Influence of Compressor Exit Conditions on Combustor Annular Diffusers Part II: Flow Redistribution

Abstract

In gas-turbine engines the velocity of air, issuing from the compressor, must be reduced in order to permit effective operation of the downstream combustor. This is partly achieved by locating an annular diffuser behind the compressor outlet guide vanes (OGVs) and, in modern systems, the inlet of this diffuser is usually located at the trailing edge of the blade row. This paper is concerned with some of the interactions that occur between these components and, in particular, the e ow redistribution that occurs along the diffuser length due to the e ows generatedbytheOGVbladepassageandupstreamrotor.Amainlyexperimentalinvestigationhasbeenundertaken, on a fully annular facility, which incorporates a single-stage axial e ow compressor and simulated e ame tube. In addition, immediately downstream of the OGV row a constant-area passage, ordiffusers of area ratio 1.45 or 1.60, can be incorporated. The OGV blade row produces a proe le that, as a result mainly of the blade wakes, contains an excess of kinetic energy relative to that of a uniform proe le. The mixing out of these wakes therefore enhances the pressure rise within the downstream diffuser. Measured mean velocity data are used to determine the path of streamlinesalongeachdiffuserandindicateregionswherehigh-energye uidisbeingconvected,towardeachcasing, and low-energy boundary layere uidisbeing removed.Thisisbecauseoftheremnantsofthee owsgeneratedwithin each OGV passage. The mean momentum equation along each diffuser is then used to indicate that such e ows signie cantly offset the changes in momentum, within each boundary layer, that are associated with the applied pressure gradient. Such effects are therefore thought signie cant in terms of reducing the boundary-layer growth and delaying e ow separation from the casings.