed

Total Pressure Rise

Surge Limit

Choke Limit

Flow Rate

- Use CFD to explore and understand compressor stall and surge
- Develop and test control strategies (air injection) for centrifugal compressors
- Apply CFD to compare low-speed and high-speed configurations

Motivation and Objectives

Compressor instabilities can cause fatigue and damage to entire engine

Operating Point

Pressure Rise

Peak

Performance

Pressure Rise

Limit Cycle

Oscillations

Flow Rate

Flow Rate

Period of

Mild Surge Cycle

Period of Deep Surge Cycle

Flow Rate

Flow Rate

Flow Reversal

Time

Time

What is Surge?Mild Surge

Deep Surge

Movable Plenum Walls

Guide Vanes

Air-Injection

How to Alleviate Surge- Diffuser Bleed Valves
- Pinsley, Greitzer, Epstein (MIT)
- Prasad, Neumeier, Haddad (GT)
- Movable Plenum Wall
- Gysling, Greitzer, Epstein (MIT)
- Guide Vanes
- Dussourd (Ingersoll-Rand Research Inc.)
- Air Injection
- Murray (CalTech)
- Weigl, Paduano, Bright (NASA Glenn)
- Fleeter, Lawless (Purdue)

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

Numerical Formulation (Flow Solver)Reynolds-averaged Navier-Stokes equations in finite volume

representation:

- q is the state vector.
- E, F, and G are the inviscid fluxes (3rd order accurate). R, S, and T are the viscous fluxes (2nd order accurate).
- A one-equation Spalart-Allmaras model is used.
- Code can handle multiple computational blocks and rotor-stator-interaction.

Boundary Conditions (Flow Solver)

Periodic boundary

at clearance gap

Solid wall boundary

at impeller blades

Solid wall boundary

at compressor casing

Inflow

boundary

Periodic

boundary

at diffuser

Solid wall boundary

at compressor hub

Outflow boundary

(coupling with plenum)

Periodic boundary

at compressor inlet

- up(x,y,z) = 0
- pp(x,y,z) = const.
- isentropy

.

mt

ap, Vp

Outflow boundary

.

mc

Outflow Boundary Condition (Flow Solver)Conservation of mass and isentropic expression for speed of sound:

DLR High-Speed Centrifugal Compressor

- Designed and tested by DLR
- High pressure ratio
- AGARD test case

DLR High-Speed Centrifugal Compressor

- 24 Main blades
- 30 Backsweep
- Grid 141 x49 x 33 (230,000 nodes)
- A grid sensitivity study was done with up to 1.8 Million nodes.
- Design Conditions:
- 22,360 RPM
- Mass flow = 4.0 kg/s
- Total pressure ratio = 4.7
- Adiab. efficiency = 83%
- Exit tip speed = 468 m/s
- Inlet Mrel = 0.92

Validation Results (Design Conditions) Static Pressure Along Shroud

- Excellent agreement between CFD and experiment
- Results indicate grid insensitivity => Baseline Grid is used subsequently

C

B

A

Off-Design ResultsPerformance Characteristic MapComputational and experimental data are within 5%

Fluctuations at 3.2 kg/sec are 23 times larger than at 4.6 kg/sec

A: 4.6 kg/sec

20

20

10

10

Pressure Rise Fluctuations (%)

Pressure Rise Fluctuations (%)

0

0

-10

-10

-20

-20

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

Mass Flow Fluctuations (%)

Mass Flow Fluctuations (%)

C: 3.4 kg/sec

D: 3.2 kg/sec

20

20

10

10

Pressure Rise Fluctuations (%)

Pressure Rise Fluctuations (%)

0

0

-10

-10

-20

-20

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

Mass Flow Fluctuations (%)

Mass Flow Fluctuations (%)

Off-Design Results (High-Speed) Performance Characteristic MapLarge limit cycle oscillations develop near surge line

Off-Design Results (High-Speed) Mass Flow Fluctuations

Mild surge cycles develop

Surge amplitude grows to 60% of mean flow rate

Surge frequency = 90 Hz

(1/100 of blade passing frequency)

0.04RInlet

5°

Impeller

Compressor Casing

RInlet

Compressor Face

Rotation Axis

Injected Fluid Sheet

Yaw Angle b

Main Flow

Air Injection SetupSystematic study:

injection rate and yaw angle were identified as the most sensitive parameters.

Related work: Rolls Royce,

Cal Tech, NASA Glenn /MIT,

Air Injection Results (Steady Jets)Different Yaw Angles, 3% Injected Mass Flow Rate

Optimum yaw angle of 7.5deg. yields best result

Mass Flow (kg/sec)

Rotor Revolutions, wt/2p

Reduction in Surge Amplitude (%)

Positive yaw angle is measured in opposite direction of impeller rotation

Yaw Angle (Degree)

Surge amplitude/main flow = 8 %

Injected flow/main flow = 3.2 %

Yaw angle = 7.5 degrees

Air Injection Results (Parametric Study)- An optimum yaw angle exists.
- A reasonable amount (~3%) of injected air is sufficient to suppress surge.

Without Phase Angle Adjustments

Air Injection Results (Pulsed Jets)- Surge fluctuations decrease as long as the injection phase was lagged 180 deg. relative to the flow => suggests feedback control
- reduction in external air requirements by 50% (compared to steady jets)

Nondim. Surge Fluctuations (%)

Rotor Revolutions, wt/2p

Air Injection Results (Pulsed Jets)

- 1.5% injected mass is sufficient to suppress surge
- High-frequency jets (winj = 4wsurge) perform better than low-frequency jets (winj = wsurge)

Nondim. Surge Fluctuations (%)

Rotor Revolutions, wt/2p

Probe

Jet

Air Injection Results (Pulsed Jets)Vorticity Magnitudes Near Leading Edge Tip- Increased amounts of mixing enhance the momentum transfer from the injected fluid to the low-kinetic energy particles in the separation zone

Air Injection Results (Pulsed Jets)Shear Stresses Near Leading Edge Tip

- High-frequency actuation leads to significantly larger shear stress levels.
- Produce smaller but intense turbulent eddies.
- Enhances the mixing at small length scales.

Jet

Conclusions

- A Viscous flow solver has been developed to
- obtain a detailed understanding of instabilities in centrifugal compressors.
- determine fluid dynamic factors that lead to stall onset.
- Steady jets are effective means of controlling surge:
- Alter local incidence angles and suppress boundary layer separation.
- Yawed jets are more effective than parallel jets.
- An optimum yaw angle exists for each configuration.
- Pulsed jets yield additional performance enhancements:
- Lead to a reduction in external air requirements.
- Jets pulsed at higher frequencies perform better than low-frequency jets due to enhanced mixing at small length scales.

Recommendations

- Perform studies that link air injection rates to surge amplitude via a feedback control law.
- Use flow solver to analyze and optimize other control strategies, e.g. inlet guide vanes, synthetic jets, casing treatments.
- Employ multi-passage flow simulations to study rotating stall and appropriate control strategies.
- Study inflow distortion and its effects on stall inception.
- Improve turbulence modeling of current generation turbomachinery solvers. Analyze the feasibility of LES methods.

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