Experimental Study of Centrifugal Impeller Discharge Flow in Vaneless and Vaned Diffusers

The flow discharged from centrifugal impellers is highly distorted and its behavior in the diffuser could be expected to have a determining effect on the performance of the compressor. The present work sets out to investigate this. The experimental work reported in this paper was designed to investigate the interaction between the vaned diffuser and the impeller. Unsteady measurements of velocity and wall static pressure were made at numerous positions in a vaned diffuser using an on-line data logging system. Experiments were carried out at a range of flow coefficients for three diffusers with 10, 20, and 30 vanes set at each of three different radius ratios, 1.04, 1.1, and 1.2. A limited number of experiments were also carried out with restaggered diffuser vanes and as a reference case extensive measurements were made in a vaneless diffuser build. The impeller, which was designed for a pressure ratio of 4.6, was run at low speed (3000 rpm) after modification to make its overall diffusion equivalent to that at the high speed for which it was designed. The circumferential distortion from the impeller was attenuated very rapidly in the entrance region of the diffuser vanes and suprisingly had only minor effects on the flow inside the vaned diffuser passage. The effect of the diffuser vanes on the flow discharged from the impeller was evident and reversal of flow back into the impeller was detected when the diffuser vanes were close to the impeller and the flow rate was not very high. The time-mean total and static pressure at impeller outlet were found to vary over the pitch of a diffuser vane, and a variation in the strength of the impeller wake was also observed.