A Study of Diffuser/Rotor Interaction in a Centrifugal Compressor

Abstract

Experimental flow data recorded on a low speed centrifugal compressor rig are compared with theoretical predictions of the diffuser field. Results for four diffuser geometries are presented covering variation of the blade number, stagger angle, and radial separation of the rotor and diffuser. The theoretical predictions are based on an extension of the Martensen surface singularity theory. The diffuser is found to play a major role in establishing the circumferential variation in the mean flow at impeller outlet. Large pitch wise variations are observed, which may be explained by consideration of the flow round the leading edges of the diffuser blades. Experiments and potential flow theory show good agreement, except for cases where the diffuser shows signs of separation. These are clearly identified from the theoretical predictions by Carter's lift coefficient criterion. Although the absolute flow at rotor outlet is markedly time-dependent owing to the passage of the rotor, it would appear that the averaged flow conditions may be predicted by the steady-state singularity program. Unstalled diffuser flows are accurately predicted and stalled flows, which are generally to be avoided, may be anticipated from the lift coefficient criterion. The designer thus has the option of reducing pitchwise velocity and pressure variations at rotor outlet to acceptable amplitudes.