Aero-thermal characteristics and film cooling of transonic turbine blade tips
Keywords: heat transfer coefficient
CFD
pressure sensitive paint
film cooling
PSP
experimental
turbine blade tip
film effectiveness

Film cooling is essential to protect high pressure turbine blade tips from the high speed, high temperature overtip leakage flows, which are forced through the tip clearance gap. Careful design of the blade tip and cooling strategy is necessary to reduce the resulting aerodynamic inefficiencies, high thermal loads and tip degradation. Only with an in-depth knowledge of the overtip leakage flow and its interactions with film coolant can this be achieved, in order to improve the efficiency and prolong the in-service life of unshrouded turbine blade tips.
This thesis presents a combined experimental and computational study of cooled and uncooled tips at engine-representative aerodynamic conditions. A high speed linear cascade was employed for the experimental side, which was overhauled for improved measurement of film cooling effectiveness via implementation of the pressure sensitive paint technique. To the author's knowledge, this is the first time this technique has been used to measure film cooling effectiveness on blade tips in a high speed linear cascade.
The aero-thermal performance of a series of uncooled tips was tested experimentally and the study expanded on computationally. Tip heat transfer coefficient was shown to be determined by the local behaviour of the overtip leakage flow and exhibited a strong dependence on the tip geometry and tip clearance. At increased tip clearance, aerodynamic losses were enhanced, whereas the heat transfer coefficient reduced on the rims but increased in the tip cavity; a common behaviour for all tips tested due to the generalised flow field. Results indicated the relative advantage of using a winglet tip for aerodynamic performance and a suction side cut-back squealer tip for improved thermal performance at the tip trailing edge. The effects of casing motion were also investigated computationally. The casing motion induces a cavity vortex which increased the local heat transfer coefficient, however there was no effect on the pressure side rim.
The impact of film cooling on two transonic squealer tips was examined experimentally, showing a dependence of the film cooling effectiveness and heat transfer coefficient on both the tip clearance and coolant mass flow rate. To the author's knowledge this study is a first of its kind examining these factors. Coolant injection was shown to locally enhance tip heat transfer coefficients, however areas of high film effectiveness on the pressure side squealer rim coincide strongly with a net heat flux reduction and with low heat transfer coefficient due to blockage by the coolant. Comparison of the two squealer designs showed improved cooling performance by a suction side cut-back squealer due to careful placement and angling of the pressure side near-tip film holes. Numerical methods showed strong interactions of film coolant and the overtip leakage flow producing vortical flow structures. Higher film effectiveness levels were gained from the use of larger diameter dust holes due, in part, to the behaviour of the coolant jet.