Inter-turbine diffusers provide continuity between H.P. and LP turbines whilst diffusing the flow upstream of the L.P. turbine. Increasing of the mean turbine diameter offers the potential advantage of reducing the flow factor in the following stages leading to increased efficiency. The flows associated with these inter-turbine diffusers differ from those in simple annular diffusers both as a consequence of their high-curvature S-shaped geometry and of the presence of wakes created by the upstream turbine. It is shown that even the simplest two-dimensional wakes result in significantly modified flows through such ducts. These introduce strong secondary flows demonstrating that full three-dimensional, viscous analysis methods are essential for correct performance modelling.

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