The aim of the present work is to increase the limit of stability and improve the performance of an actual aircraft turbocharger compressor with different casing modifications. Three schemes of modifications in the shroud side of the compressor casing through the vaneless region; circumferential groove, protrude and combined of groove and protrude, were studied. The time variations of wall static pressure were observed using couple of pressure transducers with high frequency response in the vaneless region at different compressor operating conditions. Stall initiation and surge triggering were detected by analyzing both of the fluctuations of pressure signals and the power spectrum density (PSD) which deduced by using the Fast Fourier Transformation analysis (FFT). The number and speed of stall cells relative to the impeller speed were investigated. The flow angles, that are representing the stall initiation for the original compressor, were studied theoretically and experimentally. Both the theoretical and the experimental results were compared with those experimentally obtained by another author and show good agreements.
The present measurements show that the inception of unsteady flow which leads to rotating stall initiation hence surge trigger appears at the vaneless region between the impeller exit and the diffuser vane leading edge. The modified casing by one way of the three-presented schemes can be used to increase the limit of stability for low speed compressor at different operating conditions. The compressor with groove height Hg = 0.2 and depth Tg = 0.2 gives about 55% and 39% improvement in stall margin, but unfortunately with decrease in the pressure coefficient at low flow rates. While the compressor achieves improvement between 14% and 26% in the range of stable operating based on surge margin and about 13% in pressure coefficient. Modification utilizing combination of groove and protrude achieves improvements of about 28% in stall margin, 22% in surge margin and 4% in maximum pressure coefficients