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.
Highlights
The axial fans employed in automotive cooling systems are commonly provided with a rotating shroud, i.e., a ring connecting the blade tips, see Longhouse [1]
This ring improves the volumetric efficiency and strengthens the whole assembly, but it often increases the radiated noise. The reason for this are large-scale flow structures contained in the leakage flow released from the gap between the rotating ring and the stationary enclosure, which are eventually reingested by the rotor
The leakage flow noise often constitutes the major contribution to the overall radiation and its main features are known, but very few works related to it are available in the literature, e.g., see Fukano et al [2], Fukano and Jang [3], Piellard et al [4], Magne et al [5], Moreau and Sanjosé [6], Zenger et al [7], Na et al [8], Canepa et al [9,10,11,12], Zhu et al [13] and Sanjosé and Moreau [14]
Summary
The axial fans employed in automotive cooling systems are commonly provided with a rotating shroud, i.e., a ring connecting the blade tips, see Longhouse [1]. Pressure risean was observed, due the patterns on both gap geometry and operating conditions, a further, unexpected dependence on both related rotor deformation This raised a number of questions on the features of the phenomenon and rotational speed and of pressure rise was observed, dueand to the related[15]. Canepa et al.This [10],raised which aallows number of questions on the features of the phenomenon and on the validity of the scaling the results of acoustic measurements related to different rotational speeds.
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