The Effect on Combustor Diffuser Performance of Structural OGV/Pre-Diffuser Systems

Abstract

An experimental investigation has been carried out to assess the aerodynamic effects of locating radial struts within the pre-diffuser of a modern combustor dump diffuser system. Engine representative inlet conditions were generated by a single stage rotor, with the diffuser system incorporating various compressor outlet guide vane (OGV)/pre-diffuser assemblies and an annular flame tube with representative porosity. Stagnation and static pressure measurements were obtained at numerous locations and included assessment of the upstream pressure field, associated with the struts, which impacts on the rotor and OGV aerodynamics. Measurements were also obtained within the feed annuli, surrounding the flame tube, with attempts also being made to assess the stagnation pressure distributions presented to a simulated flame tube burner. Initial tests were performed with an OGV row attached to a conventional 1.45 area ratio pre-diffuser, this providing the datum to which all other systems were assessed. These included systems with thin or thick struts with the strut blockage, at pre-diffuser exit, being 5% and 11% of the gas passage area respectively. For the geometries tested it was shown that the method of adjusting each pre-diffuser passage area, to account for the strut blockage, was successful in providing similar levels of reduced kinetic energy at pre-diffuser exit. Despite this, however, the presence of strut wakes and their effect on the dump cavity flow produced increases in stagnation pressure loss. These loss variations were evaluated for both the feed annuli and burner flows, with the magnitudes depending on whether the struts were aligned or midway between burners. Also assessed was the impact of the increased circumferential flow non-uniformity that was observed for the flow within the inner feed annulus. A beneficial effect produced by the struts was the significant reductions in flow swirl, within the diffuser system, relative to the datum. This improved axial alignment of the flow, provided a more uniform pressure distribution to the burners and a more stable feed to the various flame tube features.