The performance of a branched annular diffuser system

Low-speed tests have been carried out on a branched annular diffuser system having a geometry similar to that employed in some gas turbine engine combustion systems. The system comprised a straight walled pre-diffuser followed by a sudden area expansion in which the flow was divided between two concentric annuli separated by a bluff body simulating a combustion chamber. The overall geometric area ratio was maintained at 2.0 and all tests were carried out with fully developed flow at inlet. The design flow split between the outer and inner annuli was 2.15:1. The system was tested with five different pre-diffuser geometries to show the effect of increasing the area ratio, increasing the included angle and canting the pre-diffuser. For each pre-diffuser geometry the influence of varying the flow split and the axial distance between pre-diffuser outlet and combustion chamber head (dump gap) were investigated. In addition to determining the overall performance characteristics, the pressure losses for the inner and outer flow fields were calculated and the losses further sub-divided in order to identify regions of high loss. [Continues.]