Paper 16: The Performance of Annular Diffusers

Abstract

Tests have been carried out to investigate the influence of inlet conditions on the performance of symmetrical annular diffusers having 4:1 area ratio and 2·5° and 5° wall angles. The conditions at inlet were varied by using either approach lengths or annular gauzes. Tests on the 2·5° diffuser showed that the ‘efficiency’ deteriorated with increase in outer wall momentum thickness. In addition to the overall performance characteristics a detailed study has been made of the boundary layer growth in the 5° diffuser. A theoretical analysis, based on the assumption of power law velocity profiles, of the flow in conical and annular diffusers operating with fully developed flow at inlet, has been carried out. While it is necessary to estimate the loss in stagnation pressure along the streamline of maximum velocity, the method does not require the use of an empirical auxiliary equation for the variation of the shape parameter. Comparison with experimental data shows good agreement for the point of separation and the axial static pressure distribution.