Turbofan engines are connected to the airframe by a pylon which crosses the bypass duct downstream of the fan. The presence of the pylon generates a potential field that influences the upstream flowfield. Distortion generated by the pylon can be detrimental to the fan stability and integrity and as such must be controlled. This work presents a new method for the two-dimensional design of fan outlet guide vane cascades in the presence of the potential flow distortion caused by the pylon. The method, based on surface singularities, is capable of mitigating the distortion while preserving the performance of the nominal cascade. Design targets are achieved by manipulating both airfoil geometry and position within the cascade. As the distortions caused by the pylon
The growth in diameter of turbofan engines exacerbates problems related to the interaction of the Outlet Guide Vanes (OGV), pylon and intake because it reduces the ratio between components gaps and disturbance wavelength. The main components of this interaction are the potential fields generated by the intake and by structural components in the bypass, the pylon and the Radial Drive Fairing (RDF). The OGV bladerow and the fan are immersed in these potential fields and suffer performance degradation as well as integrity issues as a result. Simple actuator-disc analysis shows that a uniform OGV cascade amplifies the effect of the pylon potential flow. Therefore, a number of methods have been proposed over the years to compute OGV exit flow angle patterns that result in an approximately circumferentially uniform static pressure field at fan exit. Within actuator disc approximations, the determination of the optimal exit flow angle pattern can be accomplished analytically but little information is obtained on how the geometry of the vanes ought to be modified. Consequently, it is not difficult to generate by this method OGV cascades that stall or choke locally. More recent contributions use CFD computations coupled to optimization methods to determine OGV patterns that reduce the distortion at the fan exit, while minimising some measure of OGV loss. Whilst in principle more rational, these methods encounter practical difficulties due the computational power needed to obtain reliable loss estimates while exploring large design spaces. In this paper the view is taken that the performance of the OGV bladerow can be preserved during the optimization process if the loading distribution of each vane is made to match the loading distribution of the nominal vane (i.e. the aerodynamic design intent with axisymmetric inlet and exit flow). As loading distributions are readily available from inviscid-type analysis, the generation of optimal OGV patterns can be accomplished with very reasonable computational expense using a method based on the model described in part I of this paper.
Flow in annular ducts is sensitive to the presence of downstream blockages which can cause flow non-uniformities propagating far upstream of the blocking body. These effects can be exacerbated in swirling flows where a cascade of uniform guide vanes is present upstream of the blockage. This work uses two- and three-dimensional boundary singularity methods to model and optimise a guide vane cascade geometry to minimise the upstream velocity distortion. Starting from a uniform cascade, the geometry is modified to provide a uniform upstream velocity distribution and minimised blade-to-blade loading in two dimensions. The new geometry is then extrapolated to a three-dimensional annulus. A three-dimensional tool is used to further modify the geometry in three dimensions to minimise the velocity distortion in the whole annulus upstream of the cascade.
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