Experimental research into relation between propeller fan's flow field and noise (relationships between difference of tip flow behavior in each operation point and its noise)

The difference of the tip flow behavior in each operation points and the relation to the noise of the propeller fan with half-open-type casing were investigated through the measurement of the flow field of downstream by use of hot-wire probe and the pressure fluctuation on the casing wall with small pressure sensors. As a result, following conclusions were obtained. As flow ratio becomes smaller, the distance from the blade to tip vortex increases, and tip vortex interferes with casing more upstream. Velocity fluctuation at the downstream near the blade tip and pressure fluctuation on the casing wall have high correlation with the noise less than 200 Hz, which indicates the tip vortex becomes the noise source less than 200 Hz. The rate of contribution of the noise less than 200 Hz to overall noise increases as flow ratio becomes smaller, and the control of the tip vortex is more important in small flow ratio to reduce the fan noise.