| Summary: | The leading edge region of gas turbine blades and vanes experiences high thermal and mechanical
stresses and has to be properly cooled. External cooling of the leading edge region
is typically achieved by a film cooling technique. An investigation into the film cooling effectiveness
of three different large scale leading edge geometries is presented in this study. One
of the geometries investigated represents an original design and is an example of an improved
film cooling layout. All geometries used have four rows of cooling holes placed symmetrically
about the geometrical leading edge, but the layout of the cooling holes is different from one
leading edge geometry to another. A broad range of variables is considered including mass
flow ratio, coolant density, and jet Reynolds number. Film cooling effectiveness measurements
were made in a low speed wind tunnel environment using a flame ionization detector
technique and the mass/heat transfer analogy. These measurements significantly extend the
insight into the effects of hole geometry on the film cooling characteristics of the leading
edge of turbine blades and provide new data for design purposes.
The effect of geometry is more important for the case of double row injection where
spanwise-averaged film cooling effectiveness is improved by the use of compound angle holes.
The spanwise-averaged film cooling effectiveness is higher at lower mass flow ratios and
decreases typically as the mass flow ratio increases. At higher mass flow ratios, the newly
designed leading edge geometry produces higher spanwise-averaged film cooling effectiveness
than the other two geometries investigated thus providing the necessary backflow margin at
operating conditions more relevant to gas turbine use.
For the case of single row injection, the effects of geometry scale reasonably well when the local mass flow ratio is used in the analysis of the spanwise-averaged film cooling effectiveness
immediately downstream of the injection holes. The local momentum flux ratio is a more
appropriate scaling parameter when coolants with different densities are used.
A film cooling effectiveness correlation was also developed for one of the geometries
investigated based on an area-averaged film cooling effectiveness and on a newly defined
blowing parameter. This correlation accounts implicitly for the particular geometrical layout
used and explicitly for the main injection parameters investigated. The results can be now
more directly used in existing design procedures.
A new experimental technique based on wide-band liquid crystal thermography and transient
one-dimensional heat conduction has been developed and implemented. The technique
combines a real-time, true colour imaging system with the use of a wide-band liquid crystal
and multiple event sampling for the simultaneous determination of the film cooling effectiveness
and heat transfer coefficient from one transient test. A comparison of different image
capture techniques is also presented and computer codes are developed for data processing.
For a test case of compound angle square jets in a crossflow, very good agreement was
obtained between the film cooling effectiveness calculated from the transient heat transfer
experiments and the film cooling effectiveness measured in isothermal mass transfer experiments
using a flame ionization detector technique. This new approach has been developed
as a major part of this thesis and represents a significant contribution to the use of liquid
crystal thermography in film cooling applications.
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