Tip clearance noise of axial flow fans operating at design and off-design condition

The noise due to tip clearance (TC) flow in axial flow fans operating at a design and off-design conditions is analyzed by an experimental measurement using two hot-wire probes rotating with the fan blades. The unsteady nature of the spectra of the real-time velocities measured by two hot-wire sensors in a vortical flow region is investigated by using cross-correlation coefficient and retarded time of the two fluctuating velocities. The results show that the noise due to TC flow consists of a discrete frequency noise due to periodic velocity fluctuation and a broadband noise due to velocity fluctuation in the blade passage. The peak frequencies in a vortical flow are mainly observed below at four harmonic blade passing frequency. The discrete frequency component of velocity fluctuation at the off-design operating conditions is generated in vortical flow region as well as in reverse flow region. The peak frequency can be an important noise source when the fans are rotated with a high rotational speed. The authors propose a spiral pattern of velocity fluctuation in the vortical flow to describe the generation mechanism of the peak frequency in the vortical flow. In addition, noise increase due to TC flow at low flow rate condition is analyzed with relation to the distribution of velocity fluctuation due to the interference between the tip leakage vortex and the adjacent pressure surface of the blade.