In modern gas turbine engines, up to 20% of the core airflow is bled off from various compressor stages to facilitate internal cooling, bearing chamber and rim sealing, as well as axial load management. As this secondary airflow makes no direct contribution to engine thrust, there are strong economic incentives to reduce the quantity and quality of offtake air and maximise its effective use. Secondary airflows are commonly bled off via circular drillings in the compressor rotor, thereby augmenting their swirl velocity. This results in the creation of vortices within the rotor cavity and strong radial pressure gradients opposing inflow. In the present work the relative performance of a series of non-circular offtake passages has been assessed using CFD techniques. The results of this work demonstrate the degree of control that may be exercised over swirl uptake, which can be used to suppress the creation of vortices in rotor cavities.

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