Mesures de pressions et de vitesses dans un ventilateur centrifuge (rotor et stator) avant et après l'apparition et la disparition des décollements tournants

Pressure and speed measurements in a centrifugal fan (rotor and stator) before and after the appearance and disappearance of rotating stalls

E.N.S.A.M. Lille

Abstract

A centrifugal fan research facility, designed for pressure measurements on impeller blades is described in this paper. At low flow rates, a zone of discontinuity can be observed in the variations of head coefficient µ, power coefficient r and efficiency n with the flow coefficient δ. A rotating stall may occur in this area. The frequency response of the blade pressure measuring device has been determined experimentally, by using the time response to a pressure step. The blade pressure fluctuations increase when the rotating stall appears, especially on the suction side at the inlet of the blade. The relative speed of the rotating stall has been determined by carrying out a frequency analysis of pressure fluctuations. The absolute speed of the rotating stall has also been determined by the frequency analysis of the signal of a hotwire anemometer at impeller inlet. A bette knowledge of the rotating stall (speed, number and amplitude of the stall zones) can be reached by using two hot-wire anemometers on two radii near the inlet of the impeller. The determination of the periodicity of the anomemeters' output signals and their time shifting can be determined by using the intercorrelation function. The frequency and the number of stall zones with the variations of the flow coefficient Il have been studied. The fact that those characteristics do not change when the speed of the fan changes from 1000 rpm to 2500 rpm has been verified. The blade to blade velocity distribution on the whole width of the channel has been obtained by using a probe with two hot wires at impeller outlets. The flow structure with jet and wake can be observed at the centre of the channel and the flow is more uniform near hub and shroud.