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Demands for high efficient and environmental friendly aircrafts drive civil engines towards higher bypass ratios and smaller axial length. Both of these lead to great increase in the mean rise angle of inter-turbine ducts (ITDs), which determines the high curvature and high diffusion rate of its internal flow fields. In order to reduce the flow loss of aggressive ITDs, it is necessary to study the complex flow mechanism in ITDs, explore the factors influencing the flow and performance in ITDs, and provide technical supports to develop advanced turbine design technologies for highbypass turbofan engines. In this paper, numerical simulations of a typical aggressive ITD with struts between high and low pressure turbines of a large civil engine are carried out, and a method for comprehensively evaluating the aerodynamic performance of ITDs considering compressibility, area ratio, outlet flow angle is proposed. With this method, the influence of geometric parameters on ITDs flow structure and performance is discussed. The study shows that the area ratio has important influence on the flow field inside the ITDs, and the optimization of area distribution can improve the aerodynamic performance of ITDs. Finally, the influence of inlet aerodynamic conditions such as inlet Mach number and turbulence intensity on flow structure and performance of ITDs is analysed. INTRODUCTION The inter-turbine duct (ITD) is often used to connect HP (high-pressure) and LP (low-pressure) turbines in two-spool turbofan engines. In order to shorten axial length to reduce weight and increase bypass ratio to pursue efficiency, more aggressive ITDs with larger mean rise angles are in burning demand. Owing to the geometry of high curvature and large mean rise angle, the flow has a risk of significant separation which limiting the aerodynamic performance of ITDs. Thus, it is important to study the complex flow mechanism in ITDs. Work on traditional ITDs can trace back to (Sovran and Klomp, 1967), which proposed the reference sample table S&K diagram with great engineering value for the initial design of ITDs. They carried out detailed experimental research on three types of linear diffusers, and uncovered the relationship between the inlet-to-outlet area ratio and length to height ratio in order to maximum the static pressure lift. However, (Couey et al., 2010) pointed out that the design scope of ITD today is quite different from the experimental data of (Sovran and Klomp, 1967), and supplemented the influence of mean rise angle on aggressive ITD performance. Extensive researches have been made to reveal the physical mechanism of flow in ITD. The effects of mean rise angle, area rate, structural struts, upstream swirls and wakes have been particularly discussed. (Dominy et al., 1995, 1996, 1998) studied the influence of upstream swirl and wake on flow in ITD channel through experiment and numerical simulation. The results show that the upstream swirl and wake can significantly change the flow structure and the development of secondary flow in ITD channel, although they have little effect on the static pressure coefficient. (Hu et al., 2011; Zhang et al., 2011) studied the influence of inlet swirl distribution on the flow development, finding that the flow loss distribution inside ITD is closely related to the counter-rotating vortex pairs in both shroud and hub regions. In recent research, (Zhang et al., 2018) studied the effects of mean rise angle and area ratio on ITD aerodynamic performance by testing four ITD configurations by varying outlet-to-inlet area ratios and mean rise angles. They found that the adverse pressure gradient near the first bend increased with the increasing of mean rise angle, causing a

Effects of Geometric Parameters and Inflow Conditions on Flow in Aggressive Inter-Turbine Ducts