Introduction

In this chapter we briefly survey some of the other important features of the flows through turbomachines. We begin with a section on the three-dimensional characteristics of flows, and a discussion of some of the difficulties encountered in adapting the cascade analyses of the last chapter to the complex geometry of most turbomachines.

Three-Dimensional Flow Effects

The preceding chapter included a description of some of the characteristics of two dimensional cascade flows in both the axial and radial geometries. It was assumed that the flow in the meridional plane was essentially two-dimensional, and that the effects of the velocities (and the gradients in the velocity or pressure) normal to the meridional surface were neglible. Moreover, it was tacitly assumed that the flow in a real turbomachine could be synthesized using a series of these two-dimensional solutions for each meridional annulus. In doing so it is implicitly assumed that each annulus corresponds to a streamtube such as depicted in figure 4.1 and that the geometric relations between the inlet location, r1, and thickness, dr1, and the discharge thickness, dn, and location, r2, are known a priori. In practice this is not the case and quasi three-dimensional methods have been developed in order to determine the geometrical relation, r2(r1). These methods continue to assume that the streamsurfaces are axisymmetric, and, therefore, neglect the more complicated three-dimensional aspects of the flow exemplified by the secondary flows discussed below (section 4.6).