Aerodynamic and heat transfer measurements in turbine tip leakage flow models
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
The heat transfer study utilised a mesh heater to generate the transient required for
solution of Fouriers' 1-0 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 venacontracta
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%