The ability to design S-shaped ducts with high aerodynamic loading is advantageous from a performance and/or weight saving perspective. However, the radial pressure gradients required to turn the flow produce strong pressure gradients in the axial direction. This promotes the likelihood of flow separation from the inner casing as the loading is increased. The current paper presents a novel approach to accommodating the increased loading by bleeding an amount of air from the critical inner casing. The process through which the air is bled re-energizes the boundary layer sufficiently to enable it to remain attached despite the high duct loading. A bled duct is numerically developed and experimentally evaluated using a fully annular isothermal facility, with representative inlet conditions provided by a single stage axial compressor. The measurements indicate successful operation of this new design concept with a reduction in the overall system length, compared to a conventional design, of approximately 30% and a reduction in loss of approximately 20%. The data also demonstrate, to a limited degree, the ability to control the flow distribution at duct exit ultimately improving flow uniformity. Furthermore, the pressure of the bled flow is higher than at rotor exit where, in current engine architectures, flow is typically removed from the main gas path. In other words current engine bleed locations could be replaced by a bleed flow within the transition duct, and this flow is of sufficient pressure to meet the existing requirements associated with cooling, sealing and/or zone ventilation.
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