1. Introduction

A radial compressor can be divided into different parts, as shown in Figure 1.1. The flow is aspirated from the inlet plenum and after being deflected by the inlet guide vanes (IGV), it enters the inducer. From there on the flow is decelerated and turned into the axial and radial directions before leaving the impeller in the exducer. The presence of a radial velocity component is responsible for Coriolis forces, which, together with the blade curvature effect, tends to stabilize the boundary layer at the shroud and suction side of the inducer (Johnston 1974; Koyama et al. 1978). The boundary layer becomes less turbulent and will more easily separate under the influence of an adverse pressure gradient.