A rectangular model simulating four sectors of a combustion chamber was used to compare the performance of a standard dump diffuser, of overall length 180 mm, with that of a faired design 25.5 mm shorter. The performance of each system was assessed in terms of total pressure loss and static pressure recovery between prediffuser inlet and the annuli surrounding the flame tube. Since the program objective was to test design concepts only, no allowance was made for the presence of burner feed arms or flame tube support pins. In addition, tests were performed with relatively low levels of inlet turbulence and no wake mixing effects from upstream compressor blades. Relative to the dump design, the mass weighted total pressure loss to the outer and inner annuli was reduced by 30 and 40 percent, respectively, for the faired diffuser. Measurements around the flame tube head were used to identify regions of high loss within each system and account for the differences in performance. Within a dump diffuser the flow separates at prediffuser exit resulting in a free surface diffusion around the flame tube head and a recirculating flow in the dump cavity. This source of loss is eliminated in the faired system where the flow remains attached to the casings. Furthermore, the faired system exhibited similar velocity magnitudes and gradients around the combustor head despite its shorter length.

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