6.1 INTRODUCTION

Air flowing through the annulus of an axial compressor performs the relatively more difficult task of going against an adverse pressure gradient. As the compression ratio increases, the danger of stall troubles rises correspondingly, which is all too prevalent in axial compressors. In individual airfoils stalling is encountered when the difference between the flow direction and the blade's angle of incidence becomes excessively large. The stability of the flow is jeopardized by the very fact that the movement is taking place in the same direction as that of the increase in pressure. Flow reversals occur under the right conditions of mass flow and rotor speed, which vary from those for the blade's design.

The assumption of a two-dimensional flow in the compressor's annular area, meaning that radial flow of the fluid is ignored, is reasonable for stages in which the blade height is small relative to the mean diameter of the annulus. The ratio of diameters at the hub and at the tip is greater than 0.8 in the later stages of a compressor. The front stages, however, have lower values of hub-to-tip ratio, and may be as low as 0.4 for the first stage of an aviation engine to accommodate a large mass flow through the machine with a limited frontal area. Consequently, the annulus also has a gradually reducing cross section and so the streamlines will not lie on a surface of revolution parallel to the axis of the rotor. The result is that the flow...