Handbook of Turbomachinery Second Edition Revised - Ventech!

thereby increasing both the skin-friction and heat-transfer coefficients. The
application of a layer of thermal barrier coating also results in thicker
trailing edges, with associated aerodynamic penalties. At given internal heattransfer
coefficient and temperature of the coolant, the temperature gradient
across the thickness of the coating is driven primarily by the gas temperature
and external heat-transfer coefficient. The greatest benefit from application
of TBC or the highest temperature gradient through the coating can be
gained in the environment where the large temperature difference between
hot gas and cooling air is combined with high heat-transfer coefficients on
both sides. This environment exists often in certain areas of airfoils, end
walls, and combustor liners that are cooled by backside convection. The
greatest concern affecting the use of thermal barrier coatings is their life,
caused by their brittleness and flaking due to differential thermal expansion
relative to the airfoil base material. Recent advancements in materials for
TBC and improved coating application techniques have provided significant
extension of their life. This allows a lesser conservatism in life prediction for
coated nozzles and blades, including thermal resistance of the coating in the
component temperature analysis. Selection of the coating thickness for
blades should also consider the fact that coating strength is very low relative
to the base alloy causing increased inertial (centrifugal) stresses in the blade
material. Design of airfoils with thermal barrier coatings must obviously
weigh all of these factors.
Airfoil Cooling Development Process
Figure 7presents atypical development process for airfoil cooling. The
starting point for the airfoil cooling system design specifies the component
life requirements, which dictate an acceptable metal temperature limit.
Local metal temperature of the airfoil is driven mainly by a combination of
three factors:
1. External heat load to the surface from the main-stream gas by
convection through a boundary layer or through a film cooling
layer (when cooling air is discharged through the film holes)
2. Internal convective cooling of the part by cooling air
3. Thermal conduction and heat sink within the component
A preliminary turbine aerodynamic design provides the airfoil geometry,
estimates cooling air flows, and specifies pressures, temperatures, and
velocities of the main-stream flow. Previous experience is essential for
correct assumptions in the analysis and cooling concept selection.
Recent advancements in computer-based analytical methods, and
particularly in computer graphics, provide sophisticated tools for con-
Copyright © 2003 Marcel Dekker, Inc.