Handbook of Turbomachinery Second Edition Revised - Ventech!

flow metering with good mechanical strength, has been widely adopted in
one form or another.
Modern cooling techniques include angled effusion cooling (EC) using
multiple rows of small holes drilled through the liner wall at a shallow angle
to its surface. With this scheme, the cooling air flows through the liner wall,
first removing heat from the wall by convection–conduction, and then
providing a thermal film barrier between the wall and the hot combustion
gases. EC is presently considered to be the most promising option among
advanced combustor-cooling techniques that are being actively developed
for the new generation of industrial and aero engines. For some advanced
aero engines, it has reduced the conventional cooling air requirement by
30%. The main drawback of EC is an increase in liner weight of around
20%, which stems from the need for a thicker wall to achieve the required
hole length and provide buckling strength.
A common alternative to increasing the efficiency of cooling
techniques is to spray protective and thermal barrier coatings on the inner
liner wall. As it has for the past 60 years, the search continues for new liner
materials that will allow operation at higher temperatures. Current
production liners are typically fabricated from nickel-based alloys such as
Haynes 230.
Application of combined impingement-film or impingement-effusion
techniques are often considered when a higher cooling effectiveness is
required. These techniques require a double-walled liner design where the
outer (in relation to the gas path) wall in the double-walled region is
perforated. The advantage of the method derives from its use of cooling
air to serve a dual purpose. First, the air is shaped into multiple small jets,
which provide impingement cooling to the front (primary zone) section of
the liner wall, and then the jets merge to form an annular sheet, which
operates in a conventional film-cooling mode to cool a downstream
section of the inner liner wall. Another advantage of impingement cooling
is that the impingement jets can be positioned to provide extra cooling on
liner hot spots. The higher cooling effectiveness of these techniques comes
with certain penalties in terms of cost, weight, and higher pressure losses
that affect overall engine efficiency. Another concern stems from the
significant difference in temperature between the two walls, leading to a
differential expansion that might result in buckling of the inner wall if the
local hot spots become too severe. Also, the high heat-transfer coefficients
that are normally associated with impingement cooling cannot be realized
fully in the downstream section, because the film of air discharged from
the upstream protects the downstream section, reducing the inner wall
metal temperature and thus lowering the effectiveness of impingement
cooling.
Copyright © 2003 Marcel Dekker, Inc.