Turbine cooling methods

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4.1-1 Introduction
Turbine cooling methods have provided the
ability to increase turbine inlet temperatures above
melting temperatures of turbine airfoil components with
not only airfoil survivability, but also extending airfoil
life. These cooling methods can be broadly classified into
internal and external methods. Internal cooling methods
include the use of geometric features placed in the flow
path of internal channels within the turbine airfoils to
promote turbulence, thereby enhancing convective heat
transfer coefficients. These geometric features generally
include ribs, pin fins, and impingement holes. External
cooling methods include the use of film-cooling holes
that are placed in the surface of the airfoils with the hole
shapes and hole placement being the design issue.
Because the flow fields across turbine vanes
and blades vary relative to the position on the airfoil,
one would expect that the cooling design would vary.
Consider that the flow at the airfoil mid span is primarily
two-dimensional while the flow at the airfoil edges is
clearly influenced by the inner hub and outer casings
of the turbine. The flows influencing the inner hub
and outer cases often contain vortices that give rise to
velocity components that are orthogonal to the primary
flow direction. Not only do the cooling schemes vary in
these regions, but the methods that are used to analyze
these various sections also vary. Because these cooling
schemes are relatively complex, the analysis methods
employed are not straightforward.
Section 4.1 is aimed at providing the reader
with methods that are currently used to analyze complex
turbine cooling schemes as well as a background for
understanding relevant effects on the different cooling
methods. Turbine airfoil geometries have also evolved
over the years to reduce pressure losses across each stage
resulting in three-dimensional airfoil designs. Section
4.2 provides the reader with an understanding of three
dimensional airfoil geometries.
- Karen Thole