The demand of further increased bypass ratios for turbofan engines will lead to low pressure turbines with larger diameter and lower rotational speed in conventional high-bypass aero engine architectures. Due to that, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at a larger diameter without any severe loss generating separation or flow disturbances. To reduce costs and weight this turbine duct has to be as short as possible. This results in super-aggressive (very high diffusion) s-shaped geometries where the boundary layers are not able to withstand the strong adverse pressure gradient which results in flow separation. This paper describes the flow through a fully separated duct as a baseline configuration. In a next step the influence of passive flow control devices onto this separation has been investigated. Therefore, low-profile vortex generators were applied within the first bend of this s-shaped intermediate turbine diffuser in order to energize the boundary layer and further reduce or even suppress the occurring separation. This configuration was investigated downstream a transonic turbine stage. Measurements were performed by means of five-hole-probes, static wall pressure taps and oil flow visualization at the duct endwalls. For a better understanding of the flow behavior the vortex generators were also investigated in a two-dimensional rectangular s-shaped duct using the same Mach number level. Results showed that the vortex generators reduce the separation in the 2D-duct but have no distinct influence on the separation within the turbine duct due to wakes as well as strong secondary flow effects. This work is part of the European project AIDA (Contract: AST3-CT-2003-502836).

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