In a high bypass ratio aircraft engine, the high-pressure and low-pressure turbines are connected by an intermediate turbine duct (ITD). The aerodynamic performance of the ITD is affected by the incoming flow from the high-pressure turbine. This paper investigates the effects of an incoming wake or/and a near casing streamwise vortex on the flow field and loss of an ITD. For the case with only an incoming wake, the wake interacts with the boundary layer, forming a pair of vortices and causing additional loss. With an incoming streamwise vortex, the casing boundary layer interacts with it and a loss core forms near the casing. When both a wake and a streamwise vortex are present at inlet, apart from interacting with the boundary layer, the wake and the streamwise vortex could interact with each other. It is found that the distance between the wake and streamwise vortex has a major effect on the flow pattern and aerodynamic loss of ITD. Three different distances between the wake and the incoming streamwise vortex are investigated. When the distance between the wake and incoming streamwise vortex is large, the two flow structures develop relatively independently and the combined effect is small. As the distance between them reduces, the flow structure induced by the wake interacts with the incoming streamwise vortex, and suppresses the loss production. However, for the case with the shortest distance, the interaction enhances the loss generation. A simplified analytical model is proposed to explain this loss mechanism.

In high bypass ratio engines, the flow exits the inter-turbine duct and enters the low-pressure turbine. This paper aims to understand the effects of the boundary layer at the exit of inter-turbine duct on the endwall secondary flows and loss of the first blade row in a low-pressure turbine. From the Navier-Stokes equations, the loss is decomposed into the parts generated by the mean vortex as well as turbulence theoretically. The result of computational fluid dynamics shows that the incoming boundary layer from the inter-turbine duct increases the total pressure loss coefficient by 14% compared to the case with uniform inlet condition. Although the distribution of the secondary vortices is strongly affected by the inlet boundary layer, the loss generated by the mean vortex within the blade passage is hardly affected. The analysis based on the turbulent dissipation shows that the dominant factor leading to the loss increase is the turbulent dissipation downstream of the blade trailing edge near the hub. The mixing process of the wake and the strong counter-rotating vortex pair increases the turbulent dissipation significantly. It is also found that a simplified incoming boundary layer defined by the Prandtl’s one-seventh power law can not reproduce the complex effects of the incoming boundary layer from the inter-turbine duct.

The integration design of the intermediate turbine ducts (ITDs) with the first row of the low-pressure turbine vane can significantly reduce the length of the turbine section, thus reducing the weight and drag of the aeroengine. This paper investigates the effects of the hub profile on the aerodynamic performance of integrated ITDs (IITDs). The flow features and loss mechanism of four IITDs are studied by experimental, numerical and theoretical methods. In the baseline case, an open corner separation occurs near the hub-suction surface corner, which results in a significant loss. The loss is broken down into the parts generated by the mean vortex and turbulence theoretically. The open corner separation causes significant turbulence loss. To reduce the size of the separation zone, the positive radial/spanwise pressure gradient near hub is increased by moving the hub profile near the vane rear part slightly downward. As a result, a small closed corner separation with three-dimensional topology occurs instead of the open corner separation in the baseline case. The corner shape factor is defined to quantitatively describe the closed corner separation. When the hub profile moves further downward, the loss due to the corner separation reduces, but the loss generated in the vane passage away from hub increases mainly due to the mixing as the low-momentum flow near the hub transports towards the mid span. The change of the overall loss is subject to the combination of the two effects, and should be balanced during the design process.