Computational Analysis of Stall and Separation Control in Centrifugal Compressors

Computational Analysis of Stall and Separation Control

in Centrifugal Compressors

ABSTRACT

A three dimensional compressible Navier-Stokes code has been used to model the steady and unsteady flow field within a low-speed centrifugal compressor tested at NASA Lewis Research Center. Near design conditions, the performance map, pressure field and the velocity field were in good agreement with measured data. At off-design conditions, the calculations show that flow reversal first occurs near the blade leading edge. If left unchecked, the reversed flow region grows spatially and temporally. Injection of air upstream of the compressor face was found to modify the local flow near the blade leading edge, and suppress rotating stall and surge. Even a small amount of air, typically around 5% of the total mass flow rate, was sufficient to extend the useful operating range of the compressor.

INTRODUCTION

Centrifugal Compressors are currently being used in turbine engines powering tanks and rotorcraft. This is mainly due to their ability to produce high pressure ratios with fewer stages than axial compressors, leading to an overall reduction in size and weight. However, the useful operating range of a centrifugal compressor is limited by the unsteady flow phenomena that occur at low mass flow rates. These aerodynamic phenomena are rotating stall and surge.

Rotating Stall is a local phenomenon in which a circumferentially uniform flow pattern is disturbed, and some of the blades stall. The stalled regions move from blade to blade, and appear to rotate about the shaft axis, at an angular velocity which is 1/3 to 1/2 that of the shaft angular velocity. Rotating stall may lead to vibrations and fatigue of the compressor blades, and is often a precursor to surge. It is to be avoided at all costs.

Surge is a global 1-D instability that can affect the whole compression system. It is accompanied by low frequency fluctuations in pressure and mass flow rate....