A detailed experimental and numerical investigation of the effect of leading-edge fillet on the endwall secondary flow was performed in a highly loaded low-pressure turbine cascade. The objective is to gain further understanding on the reduction mechanism of endwall secondary flow losses by leading-edge fillet. The results suggest that the numerical results offer a reliable prediction for the endwall secondary flow structure together with suction surface separation bubble. A pressure-side corner separation vortex is found in the front part of pressure surface/endwall junction, where an inverse flow goes upward and over the leading edge to suction side of the same blade. This inverse flow also takes part in the interaction between endwall secondary flow and separation bubble. However, the leading-edge fillet can fill the region of corner separation vortex, and the inverse flow is eliminated. Combined with the reduction of cross-pressure gradient achieved by this fillet, the loss coefficient and flow angle deviation at exit plane are decreased. Besides, the leading-edge fillet will reduce the intensity of leading-edge horseshoe vortex and thus decrease the loss of passage vortex and shed vortex. It is these three aspects introduced by leading-edge fillet that achieves an effective reduction of the endwall secondary loss in turbine.

In modern turbine rotors, tip-leakage flow is a common phenomenon that accounts for about 1/3 of the stage loss. Studies show that as the imposed load increases, a shock wave appears in the tip region, which causes a significant interference on the leakage vortex. In the present study, numerical simulations are carried out to investigate the influence of the shock wave on the loss and breakdown of the tip-leakage vortex. The obtained results indicate that with no effective control on the flow, the loss of the leakage vortex has an approximate exponential growth up to about 10 times as the outlet Mach number increases from 0.67 to 1.15 and the corresponding proportion in the total loss increases sharply to 30.2%. It is found that the stagnation position of the breakdown changes with the backpressure and the amplitude of variation along the axial direction is up to 0.13 C_x. It is inferred that the breakdown of the leakage vortex core may be affected by the periodical passing of downstream blade and the induced pressure fluctuation may result in additional vibration in this rotor blade. The leakage vortex is unstable in supersonic flow with a shock wave and it may transfer to a flow with a low-velocity bubble in its core region. It is concluded that the leakage vortex breakdown mainly originates from interferences of the shock wave, while the internal cause of such breakdown is the centrifugal instability of the vortex.

In order to reduce the losses caused by tip-leakage flow, axisymmetric contouring is applied to the casing of a two-stage unshrouded high pressure turbine (HPT) of aero-engine in this paper. This investigation focuses on the effects of contoured axisymmetric-casing on the blade tip-leakage flow. While the size of tip clearance remains the same as the original design, the rotor casing and the blade tip are obtained with the same contoured arc shape. Numerical calculation results show that a promotion of 0.14% to the overall efficiency is achieved. Detailed analysis indicates that it reduces the entropy generation rate caused by the complex vortex structure in the rotor tip region, especially in the tip-leakage vortex. The low velocity region in the leading edge (LE) part of the tip gap is enlarged and the pressure side/tip junction separation bubble extends much further away from the leading edge in the clearance. So the blocking effect of pressure side/tip junction separation bubble on clearance flow prevents more flow on the tip pressure side from leaking to the suction side, which results in weaker leakage vortex and less associated losses.

As modern turbines are designed with low aspect ratio and high blade loading, secondary flow interactions become more important. In the present work, numerical simulation is performed in a two-stage high-pressure turbine with divergent meridional passage to investigate the transport and interaction of secondary vortex from the first stage rotor within the second stage’s stator. Scale-Adaptive Simulation model coupled with Shear Stress Transport model (SAS-SST turbulence model) is used to capture the flow structures caused by the interaction in the second stator. Coupled with the passage vortex of the first rotor, the shed vortex rotates opposite in the direction and has comparable strength. As both of these vortices convect downstream to the stator bladerow, each deforms into two legs on the pressure and suction sides in the passage. In the passage due to the cross pressure gradient by blade loading, all the low-momentum fluid contained in these vortices moves towards the suction side. Besides, with the existing static pressure gradient in radial direction and vortex dynamics, the suction-side leg and the pressure-side leg move in different radial directions. The suction side leg of incoming passage vortex moves towards the endwall along the suction surface and interacts with the developing passage vortex of the second stator. The incoming shed vortex moves towards the midspan and rolls up the boundary layer fluid from suction surface. Due to the interactions between the incoming shed vortices from the hub and casing and the boundary layer of second stator, two counter-rotating vortices are formed near the midspan. Additional high loss is found there at the outlet plane, which has a comparable magnitude to the endwall secondary loss. The pressure side leg of the incoming passage vortex remains in a certain span with that of the incoming shed vortex and is not engulfed by the developing passage vortex.