7 Detailed Airfoil Design for Axial-Flow Turbines

The basic airfoil geometry is established by the preliminary design procedure of chapter 6 to match the desired inlet and exit velocity triangles selected for the blade row. Some refinement may be appropriate, based on the more precise flow profile prediction capability of the performance analysis of chapter 5. The selection of some airfoil characteristics will be strongly influenced by the need to achieve adequate mechanical integrity for the intended application. Leading and trailing edge radii, blade thickness distributions, blade taper in the radial direction, etc. are all strongly influenced and possibly dictated by mechanical integrity issues. Many of those issues cannot be fully qualified until the airfoil is designed. Hence, detailed airfoil design is really a collaborative effort between the aerodynamic and mechanical designers. This chapter describes techniques to define airfoils that provide the desired blade characteristics. But it should be recognized that adjustment of some of those characteristics is to be expected as the detailed airfoil design progresses.

Detailed design of axial-flow turbine airfoils involves defining a smooth profile shape that satisfies the basic constraints imposed by the velocity triangles, namely the blade camberline inlet angle, β1, and the throat-to-pitch ratio, o∕s. Assumed leading and trailing edge geometry reduces the problem to connecting a series of defining points while matching slopes at those points. Airfoil shapes may be defined by simple circular-arc segments, e.g., Fielding [69], spline-connected polynomial segments, e.g., Ye [72], or a combination of both, e.g., Pritchard [73]. The method described in this chapter is quite similar to Ye's method but with several modifications and extensions to make it more general and much easier to use.