Effects of swirl on flow separation and performance of wide angle diffusers

Experimental investigations have been carried out to determine whether the introduction of a circumferential velocity component can produce worthwhile improvements in the performance of, and eliminate flow separation in, wide angle conical diffusers. The swirl generator is a 24 flat-bladed, radial intake type. Systematic experimentation has been carried out for one diffuser configuration fitted with a tailpipe (16.5° and 4.4 area ratio) using varying strengths of inlet swirl and introducing the dissipated mechanical energy as the main criterion of diffuser performance. The best inlet swirl strength produced about 60% reduction of the total diffuser losses in swirl-free flow. The analysis of these results, together with information obtained from flow visualisation experiments, suggests that increasing the swirl beyond an observed threshold completely eliminated flow separation, but it also gave rise to a central zone of recirculating flow and hence additional dissipative losses. We conclude that the optimum improvement achievable in wide angle diffuser performance using swirl does not require the addition of more energy than it saves