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.

An Investigation of Unsteady Impeller-Diffuser Interactions in a Centrifugal Compressor

“…This lends credibility to the belieff that the vaned diffuser increases performance by affecting the aft impeller and vaneless space flewfields (Fisher and Inoue, 1981), thereby reducing secondary flows and separation. Thus, if a vaned diffuser is utilized to increase efficiency, the vane stagger should be equal to the absolute flow angle at the operating point, minimizing the loading.…”

“…Optimum values given range from 1.1 to 1.15 (Rodgers, 1982;Bammert et al, 1983;Fisher and Inoue, 1981;Inoue and Cumpsty, 1984). It is generally concluded in the literature that vaned diffusers become less effective at larger radius ratios.…”

“…Fisher and Inoue (1981) made instantaneous measurements in the vaneless space of a low speed compressor stage. However, only time averaged data were compared to steady flow predictions from conformal mapping and Martensen singularity theory (Gostelow, 1984), with good correlation obtained.…”

“…Examples of these comparisons include Yoshinaga (1980), and Fisher and Inoue (1981). Faulders (1956) discusses the application of potential flow theory and conformal mapping to existing steady cascade data to predict diffuser performance.…”

Fluid Flow Analysis of a Single-Stage Centrifugal Fan with a Ported Diffuser

“…They found that the fluid near the low pressure side of the blade had higher velocity. Fisher and Inoue (1981) experimentally studied the interaction between the impeller and four different radial diffusers. Their results showed that the radial diffuser played a major role in establishing the circumferential variation in the mean flow at the outlet of the impeller.…”