9.5 Axial-Flow Turbine Analysis

The mass flow of a gas turbine engine, which is limited by the maximum permissible Mach number entering the compressor, is generally large enough to require an axial turbine (even for centrifugal compressors). The axial turbine is essentially the reverse of the axial compressor except for one essential difference: the turbine flow operates under a favorable pressure gradient. This permits greater angular changes, greater pressure changes, greater energy changes, and higher efficiency. However, there is more blade stress involved because of the higher work and temperatures. It is this latter fact that generally dictates the blade shape.

Conceptually, a turbine is a very simple device because it is fundamentally no different from a pinwheel that spins rapidly when air is blown against it. The pin-wheel will turn in one direction or the other, depending on the direction of the impinging air, and a direction can be found for the air that causes no rotation at all. Thus it is important to properly direct the airstream if the desired motion and speed are to be obtained.

A modern turbine is merely an extension of these basic concepts. Considerable care is taken to establish a directed flow of fluid of high velocity by means of stator blades, and then similar care is used in designing the blades on the rotating wheel (the vanes on the pinwheel) so that the fluid applies the required force to these rotor blades most efficiently. Conceptually, the turbine is a cousin to...