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 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 Lines of Constant Rotational Speed Efficiency Choke Limit Surge Limit Flow Rate Total Pressure Rise Desired Extension of Operating Range

Motivation and Objectives Compressor instabilities can cause fatigue and damage to entire engine

What is Surge? Mild Surge Deep Surge Pressure Rise Flow Rate Mean Operating Point Limit Cycle Oscillations Pressure Rise Flow Rate Peak Performance Time Flow Rate Period of Deep Surge Cycle Flow Reversal Time Flow Rate Period of Mild Surge Cycle

How to Alleviate Surge Diffuser Bleed Valves Movable Plenum Wall 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) Movable Plenum Walls Guide Vanes Air-Injection

Numerical Formulation (Flow Solver) Reynolds-averaged Navier-Stokes equations in finite volume representation:   t q  dV  E ˆ i F j G k      n dS  R S T 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

Outflow Boundary Condition (Flow Solver) Plenum chamber up(x,y,z) = 0 pp(x,y,z) = const. isentropy ap, Vp mc . mt Outflow boundary Conservation of mass and isentropic expression for speed of sound:

DLR High-Speed Centrifugal Compressor 40cm Designed and tested by DLR High pressure ratio AGARD test case

DLR High-Speed Centrifugal Compressor 24 Main blades 30 Backsweep Grid 141 x 49 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

Off-Design Results Performance Characteristic Map B C D Computational and experimental data are within 5% Fluctuations at 3.2 kg/sec are 23 times larger than at 4.6 kg/sec

Off-Design Results (High-Speed) Performance Characteristic Map -30 -20 -10 10 20 30 Mass Flow Fluctuations (%) A: 4.6 kg/sec Pressure Rise Fluctuations (%) -30 -20 -10 10 20 30 Mass Flow Fluctuations (%) B: 3.8 kg/sec Pressure Rise Fluctuations (%) Large limit cycle oscillations develop near surge line -30 -20 -10 10 20 30 Mass Flow Fluctuations (%) C: 3.4 kg/sec Pressure Rise Fluctuations (%) -30 -20 -10 10 20 30 Mass Flow Fluctuations (%) D: 3.2 kg/sec Pressure Rise Fluctuations (%)

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)

Air Injection Setup Systematic study: 0.04RInlet Casing 5° Rotation Axis Impeller RInlet Yaw Angle b Main Flow Injected Fluid Sheet Compressor Face Compressor Casing Systematic 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)

Air Injection Results (Parametric Study) Optimum: Surge amplitude/main flow = 8 % Injected flow/main flow = 3.2 % Yaw angle = 7.5 degrees An optimum yaw angle exists. A reasonable amount (~3%) of injected air is sufficient to suppress surge.

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) With Phase Angle Adjustments Without Phase Angle Adjustments 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

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 Numerical Probe Jet

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. Area of Interest 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.