Numerical and Experimental Studies of Single and Tandem Low-Solidity Cascade Diffusers in a Centrifugal Compressor

Abstract

In order to experimentally determine the performance characteristics of low-solidity cascade diffusers, three kinds of diffusers, a vaneless diffuser, a single and a tandem low-solidity cascade diffuser were attached to a medium specific speed centrifugal compressor stage and tested on a closed-loop test stand. The three-dimensional incompressible viscous flow analysis method, which had recently been established in our laboratory, was used to calculate the internal flow conditions inside of these diffusers. Both the single and tandem low-solidity cascade diffusers performed better than the vaneless diffuser. In particular, the tandem low-solidity cascade diffuser showed an increase in static pressure recovery coefficient of greater than 15% at the design point, and an increase greater than 40% at the lower flow rate, as compared with the pressure recovery of a vaneless diffuser. The total-to-static overall compressor stage efficiency was improved by 4% to 10% from 100% to 70% flow rate by using the tandem diffuser. The measured blade to blade static pressure distribution inside the low-solidity cascade diffusers was compared with the numerical results obtained via 3D viscous incompressible flow analysis, and the authors found that the static pressure recovery was qualitatively well predicted by this flow analysis method.