Handbook of Turbomachinery Second Edition Revised - Ventech!

P? is total inlet pressure.
Tc=T? is local mixing cooling layer and main-stream temperatures
ratio.
mc=m? is the coolant to total main-stream mass flows ratio.
a is the angle of injection.
The loss in total pressure of the main-stream flow due to injection decreases
with decreasing angle of injection is directly proportional to the local mainstream
Mach number squared and is strongly influenced by the blowing
rate. When the injection angle decreases, the effect of coolant-to-mainstream
temperature ratio Tc=T? becomes less pronounced for highertemperature
ratios and more evident for lower ratios. Limited test data have
shown this loss prediction to be reasonable and consistent with test results.
The important end result of this method is that it gives a certain guidance in
the optimum location of film cooling holes. It is apparent that film cooling
near the stagnation point and on the pressure surface of airfoils (where the
Mach numbers are low) will give small total pressure losses, whereas film
cooling on the suction side near the throat (where Mach numbers are high)
will give high total pressure losses.
It is quite often argued that the trailing-edge ejection of the coolant
flow might reduce the aerodynamic losses by filling in the wake. Several
investigations showed a positive effect of cooling discharge into the trailingedge
wake, especially when significant amount of high momentum flux
cooling flow had been discharged. However, due to manufacturing and cost
considerations, most of the advanced airfoils are designed with air
discharged on the pressure side immediately upstream of the trailing edge.
Significant attention over the last few years was paid to the penalties
associated with end-wall film cooling. Some of the studies have indicated
that when film is properly introduced upstream of the stagnation plane of
the airfoils, it might control the secondary flow formation such as a
horseshoe vortex initiated at the fillet junction between an airfoil and the
end wall (more details about end-wall cooling and secondary flow control
can be found later in this chapter).
An important role in cooling penalties is also played by the cooling air
that cools turbine discs and buffers disc cavities from hot gas ingress. The
amount and manner of discharging this air into the main stream can
significantly affect the performance losses. A recently performed study [11]
indicates that the most beneficial location and direction for discharging the
air from disc cavity is an immediate vicinity of the vane trailing edges near
the inner end wall at a velocity vector that closely matches the nozzle exit
velocity.
Copyright © 2003 Marcel Dekker, Inc.