Assessment of Flow Control Strategies for Improving Centrifugal Compressor Efficiency

Abstract

This paper describes a preliminary assessment of two flow control strategies for improving the adiabatic efficiency of centrifugal compressors for aero-engine applications. Given that the diffuser loss and pressure recovery play and important role in centrifugal stage efficiency, a centrifugal compressor with “fishtail” pipe diffusers is chosen for the study. This type of diffuser, which is among the most efficient diffusers, turns the flow directly from a high-swirl radial flow toward an axial flow, thus providing a smaller outer compressor diameter. As such, they are ideal for aero-propulsion applications. Past researches indicate that the diffuser performance is very much dependent on the impeller exit flow (diffuser inlet flow) uniformity. Two passive candidate flow control strategies that could improve impeller exit flow uniformity are proposed, namely slots casing treatment near the impeller radial bend and flow recirculation with injection in this area. They are aimed at attenuating the significant low-momentum region near the shroud that grows from the radial bend to the impeller exit. Iterations of the two proposed flow control strategies were evaluated through unsteady RANS CFD simulations on a low-speed centrifugal compressor stage with fishtail pipe diffusers. A comparison in terms of component and stage performance as well as an analysis of the flow field was carried out from the simulation results of the early iterations of the two flow control strategies. They show that both strategies have good potential for improving impeller exit flow uniformity and reducing losses in the fishtail pipe diffusers. However, the casing treatment strategy is more promising for improving stage efficiency due to lower penalty in impeller efficiency.