Influence of a Closely Coupled Throttle on the Stalling Behavior of a Radial Compressor Stage

Abstract

On the basis of the classical discussion of the stability of compressor blade rows, a treatment is given of the stability of a stage consisting of a radial impeller followed by a vaned radial diffuser with a small radial gap between them. At the exit of the diffuser a radial array of adjustable vanes serves either to throttle the flow or to offer no resistance. In the latter case, throttling takes place by means of restrictions at inlet situated well upstream. The theory demonstrates that instability in the former case can only occur at the point of static instability, i.e., when the stage pressure characteristic slope becomes so positive that the throttle pressure characteristic is tangential to it. With the conventional throttling arrangement, instability is predicted as expected, when the stage characteristic has zero slope. Experimental confirmation of these predictions was obtained from tests on a slow speed radial compressor stage employing the above arrangement at exit, dischanging to atmosphere. When the vanes were used for throttling no rotating stall appeared but with throttling situated remotely from the stage a vigorous rotating stall mode developed.