Note that the IGV also adds no energy to the flow. It is designed to add swirl in the direction of rotor motion to lower the Mach number of the flow relative to the rotor blades, and thus improve the aerodynamic performance of the rotor. We begin by unwrapping the compressor. Here we have assumed that the area of the annulus through which the flow passes is nearly constant and the density changes are small so that the axial velocity is approximately constant. Figure 9.6 Velocity triangles for an axial compressor stage.
Primed quantities are in the relative frame, unprimed quantities are in the absolute frame.
This behavior is represented schematically in Figure 9.7. We can apply the same analysis techniques to a turbine. Again, the stator does no work. It adds swirl to the flow, converting internal energy into kinetic energy. The turbine rotor then extracts work from the flow by removing the kinetic energy associated with the swirl velocity.