Gas turbine inlet temperatures have

Gas turbine inlet temperatures have been increasing over the years
due to positive economies of higher efficiency at higher firing temperatures.
In the 1960s, material properties limited gas turbine firing
temperatures and turbine blade temperatures to around 800C.
Modern firing temperatures are closer to 1500C though blades
(comprised of super-alloys) cannot be allowed to exceed temperatures
of 900C, to minimize the negative effects of thermal stresses.
This large differential in temperature between the hot gas and the
blade surface results in a very significant thermal load, as represented
in Fig.1. Advanced blade cooling technologies (for instance,
those shown in Figs.2 are instrumental in allowing for this
large thermal load. (Han et al. (2000)).
Advanced cooling technologies include blade internal cooling
(Fig.2(b)) and external cooling (Fig.2(a)). Secondary coolant air is
bled from the turbine compressor stage (typically at a much lower
temperature). This air is ducted at high Reynolds numbers through
cooling channels inside the blade/vane which are equipped with
rib turbulators to remove heat from the blade. Channels roughened
with pin-fins are used in the trailing edge region of the blade; the
heavily loaded leading edge region is cooled by jet-impingement
cooling.
Corresponding author. Email: jc-han@tamu.edu.
External cooling of the turbine blade is achieved by film cooling
and the provision of an insulating thermal barrier coating.
Film cooling involves the secondary fluid being discharged into
the hot mainstream through holes drilled into the internal cooling
passages. The goal is to form an insulating film reducing contact
of the blade material with the hot mainstream.
Since the thermodynamic cost of tapping air from the compressor
is high, a thorough understanding of the flow-field and
various parametric effects is of great value to the engine designer.
Characterizing film cooling using scaled models in the laboratory
has been an aggressive academic pursuit in the past few decades.
The two layer model (Fig.3) to analyze film cooling effectiveness
stipulates that the cooling air exiting the film cooling holes
forms an insulating layer of temperature Tf on the surface of the
blade. The heat transfer between the layer and the blade surface
is given by Eq.2. These parameters ( and h) are convenient to
measure in scaled down laboratory tests.
For a surface without film cooling, the heat load is
q00
o = ho(T1