This investigation studies the leakage flows over the high pressure turbine blade tip at high speed flow conditions. There is an unavoidable gap between the un-shrouded blade tip and the engine casing in a turbine stage, where the pressure difference between the pressure and the suction surfaces of the blade gives rise to the development of leakage flows through this gap. These flows contribute to about one third of the aerodynamic losses in a turbine stage. In addition they expose the blade tip to a very high temperature and result in thermal damages which reduce the blade‟s operational life. Therefore any improvement on the tip design to reduce these flows has a significant impact on the engine‟s efficiency and turbine blade‟s operational life. At the engine operational condition, the leakage flows over the high pressure turbine blade tip are mostly transonic. On the other hand literature survey has shown that most of the studies on the tip leakage flows have been performed at low speed conditions and there are only a few experimental works on the transonic tip flows. This project aims to explore the tip leakage flows at high speed condition which is the real engine condition, both experimentally and computationally and establish a comprehensive understanding of these flows on different tip geometries. The effect of tip geometry was studied using the flat tip and the cavity tip models and the effect of in-service burnout on these two tip models was established using the radius-edge flat tip and the radius-edge cavity tip models. The experimental work was carried out in the transonic wind tunnel of Queen Mary University of London and the computational simulations were performed using RANS and URANS. As the flow approached each tip model it turned and accelerated around its leading edge in the same way as the flow turns around the leading edge of an aerofoil. In the case of the tip models with sharp edges the tip flow separated at the inlet to the tip gap. For the flat tip model the flow reattachment occurred further downstream whereas in the case of the cavity tip model the length of the pressure side rim was not sufficient for the reattachment to occur and the separated flow left the rim as a free shear layer. The cavity tip model was found to have a smaller effective tip gap and hence smaller discharge coefficient in comparison to the flat tip model. For the radius-edge tip models, no separation occurred at the inlet to the tip gap and the effective tip gap was found to be the same as the geometrical tip gap. Therefore it was concluded that the tip model with radius-edges had a larger effective tip gap and hence a greater discharge coefficient than the tip geometry with sharp edges. It was observed that in the case of the supersonic tip leakage flows, decreasing the pressure ratio PR (i.e. the ratio of the static pressure at the tip gap exit to the stagnation pressure at the inlet to the tip gap) increased the discharge coefficient Cd for the tip models with sharp edges but it decreased the Cd value in the case of the tip models with radius edges. The cavity tip model with sharp edges was found to have the smallest discharge coefficient and thus the best performance in reducing the tip leakage flows as compared to all the other tip models studied in this investigation.

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<p dir="ltr">This research presents a culmination of work into uncovering the underlying fluid dynamic behaviors of supercritical CO2 as it relates to high pressure turbine design using a combined fundamental and practical numerical and experimental analysis. The fundamental analysis of the thermo-fluid dynamic properties of supercritical CO2 boundary layers and separation is analyzed against the air counterparts for non-dimensional quantities of interest – pressure ratio, Mach number, Reynolds number – and combinations of these quantities. The coupling of density derivatives with pressure and temperature are investigated within the operating conditions of the first stage turbine of a supercritical CO2 oxyfuel power cycle. Armed with the information garnered from this analysis, a 3D optimization is run using computational fluid dynamics to investigated nearly 3000 unique blade shapes, focusing on increasing the isothermal corrected efficiency and decreasing the heat load to the blade. Three different families of blade shapes are identified from the analysis and their aerodynamic qualities discussed. A single advanced blade design is chosen for in depth analysis and experimental testing against the baseline blade from which the optimization was started. Mechanical design for the experimental campaign in the Big Rig for Aerothermal Stationary Turbine Analysis (BRASTA) is presented for a novel sector-based off-axis design and the results of the aerothermal measurements discussed. In tandem with blade design and analysis, the Tip Gap Experimental Research Article for Large Scale Injection Layouts (Tiger Lily), a canonical model for the large-scale investigation of tip flows in high Reynolds number flows, is developed and the mechanical and aerodynamic design discussed. Aerothermal analysis for different tip coolant injection configurations is performed using Improved Delayed Detached Eddy Simulation (IDDES) computational fluid dynamics analysis to resolve turbulent structures resulting from coolant injection and over tip flow interaction. Experimental investigation of Tiger Lily is presented, validating the structures and features seen in the numerical analysis. The conclusion of these investigations results in the increased understanding of the underlying fluid dynamic behaviors of supercritical CO2 in high pressure turbines.</p>