This paper presents numerical simulation of a combustor diffusion system, mainly focusing on the effects of the cowling geometry, the area ratio of the pre-diffuser and the axial length of the dump gap. The diffusion system of a full-scale single annular combustor is analyzed, using commercial software Fluent. Six different geometries are designed and numerically analyzed. Case 1 is a baseline, for which low emission technology burning in a lean mode is adopted. Cases 2, 3, and 4 are simulated to study the influence of the cowling geometry, especially the area of cowl capture plane. The study of cowling geometry shows that the best layout is case 4, owing to its least spillage of the air from the dome region into the inner and outer annuli of combustor. The normalized total pressure loss over the combustor is 3.49%. The total pressure distribution at the inlet of the main stage is more uniform than the baseline case 1. Cases 4 and 5 are also analyzed to investigate the influences of area ratio of the pre-diffuser, which varies from 1.4 in case 4 to 1.5 in case 5. The normalized combustor total pressure loss decreased to 3.30%, whose total pressure loss of pre-diffuser decreased while the static pressure recovery coefficient of the pre-diffuser increased from 0.43 to 0.52. Cases 4 and 6 are examined for the axial length of the diffuser dump gap, which is increased by 20% from case 4 to case 6. The combustor total pressure loss increases slightly, with little impact on the pressure loss of the pre-diffuser and dump gap region.

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