Behavior of Severely Asymmetric Flow in a Vaneless Diffuser

Abstract

There is controversy about the mechanism of decay of the asymmetric flow in the vaneless diffuser of centrifugal blowers. In order to clearly observe the behavior of asymmetric flow, every other flow passage of a centrifugal impeller is blocked with a punched plate so that a severely asymmetric flow is induced. The flow behavior in the vaneless diffuser is measured with unsteady flow measuring instruments as well as with conventional instruments for steady flow. The experimental results indicate that there are some flow phenomena which cannot be explained by the conception of mixing process. Such flow phenomena can be explained quantitatively as the isentropic energy exchange between relative streamlines due to the circumferential pressure variation. In addition, the wall roughness of the vaneless diffuser is changed, and the influence of the wall friction on the total pressure loss is compared with the predictions based on the two controversial hypothesis. While these data are handled, it is recognized that the time average total pressure is considerably larger than the mass average total pressure for a severely asymmetric flow. Therefore, if instruments for steady flow measurement are used near the impeller exit to measure the total pressure, the impeller efficiency may be overestimated and the diffuser efficiency may be underestimated.