<p>In large civil aero-gas turbine engines, the drive for improved efficiency, reduced fuel-burn and CO<sub>2</sub> emissions has led to engine architectures with very-high bypass ratios and compact ultra-high-pressure ratio cores. For the S-shaped ducts, connecting the compressor spools the challenge is to design for increased radius change whilst minimizing length/weight in order to reduce fuel burn. However, the aerodynamic flow in these ducts is highly complex and characterized by strong pressure gradients and streamline curvature effects. It is therefore difficult to rapidly assess aerodynamic loading in the early stages of the design process without the use of time-consuming high-fidelity CFD calculations. Indeed, no reliable knowledge-based design tools exist. Hence, the aim of the current work is to develop such a tool which can be used in a more agile, whole-engine design process to iterate to an improved duct solution before the engine geometry is fixed. This paper presents the foundations of the tool based on a novel rapid viscous-inviscid interaction method for predicting the complex flow field with S-shaped transition ducts. It is important that this is achieved rapidly enough for preliminary studies and exploration of the design space but sufficient accuracy for confidence in the design. Two-dimensional axisymmetric incompressible steady flows are considered in this semi-analytical iterative process. A potential core based on radial equilibrium with streamline curvature and implicit velocity profiles by spline reconstruction is coupled to an integral method reproducing the growth of turbulent boundary layers up to separation. A mixing length turbulence model upgraded for strongly curved flows is used to generate the shear stress distribution within the boundary layers. Finally, the wall shear stress closure is performed by inverse calculation of a composite law-of-the-wall accounting for the biased core velocity profile and the effects of the local pressure gradients. Despite the complexity of the flow in S-shaped ducts, the dominant aerodynamic phenomena are captured. The pressure recovery along the hub wall, utilised as loading parameter, is predicted with an accuracy of ±3% when compared to current CFD methods. Importantly, convergence using the new tool is achieved approximately 20 times faster than the CFD. Performance maps produced using this new method can then be fed into the next stage of the design process and used to inform detailed CFD models, better defined engine boundary conditions and final optimisation of the duct geometry.</p>

A Rapid Viscous-Inviscid Interaction Method for the Preliminary Design of S-Shaped Transition Ducts

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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