Experimental and Numerical Investigation of Vaned Hub and Shroud Wall Contoured Diffusers Designed to Improve Flexibility and Efficiency of an Open Impeller Centrifugal Compressor Stage

Abstract State-of-the-art centrifugal compressor stages are required to provide both a flexible and a highly efficient operation. To extend the stable operating range and to improve the design-point polytropic total-to-total efficiency of an open impeller centrifugal compressor stage, three vaned contoured diffusers characterized by geometric modifications of the hub and shroud wall in the vaneless space upstream the diffuser vanes and within the diffuser passages were designed. In this paper, a shroud wall, a hub wall and a hub and shroud wall contoured diffuser and a state-of-the-art baseline diffuser are experimentally examined. For the hub-contoured diffuser an operating range extension of 5.3% was measured at design stage Mach number. For the shroud-contoured diffuser an improvement of polytropic total-to-total efficiency by up to 0.3% is observed. The experimental data including normalized total-to-static pressure ratio and 5-hole-probe data is utilized to validate the numerical setup. By means of the CFD simulations the hub- and the shroud-contoured diffuser designs are analyzed and the hub-contoured diffuser’s effect on the local flow at diffuser vane leading edge is investigated. The results illustrate the local effect of the hub-contoured diffuser design on the flow field in the examined centrifugal compressor stage.