7.1 INTRODUCTION

A centrifugal compressor achieves part of the compression process by causing the fluid to flow outward in the radial force field produced by the rotation of the impeller. This portion of pressure increase differs from the pressure rise in an axial flow compressor rotor and stator, where a change from kinetic energy to thermal energy leads to compression in the diffusion process. In a radial stage, on the other hand, the change in the potential energy of the fluid is a direct consequence of the centrifugal force field of the rotor. Consequently, problems arising from the growth of the boundary layer and separation associated with adverse pressure are reduced. Because of this advantage, the centrifugal compressor has been employed to obtain a range of compression ratio and performance efficiency in turbojet engines.

Substantially higher compression ratios are achievable in a centrifugal compressor stage than in an axial stage. In an axial blade, relative flow velocity decreases from the leading edge to the trailing edge. This deceleration, or diffusion, can under proper conditions result in boundary-layer separation, resulting in an engine stall. This places a restriction on the loading capability of the axial blade. Radial compressor stages experience comparatively much less diffusion. Also, centrifugal stages are more rugged than axial blades, thus allowing them to operate at higher tip speeds. The upshot of these beneficial factors is that the pressure ratio may vary from 3.2 for a centrifugal impeller operating at 1.18 tip Mach speed to nearly 14.0 running...