| dc.description.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 (< 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 (> 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. In that sense, this work is not about a description of an automated optimization process, but rather about the physical principles that can guide such an optimization. From the CFD results and discussions, detailed guidelines to control the adverse pressure gradients (APG) on duct walls are tabulated. A geometry generation methodology which enables the design of curved annular diffusers based on the evolved guidelines, is discussed. An aggressive diffuser design space is identified with ducts of maximum slope of 50 degree and maximum divergence angle between the outer and inner walls of 10 degree for axial-length to inlet height ratio ranging from 1.25 to 2.5. Part of the identified design space for which the flow separation can be eliminated based on the evolved guidelines is demarcated. The need for flow control, possibly passive, is established for more aggressive designs.
The use of splitter blade as a passive flow control mechanism in the design of separation free aggressive annular diffuser is explored through CFD Simulations. The fundamental working principle of a splitter blade in case of a two-dimensional rectangular diffuser is argued. Using these arguments, the effects of a splitter blade and its configuration in an annular diffuser are discussed. Guidelines to choose the splitter blade configuration to control the APGs on the duct walls are provided. One or more splitter blades are employed in the duct to eliminate flow separation for all the ducts in the aggressive design space considered. Requirement of number of splitter blades in the aggressive design space is demarcated. Performance charts for the ducts in this aggressive design space are established and can be used during conceptual design of an aggressive annular diffuser.
The methods demonstrated in this work to eliminate flow separation on the duct walls were invoked in an attempt to design an open literature ITD with the reduced length. For this, ITD of Pratt & Whitney’s Energy Efficient Engine design (which is an open source geometry) was considered. Through the CFD simulations, an attempt was made to reduce the length of the duct up to 50% without having any flow separation on the duct walls. The performance parameters and the exit flow quality for the shorter ducts are presented, which could be helpful for taking trade-off decisions (trade-off between length reduction and performance drop). | en_US |
