A Numerical Investigation of the Unsteady Flow in Centrifugal Stages

Abstract

Computational Fluid Dynamics (CFD) has matured to the point where it can now be used effectively in impeller and diffuser design. The effective use of CFD for a complete centrifugal stage, however, still remains a challenge. The interaction between an impeller and its diffuser is not well understood and the level of modelling required to explain it is not really known. The goal of this paper is to present some results that can aid in this understanding. A numerical scheme is outlined that is capable of both steady and unsteady stage analysis. Results are presented for a stage consisting of an impeller and pipe diffuser using steady mixing plane analysis. Furthermore, unsteady results for a centrifugal stage consisting of an impeller and a vane diffuser are presented and compared to isolated steady analyses of each component. The results indicate that centrifugal stage performance is not well predicted using steady analysis. Differences in both the diffuser static pressure rise and the impeller pressure ratio are significant and it may be concluded that steady analysis is too unreliable for centrifugal stage predictions. Unsteady results also captured an important interaction phenomenon observed experimentally.