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Complex interactions between Over-Tip Leakage (OTL) flow and cooling injection have been the focus in turbomachinery. In the present work, a numerical investigation was carried out for such interactions between the transonic OTL flow and cooling injection with the quasithree-dimensional (Quasi-3D) models employed. The results show that, for the flat tip, the nondimensional scale of separation bubble is pretty much constant. For the squealer tip, the interaction between cooling injectio n and OTL forms a counter-rotating vortex pair (CRVP), and the thermal stripes on the suction side (SS) rim are mainly caused by the interaction between OTL and CRVP. For the given cooled flat and squealer tips, the flow was blocked by cooling injection, and which may reduce the overall OTL mass flux. Further research revealed that this blocking effect was related to the cooling hole location and orientation. Threedimensional numerical simulations provide further validation for those conclusions. INTRODUCTION In modern turbomachinery, the blade tip of the highpressure turbine (HPT) becomes one of the most vulnerable areas due to the high thermal load. The flow passing through the tip clearance between the blade tip and the casing also leads to the flow loss, namely, the OTL flow loss. It decreases the turbine efficiency. Therefore, heat transfer and aerodynamics need to be comprehensively considered to design qualified blade tips of HPT. Experimental studies have been extensively carried out to investigate the heat transfer and aerodynamic performance on the tip regions. Detailed heat transfer coefficient and static pressure distributions on different tip geometries were measured by Azad et al. (2000a, 2000b) by utilizing the transient liquid crystal technique. Kwak et al. (2003), Kwak and Han (2002a, 2002b) analysed the effects of tip gap heights, turbulence intensity and Reynolds number on the heat transfer in the tip regions of both flat and squealer tips. Christophel et al. (2005a, 2005b) experimentally investigated the effect of the pressure side (PS) blowing on the heat transfer coefficients and the adiabatic effectiveness levels. Using the 2D-PIV technique, Hu et al. (2014) reported that a good leakage control effect only happened on the condition of a small tip gap and the medium blade load. Zhang et al. (2011) showed that the heat transfer over the turbine blade tip is greatly influenced by the shock wave structure within the tip gap. After multiple reflections of shock waves, the supersonic flow finally ended with a normal shock. A strong interaction between heat transfer and aerodynamics for transonic turbine blade tips was discussed by Zhang and He (2014) as well. Hofer and Arts (2009) found that the effect of cooling on the overall mass-weighted loss coefficient was marginal for the double squealer geometries investigated in their transonic experimental study. Lots of numerical calculations have also been conducted to study the aerothermal characteristics. The effects of different tip geometries, including flat tip, double squealer tip, and suction side squealer tip, were numerically studied by Krishnababu et al. (2007). Their results illustrated the OTL mass flow rate and heat transfer to the tip increased with the increase of tip gap height. The effects of relative casing motion and cooing injection on the flow and heat transfer performances of the OTL flow were investigated by Krishnababu (2007) and Newton et. al. (2009) as well. Similar researches were conducted by Yang et al. (2006, 2013). The simulation results showed that the heat transfer weakened with increasing cavity depth, and the shallow cavity is the most effective tip geometry to decrease the overall heat load. The effects of tip gap clearance and casing recess on the heat transfer and the stage efficiency for many

Aerothermal Investigation of Transonic Over-Tip Leakage Flow with Cooling Injection for Different Tip Geometries