Aerodynamic Performance of a Coolant Flow Off Take Downstream of an Outlet Guide Vane

Abstract

Within the compression system of a gas turbine engine a significant amount of air is removed to fulfill various requirements associated with cooling, ventilation, and sealing. Flow is usually removed through offtakes located in regions where space is restricted, while the flow is highly complex containing blade wakes, secondary flows, and other flow features. This paper investigates the performance of a pitot style offtake aimed at providing a high pressure recovery in a relatively short length. For this to be achieved some prediffusion of the flow is required upstream of the offtake (i.e., by making the offtake larger than the captured streamtube). Although applicable to a variety of applications, the system is targeted at an intercooled aeroengine concept where the offtake would be located aft of the fan outlet guide vane (OGV) root and provide coolant flow to the heat exchangers. Measurements and numerical predictions are initially presented for a baseline configuration with no offtake present. This enabled the OGV near field region to be characterized and provided a datum, relative to which the effects of introducing an offtake could be assessed. With the offtake present a variety of configurations were investigated including different levels of prediffusion, prior to the offtake, and different offtake positions. For very compact systems of short length, such that the gap between the OGV and offtake is relatively small, the amount of prediffusion achievable is limited by the offtake pressure field and its impact on the upstream OGV row. This pressure field is also influenced by parameters such as the nondimensional offtake height and splitter thickness. The paper analyses the relative importance of these various effects in order to provide some preliminary design rules. For systems of increased length a significant amount of flow prediffusion can be achieved with little performance penalty. However, the prediffusion level is eventually limited by the increased distortion and pressure losses associated with the captured streamtube.