A Study of Induced Vorticity in Centrifugal Compressors

Recent investigations have moved a step closer to an understanding of real fluid phenomena in turbomachines by studying rotational flow once it has been established by viscous stresses and nonuniform exchange of energy. The present paper presents a more generalized mathematical treatment of rotational flow (Part I) and makes use of the equations developed to analyze experimentally observed phenomena in the impeller (Part II) and the diffuser (Part III) of a compressor. These studies show that induced vorticity in the impeller produces a nonuniform energy exchange which is the basis for additional rotational effects in the diffuser. Also the amount of vorticity generated varies with flow rate which increases the sensitivity of downstream components to changes in flow rate and thereby reduces compressor range. In the particular diffuser studied, the flow conditions, up to the point of separation, were all established primarily by the induced vorticity while shearing stresses next to the wall seemed to have a negligible influence. Downstream from the beginning of separation, however, an apparent shift in energy distribution is observed which could not be accounted for by the analysis. It is demonstrated that separation is detrimental to vaneless diffuser performance only if it is located at the diffuser discharge. Since the location of separation is a function of the vorticity induced, which varies with the flow rate, a widely spaced diffuser can have substantial variations in performance with a significant influence on the compressor characteristics.