Effect of Different Incoming Flow Conditions on the Performance of Axial Flow Diffusers Used for the Jet Engine

Axial flow diffuser, which is located downstream of the axial compressor in the jet engine, is always regarded as a part of the combustor component. The diffuser performance is shown to be greatly influenced by the incoming flow conditions. However, due to the independent design modules of combustor and compressor, simplified incoming flow conditions (SIFC, circumferential averaged flow parameters) are commonly used in the combustor design. Moreover few researchers have noticed the effect of accurate incoming flow conditions (AIFC, actual compressor outlet flow parameters) on the performance of the diffuser and combustor. This oversight frequently leads to unnecessary diffuser design deviations. In this paper, a 2.5-stage high-load axial compressor coupled with the downstream diffuser is numerically investigated to clarify the effect of SIFC and AIFC on the diffuser performance. The results show significant performance difference of the diffuser with the two inflow condition treatment methods, where the diffuser with AIFC presents an approximately five times higher of total pressure loss coefficient (C_p,t) than that of the SIFC (i.e. C_p,t,SIFC=0.326% (hub divergence angle α=14°) VS C_p,t,AIFC=1.61% (hub divergence angle α=7°) at the compressor peak efficiency point), and the optimal divergence angle distribution on the shroud and hub of diffuser with the SIFC and AIFC also shows a great deviation (α_OPT,AIFC=7° VS α_OPT,SIFC=14°). The greater mixing intensity is manifested in the diffuser flow under AIFC due to its circumferential upstream non-uniformity characteristics such as blade wake, which not only increases flow losses but also facilitates flow deceleration. As the compressor operates from peak efficiency point to near stall point, the total pressure loss and static pressure recovery ability of the diffuser under AIFC present more sensitive to the variations of diffuser wall divergence angle distribution. Therefore, for the purpose of improving the design art of axial flow diffuser used for aero gas turbine, the integrated consideration of diffuser and its upstream compressor is recommended due to the accurate flow conditions.