The effects of inlet conditions and expansion ratio on the onset of flow reversal in swirling flow in a sudden expansion

Experiments are reported in which the minimum swirl intensity required to produce a central recirculation zone in a swirling sudden expansion flow is determined as a function of expansion ratio and inlet conditions. Using a swirl generator which allows for independent variation of velocity profile shape and swirl number, it is shown that an inlet tangential velocity distribution with a large solid body vortex core or an axial velocity profile with a maximum on the axis will lead to a higher critical swirl.