Experiments were performed on two annular diffusers to characterize the flow separation along the outer wall. Both diffusers had the same fully developed inlet flow condition, however, the expansion of the two diffusers differed such that one diffuser replicated a typical compressor discharge diffuser found in a real machine while the other would create a natural separated flow along the outer wall. Both diffusers were tested at two Reynolds numbers, 5×104 and 1×105, with and without a vertical wall downstream of the exit to replicate the dump diffuser that re-directs the flow from the compressor outlet to the combustor. It was shown that the separation happens quite quickly within the diffuser after which the pressure recovery remains fairly constant. The results also further validate claims that CFD models can not accurately predict flows with high adverse pressure gradients.

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