Numerical Study on the Effect of Back Cavity on the Aerodynamic Behavior of a Non-Axisymmetric Casing Treatment for Transonic Axial Compressors

Abstract

For decades, aeronautic engineers have studied the potential of Casing Treatments (CT) to improve the aerodynamic operability of tip-limited compressors. Axial Slot Casing Treatment (or ASCT) has shown very good results on modern transonic compressor. One possible variant of ASCT uses a circumferential back cavity that links the slots outside the compressor main flow path. This study does a back-to-back comparison of two CT designs that differ only by the presence or the absence of the back cavity. The influence of both CT designs on transonic rotor performances and aerodynamics is presented. CT flows are detailed and analyzed. The analysis focuses on fluid exchanges between the CT and the main flow path. This study is based on CFD simulation; CFD results are, at first, confronted with available experimental data and then analyzed. Detailed CT flow analyses show that the presence of the cavity reduces the amplitude of the velocity fluctuations and allows for a more uniform redistribution of the recirculated mass flow. The presence of the back cavity reduces significantly the flow mass transfer between the main flow and the front part of the slots (situated before the rotor leading edge), as a consequence of the action of the transient vortex developing inside. In addition, the synchronization between the rotor blade passage and the extraction cycle, by the rear part of the slots, differs when back cavity is present or not. With the back cavity, the slot tends to extract mass flow mostly when it is close to the pressure side. Without the back cavity, the extraction is performed mostly in the middle of the blade channel. Because of these differences, the back cavity reduces the influence of the CT on rotor efficiency at the cost of a reduction in its capability to improve stall margin.