Thermal and aerodynamic investigation of innovative cooling designs of blade tips
Keywords: slot
transonic
slots
high-pressure
tip
shelf
rotor
cooling holes
shelves
cooling
welding beads

The blade tip is a particularly critical part of high-pressure turbine blades, as the hot gas leaking over it causes a loss of efficiency and degrades the blade. This tip degradation is often a limiting factor of engine life. For squealer designs, the pressure side rim near the trailing edge is particularly vulnerable due to its high surface area and highly complex local flow patterns, whilst also being a difficult region to cool. This thesis investigated the aerothermal performance of shroudless high-pressire tip geometries, at engine representative Mach and Reynolds numbers. The testing capability, accuracy and throughput of the Oxford High Speed Linear Cascade were improved by implementing a 4-hole fast-settling aerodynamic probe, an inlet traverse and dual component pressure sensitive paint, used to calculate temperature-corrected film cooling effectiveness distributions. Surveys were realised on the impact of tip clearance and coolant mass flow rate on the thermal performance of squealer tip geometries. Larger tip sizes were shown to increase the mass flow of overtip leakage, which negatively affected cooling, increased the aerodynamic losses and augmented the heat transfer in regions of flow impingement. Conversely, coolant jets shield the tip surfaces from the impingement of leakage flow, locally decreasing heat transfer coefficients, whilst also increasing mixing in the regions downstream of the cooling holes.
The aerothermal impact of cavity welding beads on squealer tips was examined. These real geometry features had negligible aerodynamic impact at cascade exit, but significantly affected the heat transfer at the rims and cavity of the tip. Overall, the beads acted as turbulators on the cavity floor and increased blockage by locally decreasing cavity depth. These factors led to the acceleration of both the overtip leakage over the pressure side rim, as well as of the cavity flow, which was beneficial to the adhesion of the coolant jets to the surfaces, but also raised heat transfer coefficients over the majority of the tip.
A novel pressure side cooling feature, consisting of an inclined slot inside a recessed step, was developed to enhance trailing edge tip cooling. A combined numerical and experimental study demonstrated that the concept had promising cooling performance, with improvements of upwards of 45\% over conventional cooling strategies. A large breadth of design parameters were examined, including slot inclinations, coolant ejection area, sizing of the recessed shelf and overall positioning. Key findings were that direct flow paths with large overlap with the volume of the recessed shelf are beneficial for the overall rim coverage and that the slot blowing ratio can be managed to either achieve broader coverage or larger coolant density. The highest performing geometry was a dual slot configuration, with two slots working in tandem as a single cooling solution, which achieved uninterrupted coverage of the target region of the pressure side rim.