Flow Instability Within a Diffusing, Annular S-Shaped Duct

Abstract

Experimental measurements have been made on a representative, high radius ratio, inter-turbine diffuser with and without internal load-bearing struts. It is shown that without the struts fitted the severe, local diffusion experienced within the test duct results in a casing surface flow separation. The addition of a representative row of load bearing struts which intrude into the separation zone is shown to exacerbate the flow separation by increasing the local diffusion rate. The casing surface flow separation, for the non-swirling case presented here, clearly demonstrates a low frequency instability. It has been shown previously that the propagation of wakes through a diffusing duct provides a mechanism for the creation of pressure driven secondary flows. For this duct configuration it is shown that these secondary flows act to suppress the unsteady flow separation. Direct comparisons between CFD and experimental results show good agreement especially for the case with the struts in place.