Handbook of Turbomachinery Second Edition Revised - Ventech!

unacceptable engine performance penalties. Among the various heattransfer
augmentation techniques that might be attractive for this
application is a surface with periodic concavities often called dimples,
which has been recently introduced to the industry. Application of the
dimpled back-side-cooled surface, when optimized for certain geometry,
showed significant improvement in heat transfer at remarkably small
pressure loss (Moon et al.) [117]. Fabrication of a liner wall that is smooth
on the gas side and dimpled on the cold side can result in certain
manufacturing challenges, however. Combination of this cooling method
with a thermal barrier coating (see following section) might provide
promising design alternatives to more complicated low-emission combustor
systems.
Thermal Barrier Coatings
One attractive approach to the problem of achieving satisfactory liner life is
to coat the inside of the liner with a thin layer of a very low thermal
conductivity material, which is often called thermal barrier coating (TBC).
A suitable material of low emissivity and low thermal conductivity could
reduce the wall temperature in two ways: by reflecting a significant portion
of the radiation heat flux from the flame and by providing a layer of thermal
insulation between the hot gas and the wall of a base metal. The steep
temperature drop through the TBC varies with the thermal conductivity and
thickness of the layer, and the heat flux through the layer that is greatly
affected by the heat transfer from the hot gas and to the cooling air. If the
TBC coated wall is not provided with sufficient back-side cooling, the
barrier helps little in lowering the temperature. A further benefit may be
gained if an oxidation-resistant base coat is applied, because it reduces the
oxidation constraint on the choice of liner-wall material.
An ideal TBC would be chemically inert and have good mechanical
strength, resilience to thermal shock, and resistance to wear and erosion.
Above all, it would have a low thermal conductivity and a thermal
expansion coefficient that is similar to that of the base metal. A typical
thermal spray-deposited TBC comprises a metallic base coat (such as
0.1 mm of Ni Cr AL Y), plus one or two layers of ceramic (such as yttriumstabilized
zirconium oxide ZrO2).
Recent developments in the strain tolerance of the TBC have reduced
the necessity for an intermediate coat, and two-layer coatings are now
sometimes specified for improved mechanical integrity.
Plasma flame spraying is often used to apply the ceramic and base-coat
layers because it is found to provide durable and reproducible coatings. A
typical overall coating thickness is around 0.4 to 0.5 mm, which gives metal
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