This paper describes the effect of reducing the length of a plain S-shaped compressor inter-stage duct on its performance. The investigation is aimed at understanding the physical mechanisms that determine the flow within these ducts and to assess the effect of these mechanisms in the overall duct performance. An experimental and computational investigation on the development of the boundary layers and loss generation within a datum duct (100% length) was performed. These results were used to design two S-shaped ducts with 74%, and 64% duct length of those currently used as inter-stage ducts. The duct designs were optimised for minimum stagnation pressure loss using a one degree-of-freedom design methodology. Using the duct with lengths 100%, 74% and 64% of current inter-stage ducts, an experimental investigation on the effect of duct length on the development of the boundary layers and loss generated was performed. As the duct length was reduced the loss increases mainly due to boundary layer separation. The size of the separations was found to be strongly dependant on Reynolds number. The results show that at the Reynolds numbers tested relaminarization of the boundary layer may have occurred upstream of separation.

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