Study of diffusers with relation to high speed contrifugal compressors

Abstract

The working range of high?speed centrifugal compressors, where the flow at exit from the impeller is supersonic, is very narrow. This narrow range is attributed to the performance of the diffusers that follow the impeller. The flow at entry to the diffusers is supersonic, typically with Mach numbers around 1.2 to 1.3, but it rapidly adjusts itself through a system of shock waves to a subsonic flow through the major portion of the divergent channels of the diffusers. The diffusers themselves are extremely sensitive to inlet?blockage effects, and it has been realised that the key to diffuser performance lies in the flow adjustments that occur in the region immediately prior to entry into the diffuser throat. This thesis describes a study of the flow in this region in an idealised two?dimensional diffuser with a centre body that simulates the flow in a compressor diffuser. Computations of the flow have been made and supplemented by experiments in a high?speed wind tunnel. The theoretical study is based on a calculation of the transonic flow using the method of relaxation to solve the full potential equation by a type?dependent finite?difference scheme, which automatically 揷aptures� the shock waves during computation. The shock waves appear as severe gradients of properties. When lateral injection of flow into the side of the diffuser is used to simulate the entry of radial flow from the impeller, the initial subsonic longitudinal flow accelerates to supersonic speeds followed by a shock. The flow accelerates again into the diffuser throat with expansions and associated shocks at the corners. In spite of the highly idealised geometry, the results bear a close resemblance to observations of actual flow in high?speed centrifugal compressors. The experimental results showed the importance of throat geometry in diffuser performance and the role of fairing the corners to prevent boundary?layer separation and the consequent increase in blockage. The experimental static?pressure rise in the diffusers agreed well with the results of the theoretical computation when allowance was made for boundary?layer thickening due to shock朾oundary?layer interaction.