Design of Curved Aggressive Annular Diffusers

Abstract

For economical air transportation, aero engines with lower specific fuel consumption, lower emissions and lower noise levels are required. To this end, the use of turbofan engine with high bypass ratio is an option. With the increase in bypass ratio of a turbofan engine, the flow path of the inter turbine duct (ITD) which connects the high pressure turbine and low pressure turbine, becomes more aggressive in terms of slope and curvature distribution. Further, ITD offer the potential advantage of reducing the flow coefficient (with the increased area ratio of the duct) in the following stages, leading to increased efficiency. Together with the higher duct wall slopes and increased area ratio, ITD becomes highly prone for flow separation. In general, ITD can be represented by an annular curved diffuser and during its conceptual design, classical Sovran and Klomp (SK) framework is employed. In SK ducts framework, ducts are much less aggressive in terms of wall angles (lt 20 deg), with straight wall ducts, tested in incompressible flow regimes. On the other hand, the ITD flows are known to be compressible involving curved walls at high angles (gt 30 deg). Therefore, it is immoderate to use SK s performance charts for conceptual design of the modern high slope ducts. Hence, it is required to establish the performance charts with compressible flows, curved walls and high wall angles. Having certain guidelines to design the ITD with better performance while avoiding the flow separation would be helpful during the conceptual design. Attempts are made in this work to address these aspects. Influence of curvature distribution and area-ratio distribution on the pressure fields within the curved annular diffuser are discussed through heuristic arguments. Further, these arguments were demonstrated through Computational Fluid Dynamics (CFD) simulations. The approach presented here, deals with the sensitivity of the duct performance parameters to duct wall modifications...