This thesis describes experiments performed to investigate the heat transfer and aerodynamic aspects of tip leakage flow in an unshrouded axial turbine. Experiments were performed in a transonic 2-D tunnel and in a low speed 3-D cascade. The influence of varying a number of parameters influencing the tip flow has been studied and analysed both using standard aerodynamic measurement techniques and full surface heat transfer measurements employing thermochromic liquid crystal.
The heat transfer study utilised a mesh heater to generate the transient required for solution of Fouriers' 1-D conduction equation. To the authors' knowledge this is the first such study to employ this method for tip leakage flow investigations, and the strategies used in successfully implementing it have been detailed. An improved technique for transient heat transfer analysis has been developed and extensively investigated with regards to uncertainties and the controls that can be put in place to minimise them in the design phase of the experiment.
Experiments were performed on a number of cooled and uncooled geometries. Measurements in both transonic 2-D and low speed 3-D environments displayed similar salient features. For a plain tip the leakage flow is dominated by a vena contracta which is formed when the leakage flow separates off the pressure side corner into the tip gap. The separation reduces the leakage flow through the gap but upon reattachment to the tip surface generates the highest levels of heat transfer encountered on the blade.
Squealer and cavity geometries were designed and investigated and, for the profiles studied, there was found to be a trade off between the reduction of discharge through the tip gap and the reduction of heat transfer to the tip surface. Where as the suction side squealer profiles displayed the lowest heat transfer in both 2-D and 3-D experiments the cavity profiles yielded the lowest discharge coefficient.
Coolant configurations were designed to optimise delivery of the coolant to the regions which had indicated highest heat transfer based on the results of the uncooled tests. As such coolant holes were located so as to infiltrate the reattachment region. Coolant performance relative to that of the profiled squealer tip geometries was quantified by means of the Net Heat Flux Reduction (NHFR), a technique that compiles the heat transfer and film cooling effectiveness data and transposes the experimental measurements to engine conditions. Locating coolant holes inside the separation bubble with a sub unity blowing rate was found to reduce the heat flux to the tip by up to 37%.
|Date of Award
|30 Nov 2005
|Gary Lock (Supervisor)