Edward M. J. Naylor Cecilia Ortiz Dueñas Robert J. Miller Howard P. Hodson Whittle Laboratory, University of Cambridge, Cambridge, Cambridgeshire CB3 0DY, UK Optimization of Nonaxisymmetric Endwalls in Compressor S-Shaped Ducts This paper presents a new design methodology for strutted S-shaped compressor ducts that allows for more aggressive designs while maintaining current levels of duct loss. A baseline duct geometry was selected, which had a radius change to length ratio that is 34% larger than current engine design limits. A large-scale low-speed model of the baseline duct was experimentally tested. The flow in the corner between the hub and the strut was found to separate due to the high local diffusion causing an increase in duct loss. Area ruling was applied to the baseline duct and was predicted to reduce the size and extent of the strut-hub corner separation, but the duct design was compromised. The duct loss coefficient at midpitch was predicted to increase compared with that of the baseline design. Nonaxisymmetric endwall profiling was then used on the duct wall, locally to the strut, to remove the strut-hub corner separation and thus reduce net duct loss, without compromising the duct design away from the strut. The endwall geometry was produced by numerical optimization. It was shown that the net duct loss was insen- sitive to casing profiling but highly sensitive to hub profiling. The optimal hub geometry was experimentally tested and shown to completely remove endwall strut-hub corner separation. The profiling was found to reduce the net duct loss by 16%. The paper shows that the key benefit to endwall profiling is that it can be used to safely increase the size of the design space in which aeroengine duct designers can operate. DOI: 10.1115/1.3103927 1 Introduction In aeroengines, an S-shaped annular duct is used to connect the high pressure HP compressor spool to the upstream spool. The duct is required because the spools are designed with different mean radii. The inlet to the duct is at the larger radius. Because the inlet and exit directions of the duct are approximately axial, there are two bends in the duct. The curvature in the S-shaped duct generates static pressure gradients normal to the streamlines. In the first bend, this results in a rise in the casing pressure and a fall in the hub pressure and, in the second bend, the opposite occurs. This results in the flow close to the hub wall being sub- jected to a small acceleration, followed by a large deceleration and then finally a small acceleration Fig. 1. The flow close to the casing wall is subjected to the opposite trends, with a small initial deceleration followed by a large acceleration and then a small deceleration. In addition to the pressure gradients caused by the curvature, the flow in the duct is subjected to streamwise pressure gradients generated by changes in area from inlet to exit. Typically, com- pressor ducts have a small overall area reduction, which helps to reduce the severity of adverse pressure gradients by accelerating the flow. In practice the magnitude of area change is set by the particular engine architecture. In many aeroengines large nonlifting struts intersect the S-shaped duct. These serve two purposes. First, they are a struc- tural element. Second they allow services, such as oil, cooling air, and the radial drive shaft, to traverse the duct. These constraints mean that the struts have a minimum cross-sectional area. Limi- tations on strut length, set by both upstream potential forcing lim- its and by the duct length, result in the strut having a relatively high thickness-to-chord ratio, which is typically between 0.2 and 0.3 in modern engines. The presence of the strut causes additional pressure gradients to be imposed on the duct wall local to the strut. At the front of the strut the flow stagnates. The flow is then accelerated to a velocity greater than that which occurs without the strut being present. Finally the flow is decelerated along the rear portion of the strut Fig. 1. Close to the casing wall, the deceleration caused by the rear part of the strut usually occurs in the region of the S-shaped duct where curvature has resulted in a large acceleration. This means that the deceleration is unlikely to cause boundary layer separation on the casing. Close to the hub wall, the opposite oc- curs and the deceleration at the rear of the strut usually occurs over the region where duct curvature already results in a large flow deceleration. The result is that if the combination of strut diffusion and duct diffusion is too high, a corner separation will occur in the flow between the strut and the hub wall. The nondimensional design space of annular S-ducts unstrut- ted depends on four main parameters. The four parameters that are most commonly used are R / L, h in / L, A out / A in , and R in / h in . Increasing the first three parameters by either increasing the change in radius, R, or reducing the length, L, or increasing the exit area, has a similar effect on the duct’s performance. The large deceleration on the hub wall increases in magnitude and the flow begins to separate. This is commonly referred to as increasing duct “loading.” If a strut is added to the duct an extra nondimen- sional design parameter is added. This is the thickness-to-chord ratio, t / c. As this parameter is raised the local diffusion along the strut-hub corner rises moving it toward separation. During the aeroengine design process, there is a requirement to fix the annulus line early. This encourages the designers to “play safe” so as to avoid any chance of strut-hub corner separation. This often limits the design space in which the duct designer operates. The consequence of such design decisions can lead to nonoptimal designs of the neighboring compressor stages. It can also result in an unnecessary rise in engine length and thus weight Contributed by the International Gas Turbine Institute of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 1, 2008; final manuscript received December 17, 2008; published online September 17, 2009. Re- view conducted by David Wisler. Paper presented at the ASME Turbo Expo 2008: Land, Sea and Air GT2008, Berlin, Germany, June 9–13, 2008. 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