Numerical Studies on the Effect of Slotted Casing Treatment on the Performance of a Transonic Axial Flow Compressor Shivayogi A. K.1, Nagpurwala Q. H.2, Deshpande M. D.3 1M. Sc. [Engg.] Student, Centre for Rotating Machinery Design 2 Professor, Centre for Rotating Machinery Design 3 Professor and Head, Research, M.S. Ramaiah School of Advanced Studies, Bangalore, 560 054 Online published on 18 February, 2020. Abstract Casing treatment, or incorporation of slotted casing around the rotor, is one of the few techniques to delay the inception of flow instability such as rotating stall, thus enhancing the operating range of an axial compressor. However, the improvement in stall margin through casing treatment is accompanied by a penalty of reduction in compressor efficiency. Several studies have been carried out to optimise the casing treatment configurations in terms of global compressor performance parameters, but there is no clear understanding of the flow physics, which brings about the changes in the compressor performance and stalling behaviour. Further, a given casing treatment configuration may not yield the same results for all compressor builds. This paper deals with the numerical studies on the effect of slotted casing treatment, having axial and radial skews of 15° and 45° respectively, on the overall performance and stall margin of a transonic compressor rotor. The steady state CFD simulations were performed at 100% design speed, with two axial positions of the treatment slots covering 100% and 38% of the rotor tip axial chord from leading end. The results are compared with those obtained on the base line compressor with solid casing. It is shown that the presence of casing treatment significantly alters the stall margin and rotor efficiency compared to the solid casing. Detailed study of the flow field near the rotor tip and within the slots has shown migration of low energy fluid at the rotor pressure surface into the slots and re-emergence into the main flow at the rotor suction surface. The interaction between the casing treatment slots and the blade passage flow is relatively stronger at the stall mass flow rate. A favourable restructuring and energising of the boundary layer flow near the casing in the presence of treatment slots seems to be responsible for stall margin improvement. However, the recirculation of the fluid produces large entropy gradients in the tip region across the rotor, causing a decrease in compressor efficiency. Top Keywords Transonic Axial Compressor, Casing Treatment, Skewed Slots, Stall Margin, Flow Instability. Top |