Handbook of Turbomachinery Second Edition Revised - Ventech!

holes. Cooling hole pattern is an important parameter in film cooling. In
general, decreasing the film injection angle increases the film effectiveness,
particularly when operating close to a blow-off situation. Compound angles
are used in some applications, especially where better spanwise film
coverage is required with large film hole spacing. Decreasing the hole
spacing increases the film effectiveness and also makes it more laterally
uniform. For practical situations it is recommended that film injection
angles be designed between 158 and 408 to the local airfoil tangent surface.
However, this rule does not apply to the leading-edge shower head holes
where the angle, by geometric necessity, will tend to be greater. In general
and where possible, hole spacing should be maintained at a ratio of between
3 and 4 times the hole diameter in order to provide an effective film.
Obviously, there may be other factors that will control the film hole spacing
such as the necessity to minimize flows where there is a high film hole
pressure ratio. A larger number of film hole rows requires smaller hole sizes
for a given cooling budget and might not be practical due to increased hole
blockage problem.
Blowing Ratio
In film cooling, the effectiveness can generally be correlated as a function of
the blowing ratio, as defined above, or the mass flux ratio. Increasing the
blowing ratio M up to a certain limit increases cooling effectiveness. Further
increase in the blowing ratio beyond this limit decreases film-cooling
effectiveness due to jet blow-off into the main stream. The effect of blow-off
is generally the strongest near the holes and weaker farther downstream.
The limiting value of the blowing ratio at a coolant density ratio rf=r ? ¼ 1
is between 0.4 and 0.6 for 358 injection angle. Recent studies on the effects of
density ratio, free-stream turbulence, unsteadiness, and surface roughness
show that these factors have a tendency to make the blowing ratio limit
higher. There is also a difference between concave and convex surfaces, the
limiting value being greater for the concave surface.
Early experimental work on film cooling was mostly conducted with
heated injection air, resulting in a density ratio equal to or less than unity. In
real engines, the density ratio is greater than 1.8 due to the large temperature
ratio between the gas stream and cooling air.
The coolant-to-mainstream density ratio is an important factor. As the
density ratio is increased, the film-cooling effectiveness is also increased. The
physics of the process is such that the heat transfer under the film is reduced
as the density ratio is increased for the same gas-to-coolant temperature
ratio.
Copyright © 2003 Marcel Dekker, Inc.