Velocity Measurements Downstream of the Impellers in a Multistage Centrifugal Blower

The paper presents the results of an experimental investigation on a four-stage centrifugal blower, having the aim of obtaining an accurate description of the flow field behind the impellers in several operative conditions and for different geometric configurations. Actually, the test plant allows one to change the turbomachinery characteristics assembling one, two, three, or four stages and three different types of diffuser. In this first research step, the blower has been tested in the four-stage vaneless diffuser configuration. The unsteady flow field behind the impellers and in the diffusers has been measured by means of a hot-wire anemometer. A phase-locked ensemble-averaging technique has been utilized to obtain the relative flow field from the instantaneous signals of the stationary hot-wire probes. Several detailed measurement sets have been performed using both single and crossed hot-wire probes, to obtain the velocity vectors and turbulence trends, just behind the blower impellers and in several radial positions of the vaneless diffusers. These measurements have been done at different flow rate conditions, covering unsteady flow rate phenomena (rotating stall) also. The results obtained allowed us to get a detailed flow field analysis in the multistage centrifugal blower, in relation to the geometric configuration and to the differing operating conditions.