Incidence and Deviation

Abstract

Up to this point, the relationships developed for a turbomachinery stage have been strictly correct for given velocity diagrams with known inlet and exit flow angles. We assumed that the flow is fully congruent with the blade profile. This assumption implies that the inlet and exit flow angles coincide with the camber angles at the leading and trailing edges. Based on the operation condition and the design philosophy, there might be a difference between the camber and flow angle at the leading edge, which is called the incidence angle. The difference between the blade camber angle and the flow angle at the exit is termed the deviation angle. Since the incidence and deviation affect the required total flow deflection, the velocity diagram changes. If this change is not predicted accurately, the stage operates under a condition not identical with the optimum operation condition for which the stage is designed. This situation affects the efficiency and performance of the stage and thus the entire turbomachine. In order to prevent this, the total flow deflection must be accurately predicted. The compressor and the turbine flows react differently to a change of incidence. For instance, a slight change of incidence causes a partial flow separation on the compressor blade suction surface that can trigger a rotating stall; a turbine blade is less sensitive even to greater incidence change. To obtain the incidence and deviation angle for compressor and turbine blades, we use two different calculation methods. The first method deals with the application of conformal transformation to cascade flows with low deflection as in compressor blades. The second method concerns the calculation of deviation in high loaded cascades as in turbine blades.