Understanding the Steady and Transient Behavior of the Moderately Loaded High Speed Axial Flow Compressor Stage at Off-Design Conditions

Abstract

High performance high speed transonic axial flow compressors and fans are the major components of efficient aero-engines. Engine operating envelope and performance normally depends on the compressor performance and its distortion tolerance. Even though the high speed compressors are favorite amongst modern aero-engines, they are more prone to instabilities like surge and stall, which further restricts the engine operations. The present paper deal with experimental study carried on a moderately loaded high speed single stage axial compressor and is focused on the steady state as well as transient behavior at off-design conditions. The single stage transonic compressor is designed for stage total pressure ratio of 1.35 and delivers 23 kg/s mass flow rate at corrected speed of 12930 rpm. The stall inception phenomenon is studied using high frequency miniature Kulite pressure transducers and hot wire probe. The fluctuations in the wall static pressure were also studied by placing three Kulite transducers, one at rotor inlet, second at rotor exit and the third at stator exit. The unsteady pressure signals from these Kulite transducers are processed and analysed to understand the stall inception process. The FFT was performed to identify the stall frequency and pressure oscillation amplitude. Unsteady pressure signals clearly shows the instabilities occurred at the rotor inlet and then gradually moves towards the rotor exit, while the flow at the stator exit remains undisturbed. The intensity of the instabilities along the blade span was also studied using three point Kulite rakes placed at the rotor inlet. Stable operating margin of the compressor was evaluated by deriving the compressor stage performance map. The compressor stage performance map was derived using conventional instrumentation like total pressure rakes and thermocouples. The compressor stage experienced abrupt stall at higher operating speeds. The turbulence intensity was evaluated using single component hot wire probe. Maximum stable operating range of 22% is obtained at 50% design speed. The flow parameters were also evaluated at the rotor inlet and rotor exit using aerodynamic probe at full flow and near stall conditions.