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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.

Within gas turbines the ability to design shorter aggressive S-shaped ducts is advantageous from a performance and weight saving perspective. However, current design philosophies tend to treat the S-shaped duct as an isolated component, neglecting the potential advantages of integrating the design with the upstream or downstream components. In this paper such a design concept is numerically developed in which the upstream compressor outlet guide vanes are incorporated into the first bend of the S-shaped duct. Positioning the vane row within the first bend imparts a strong radial gradient to the pressure field within the vane passage. Tangential lean and axial sweep are employed such that the vane geometry is modified to exactly match the resulting inclined static pressure field. The integrated design is experimentally assessed and compared to a conventional non-integrated design on a fully annular low speed test facility incorporating a single stage axial compressor. Several traverse planes are used to gather five-hole probe data which allow the flow structure to be examined through the rotor, outlet guide vane and within the transition ducts. The two designs employ almost identical duct geometry, but integration of the vane row reduces the system length by 21%. Due to successful matching of the static pressure field, the upstream influence of the integrated vane row is minimal and the rotor performance is unchanged. Similarly the flow development within both S-shaped ducts is similar such that the circumferentially averaged profiles at duct exit are almost identical, and the operation of a downstream component would be unaffected. Overall system loss remains nominally unchanged despite the inclusion of lean and sweep and a reduction in system length. Finally, the numerical design predictions show good agreement with the experimental data thereby successfully validating the design process.Copyright © 2011 by Rolls-Royce plc

Within the compression system of a gas turbine engine a significant amount of air is removed to fulfil various requirements associated with cooling, ventilation and sealing. Flow is usually removed through off-takes located in regions where space is restricted, whilst the flow is highly complex containing blade wakes, secondary flows and other flow features. This paper investigates the performance of a pitot style off-take aimed at providing a high pressure recovery in a relatively short length. For this to be achieved some pre-diffusion of the flow is required upstream of the off-take (i.e. by making the off-take larger than the captured streamtube). Although applicable to a variety of applications, the system is targeted at an intercooled aero-engine concept where the off-take 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 off-take present. This enabled the OGV near field region to be characterised and provided a datum, relative to which the effects of introducing an off-take could be assessed. With the off-take present a variety of configurations were investigated including different levels of pre-diffusion, prior to the off-take, and different off-take positions. For very compact systems of short length, such that the gap between the OGV and off-take is relatively small, the amount of pre-diffusion achievable is limited by the off-take pressure field and its impact on the upstream OGV row. This pressure field is also influenced by parameters such as the non-dimensional off-take 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 pre-diffusion can be achieved with little performance penalty. However, the pre-diffusion level is eventually limited by the increased distortion and pressure losses associated with the captured streamtube.