Annular vaneless diffusers have been studied both experimentally and theoretically to investigate their flow behaviour and performance. The experimental work was carried out on large models of a conical diffuser and a curved diffuser with initial contraction. Besides steady inlet conditions a simulated impeller discharge flow with a jet-wake pattern was investigated. For both conditions of steady and unsteady flow the curved diffuser gave better performance and improved stability. The jet-wake flow proved to have no significant effect upon the performance of either diffuser. The conical diffuser exhibited reversed flow on the hub which increased with mass flow reduction, and when the mass flow was low separation extended from outlet to inlet and strong low frequency pulsations of the flow were initiated. This pulsating flow phenomenon was explained as a rotating stall. Large areas of reversed flow or any flow pulsations were not detected in case of the curved diffuser. An analytical direct solution of the inviscid flow through annular vaneless diffusers of any geometry was developed on the basis of the streamline curvature approach. The solution was obtained under the assumption of a linear distribution of flow parameters. The direct solution agreed quite well with known numerical methods and with experimental results up to separation. While separation itself was well predicted, the solution failed to predict any decrease in flow distortion which was observed experimentally; this was due to the nature of the assumptions within the inviscid analysis. The direct inviscid solution was combined with a boundary layer calculation method from the literature to provide a simultaneous solution of the inviscid core and boundary layer interaction. The calculation method gave good results in the case of non-separated flows. It is felt that this calculation method could be further improved by the inclusion of separation and curvature effects in the boundary layer equations.
|Date of Award||1982|