Understanding the heat transfer of winglet tips is crucial for their applications in high-pressure turbines. The current paper investigates the heat transfer performance of three different winglet-cavity tips in a transonic turbine cascade at a tip gap of 2.1% chord. A cavity tip is studied as the baseline case. The cascade operates at engine representative conditions of an exit Mach number of 1.2 and an exit Reynolds number of 1.7 × 106. Transient infrared thermography technique was used to obtain the tip distributions of heat transfer coefficient for different tips in the experiment. The CFD results were validated with the measured tip heat transfer coefficients, and then used to explain the flow physics related to heat transfer. It is found that on the pressure side winglet, the flow reattaches on the top winglet surface and results in high heat transfer coefficient. On the suction side winglet, the heat transfer coefficient is low near the blade leading edge but is higher from the midchord to the trailing edge. The suction side winglet pushes the tip leakage vortex further away from the blade suction surface and reduces the heat transfer coefficient from 85% to 96% span on the blade suction surface. However, the heat transfer coefficient is higher for the winglet tips from 96% span to the tip. This is because the tip leakage vortex attaches on the side surface of the suction side winglet and results in quite high heat transfer coefficient on the front protrusive part of the winglet. The effects of relative endwall motion between the blade tip and the casing were investigated by CFD method. The endwall motion has a significant effect on the flow physics within the tip gap and near-tip region in the blade passage, thus affects the heat transfer coefficient distributions. With relative endwall motion, a scraping vortex forms inside the tip gap and near the casing, and the cavity vortex gets closer to the pressure side squealer/winglet. The tip leakage vortex in the blade passage becomes closer to the blade suction surface, resulting in an increase of the heat transfer coefficient.

Winglet tips are promising candidates for future high-pressure turbine rotors. Many studies found that the design of the suction-side winglet is the key to the aerodynamic performance of a winglet tip, but there is no general agreement on the exact design philosophy. In this paper, a novel suction-side winglet design philosophy in a turbine cascade is introduced. The winglets are obtained based on the near-tip flow field of the datum tip geometry. The suction-side winglet aims to reduce the tip leakage flow particularly in the front part of the blade passage. It is found that on the casing endwall, the pressure increases in the area where the winglet is used. This reduces the tip leakage flow in the front part of the blade passage and the pitchwise pressure gradient on the endwall. As a result, the size of the tip leakage vortex reduces. A surprising observation is that the novel optimized winglet tip design eliminates the passage vortex and results in a further increasing of the efficiency. The tip leakage loss of the novel winglet tip is 18.1% lower than the datum cavity tip, with an increase of tip surface area by only 19.3%. The spanwise deflection of the winglet due to the centrifugal force is small. The tip heat load of the winglet tip is 17.5% higher than that of the cavity tip. Numerical simulation shows that in a turbine stage, this winglet tip increases the turbine stage efficiency by 0.9% mainly by eliminating the loss caused by the passage vortex at a tip gap size of 1.4% chord compared with a cavity tip.

The aerodynamic performance of a cavity-winglet tip is investigated in a high-pressure turbine cascade by experimental and numerical methods. The winglet tip has geometric features of a cavity and a suction side fore-part winglet. A cavity tip is studied as the baseline case. The aerodynamic performances of the two tips are investigated at three tip gaps of 0.8%, 1.7%, and 2.7% chord. At tip gaps of 1.7% and 2.7% chord, the loss near the blade tip is dominated by the tip leakage vortex (TLV) for both tips, and the winglet tip mainly reduces the loss generated by the tip leakage vortex. In the past, it was concerned that at a small tip gap, the winglet tip could introduce extra secondary loss and show little aerodynamic benefits. The winglet tip used in the current study is also found to be able to effectively reduce the loss at the smallest tip gap size of 0.8% chord. This is because at this small tip gap, the tip leakage vortex and the passage vortex (PV) appear simultaneously for the cavity tip. The winglet tip is able to reduce the pitchwise pressure gradient in the blade passage, which tends to suppress the formation of the passage vortex. The effects of the winglet tip on the flow physics and the loss mechanisms are explained in detail.

Winglet tips are promising candidates for future high pressure turbine rotors. Many studies found that the design of the suction side winglet is the key to the aerodynamic performance of a winglet tip, but there is no general agreement on the exact design method. In this paper, a novel suction side winglet design method will be introduced. The winglets are obtained based on the near tip flow field of the datum tip geometry. The suction side winglet aims to reduce the tip leakage flow particularly in the front part of the blade passage. It is found that on the casing endwall, the pressure increases in the area where the winglet is used. This reduces the tip leakage flow in the front part of the blade passage and the pitchwise pressure gradient on the endwall. As a result, the size of the tip leakage vortex reduces. A surprising observation is that the novel winglet tip design eliminates the scraping vortex and results in a further increasing of the efficiency. The tip leakage loss of the novel winglet tip is 23% lower than the datum cavity tip, with an increase of tip surface area by only 20%. The spanwise deflection of the winglet due to the centrifugal force is small. Numerical simulation shows that in a turbine stage, this winglet tip increases the turbine stage efficiency by 0.9% at a tip gap size of 1% span compared with a cavity tip.

The thermal performance of two cavity-winglet tips with endwall motion is investigated in a transonic high pressure turbine cascade, which operates at an engine representative exit Mach number of 1.2 and an exit Reynolds number of 1.7 × 106. The numerical method is first validated with experimental data and then used to investigate blade heat transfer at three different tip clearances of 1.1, 2.1 and 3.1% chord. The effects of relative endwall motion are considered. The present results show that as the size of the tip gap increases, the heat transfer coefficient and heat load on the tip increases. The winglet geometries on the blade tip mainly affect the tip flow structure close to them. At a larger tip clearance, the size of the separation bubble above the pressure side winglet increases. The heat transfer coefficient is high on the pressure side winglet due to the flow reattachment at all tip clearances. Within the tip gap, when the size of the tip clearance increases, the size of the cavity vortex increases and the cavity scraping vortex due to relative endwall motion becomes smaller. The impingement of the both two vortexes can lead to high heat transfer coefficient on the cavity floor surface. On the blade suction surface, when the size of the tip clearance increases, the heat transfer coefficient of the cavity tip increases, but those of the winglet tips decreases. The heat transfer coefficient is high on the side surface of the suction side winglet at all tip clearances because of the tip leakage flow impingement.