Handbook of Turbomachinery Second Edition Revised - Ventech!

thickness. A problem this poses is that, in order to form a stable boundary
layer on the inner surface of the wall, the coolant flow should emerge with as
low a velocity as possible, whereas for maximum heat transfer within the
wall a high velocity is required. Although transpiration cooling is potentially
the most efficient method of liner cooling, its practical implementation has
been very limited due to the availability of required porous materials. The
porous materials developed to date have failed to demonstrate the required
tolerance to oxidation, which has led to the small passages becoming
blocked. These passages are also sensitive to blockage by foreign particles in
the air.
Effusion Cooling
The simplest approach to a practical form of transpiration cooling is a wall
perforated by a large number of small holes. Ideally, the holes should be
large enough to remain free from blockage by impurities, but small enough
to prevent excessive penetration of the air jets into the main stream.
Provided that the jet penetration is small, it is possible to produce along the
inner surface of the liner a fairly uniform film of cooling air. If, however, the
penetration is too high, the air jets rapidly mix with the hot gases and
provide little cooling of the wall downstream. Effusion cooling can be
applied to all or any portion of the liner wall, but due to the high rate of
cooling flow required it is best used for treating local hot spots in the liner
wall. Another useful role of effusion cooling is in improving the effectiveness
of a conventional film-cooling slot. As the film of air from this slot moves
downstream, its temperature gradually rises due to entrainment of the
surrounding combustion gases. Eventually, it becomes so hot that it starts to
heat the liner wall instead of cooling it. If effusion cooling is applied before
this point is reached, the injection of cold air into the film enables it to
maintain its cooling effectiveness for a longer distance downstream.
In conventional effusion cooling, the holes are drilled normal to the
liner wall. The advantages to be gained from angled effusion cooling with
the holes drilled at a shallower angle are twofold:
1. An increase in the internal surface area available for heat removal.
This area is inversely proportional to the square of the hole diameter and the
sine of the hole angle. Thus, for example, a hole drilled at 208 to the liner wall
has almost three times the surface area of a hole drilled normal to the wall.
2. Jets emerging from the wall at a shallow angle have low
penetration and are better able to form a film along the surface of the
wall. The cooling effectiveness of this film also improves as the hole size and
angle are decreased.
Copyright © 2003 Marcel Dekker, Inc.