Leakage Noise and Related Flow Pattern in a Low-Speed Axial Fan with Rotating Shroud

Abstract

The effect of rotational speed and pressure rise on the leakage flow noise radiated by a low-speed axial fan, provided with rotating shroud, has been systematically investigated. The flow in the gap region has been studied by means of particle image velocimetry (PIV) measurements taken in the meridional plane. At low blade loading, the leakage flow is restrained close to the rotor ring and, at higher loading, it forms a wide recirculation zone. In the latter conditions, an unsteady flow separation likely takes place in the blade tip region which may be observed in the instantaneous flow field only. The leakage flow noise generally increases with the blade loading, but is non-monotonic, as the overall sound pressure level (OASPL) growth is interrupted by local minima; such a trend is qualitatively independent of the rotational speed. As the loading increases, the sound pressure level (SPL) spectrum shows important modifications, since the characteristic frequency of the subharmonic narrowband humps related to the leakage noise decreases; furthermore, height and width of the humps vary non-monotonically. Such a complicated behavior is likely related to the modifications in the leakage flow pattern and also to the appearance of the flow separation at the blade tip.

Experimental noise reduction (aeroacoustical enhancement) of a large diameter axial flow cooling fan through a reduction in blade tip clearance

“…This is however consistent with literature as tip leakage vortices are reduced and more work performed at the tip. 18,21,31,32

Figure 15.Fan performance characteristic curve with tip gap reduction.

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“…This is however consistent with literature as tip leakage vortices are reduced and more work performed at the tip. 18,21,31,32 The difference in static pressure rise due to the tip gap reduction is normalized against the maximum pressure of the 4 mm (0.26%) tip gap fan configurations. This is to attain a better view on the increase in performance due to the closing of the tip gap.…”

“…16,17 Tip clearance noise is the byproduct of vortices that form due to the high-pressure air leaking around the blade tips. 18 These tip leakage vortices are ingested by the rotor, due to the reverse flow nature of these vortices, and can contribute significantly towards turbulence ingestion noise or leading-edge noise. 16,19 Turbulence ingestion noise can be countered through adding leading edge serrations to the blades.…”

“…In the case where tip leakage vortices lead to leading-edge noise, the size of the vortical structures around the blade tips can be reduced by reducing the blade tip clearance. 16,18,21 Therefor it would be beneficial to reduce the blade tip clearance and investigate the effect of a zero mm blade tip clearance on the aerodynamic and aeroacoustic performance of a large diameter axial flow cooling fan such as the M-fan.…”

Characterization of Unsteady Leakage Flow in an Axial Fan

“…Unfortunately, it causes a strong increase in the radiated noise due to the presence of coherent flow structures (Longhouse [1]) that are contained in the leakage flow released from the gap between the ring and the stationary enclosure and are eventually re-ingested by the rotor. Such a noise is often predominant in the lowto medium-frequency range and may also constitute the major contribution to the radiated power, e.g., see Canepa et al [2,3]. The qualitative features of the leakage flow are known, e.g., see Fukano et al [4], Fukano and Jang [5], Piellard et al [6], Magne et al [7], Moreau and Sanjose [8], Zenger et al [9], Na et al [10], Canepa et al [11][12][13], and Zhu et al [14].…”