Numerical Study of Compressible Subsonic Flow in Gas Turbine Annular Diffusers

Meiriele Abreu Alvarenga¹; Marcelo Pustelnik¹; Claudia Regina de Andrade¹; Edson Luiz Zaparoli¹

¹ Instituto Tecnológico de Aeronáutica - ITA

doi:10.20906/CPS/COB-2015-0099

Resumo

Annular diffusers are encountered at the compressor discharge where it gets "dumped" into the gas turbine combustor. The problem with diffusing flow is that if the diffusion happens too abruptly there will be aerodynamic losses caused by an adverse pressure gradient. Therefore, with an increase in divergence angle, stall losses arising from boundary-layer separation become more significant Usually, for aircraft engines, and also many industrial engines, the length is a crucial restriction, resulting that diffuser shape should be the shortest possible distance. In this context, the present work focuses on the numerical study of compressible subsonic flow in gas turbine annular diffusers under different Mach numbers and divergence angles. Mass, momentum and energy equations are discretized and solved employing the finite volume method. Furthermore, the turbulence effects are taken account using the realizable k- model with an enhanced wall treatment. Results showed that the annular diffuser performance is insensitive to Mach number when it is below 0.3. Between 0.3 and 0.6, the flow characteristics improve slightly because much of the pressure recovery occurs close to the diffuser inlet, thereby providing more opportunity for settling of the flow in the downstream portion of the diffuser. Above Mach number of 0.6, the pressure gradient near the inlet of a straight-walled diffuser becomes excessive and the performance begins to deteriorate. Moreover, large-scale separation can occur when the Mach number approaches 0.7, and the annular diffuser performance falls off dramatically.

Palavras-chave: annular diffuser; gas turbine; CFD Computational Fluid Dynamics