Handbook of Turbomachinery Second Edition Revised - Ventech!

Advanced large engines are clearly approaching the limit above which pure
film cooling is no longer sufficient. In the past, therefore, an iterative
solution was found in reducing the surface in need of cooling by shortening
the flame tube. This was made possible by the adoption of improved fuel
preparation systems. Over the years the ratio of the liner length to its height
dropped from about 4 for old engines to about 2 for new engines.
Cooling configurations are usually tested in model rigs, where the
cooling action is determined at defined constraints. When applying the
cooling configuration to real combustors, the exact constraints are
unfortunately not known. This applies in particular to constraints on the
hot gas side. The flow pattern and the local temperature in the flame tube,
familiarity with which is a prerequisite for determining the constraints,
cannot normally be measured. For this reason, these quantities must be
computed using 3D CFD computations for the combustor flow, where the
local release of heat is to be determined accurately enough to derive local
temperatures, velocities, and radiation load. Codes for this purpose already
exist, although the confidence in the results they provide is still limited.
However, the recent improvements in accuracy of the analytical predictions
suggest that future designs of combustor-cooling configurations should be
made in combination with CFD computations.
Transpiration Cooling
For the cooling air to be utilized much more effectively than in traditional
film-cooling-based designs, the single-layer perforated sheet must be
replaced with a multiple-layer sheet structure where the cooling air is
routed through a winding path between and through the layers. This
arrangement is referred to as ‘‘transpiration cooling.’’ This method
approaches an ideal wall-cooling system that can maintain the entire liner
at the maximum temperature of the material, avoiding cooler regions that
would represent a wasteful use of cooling air. The transpiration cooled liner
wall is constructed from a porous material that provides a large contact area
for heat transfer to the air passing through it. Because the pores are
uniformly dispersed over the surface of the wall, the tiny air jets emerging
from each pore rapidly form a protective layer of a relatively cold air over
the entire inner surface of the liner. While passing through the pores, the
cooling air removes a significant amount of heat from the wall. When this
combined convection–conduction wall heat transfer is coupled with the
protective layer of discharged film, the overall cooling effectiveness is
sufficient to counterbalance very high heat loads that include radiation from
the flame. This means that, in addition to acting as a porous medium, the
wall must also have good heat-transfer properties and be of adequate
Copyright © 2003 Marcel Dekker, Inc.