Experimental study of the leakage flow in an axial-flow fan at variable loading

The present paper reports a 2D-PIV (particle image velocimetry) study of the effect of the operating point on the leakage flow in a low-speed ring fan. First, the flow pattern has been studied at 12 operating points covering the whole characteristic curve. At very low loading, the leakage flow streams along the rotor ring and is directly reingested; then, a separation bubble attached to the ring forms that, approaching the design point, modifies in a flow streaming radially outward. As the loading further increases, a separated flow region appears in the blade tip region that finally merges with the leakage flow. A further, more detailed study has been performed at eight operating points in the neighborhood of the design one. Very small loading variations may yield the leakage flow pattern modification, but no intermittence is present during the transition, as instantaneous flow patterns of any intermediate type continuously alternate. These results provide a consistent explanation for the ones of previous acoustic measurements.