Analyses of radial compressor stability improvement by casing treatment

Centrifugal compressors used for turbochargers are required to achieve high performance within wide flow ranges. However, their operability is limited at low mass flow rates by the flow instabilities leading to rotating stall and surge, which could be delayed by a suitable casing treatment. This article presents a numerical investigation aimed at studying the effects of a type of casing treatment on the flow structure through a radial compressor and the subsequent improvement of aerodynamic stability. The considered casing treatment consists of an oblique slot recess inserted in the shroud near the impeller throat and a large annular cavity connecting this slot to the bell-mouth. The flow computations were carried out by means of the code ANSYS-CFX, considering the shear stress transport (k-omega) turbulence model, in order to investigate the effects of the cavity dimensions on improving the stability limit of the compressor. The analysis of flow structure near the stall limit has revealed that the large vortex appearing near the blade tip is being controlled by such a type of casing treatment; hence the stall onset is delayed. As consequence, the operating range and the stall margin are improved by reducing the aerodynamic blockage.