The influence of inlet conditions on the boundary layer growth and overall performance of annular diffusers

An experimental investigation of the flow behaviour in, and overall performance of, the straight-core annular diffuser under naturally and artificially generated inflow conditions is described. For naturally developed near uniform inflow diffuser overall performance, and stability limits, are theoretically predicted. In the first part of the research program the diffusers were tested over a range of naturally developed inflow velocity profiles from near-uniform to fully developed flow. The diffuser pressure recovery was observed to fall with initial thickening of the inlet boundary layers and subsequently improved as the flow became fully developed. Downstream of the diffuser exit pressure recovery continued for some two hydraulic diameters into the settling length. The measured levels of diffuser performance compared well with relevant published data but similar stability limits were not found. This was attributed to the varying inflow conditions for the published data. [Continues.]