In the computer server and communication router cabinet, due to the tight arrangement of electronic components, the heat flow density is large. For this reason, A large number of variable-speed axial-flow cooling fans are used in series or in parallel to form heat-dissipating units, which force convection heat transfer to ensure the normal operation of the equipment. However, the simultaneous high-speed operation of multiple fans brings serious noise problems. If the fan speed is reduced, the performance stability of the computer server and the communication router will be affected. How to reduce the fan aerodynamic noise in a limited space has become an urgent problem to be solved, which requires a deeper understanding of the noise characteristics of the cooling fan. This paper conducts an experimental study on the aerodynamic noise characteristics of a variable-speed axial fan commonly used in the field of electronic device heat dissipation. First, the far-field noise of the fan is measured by a microphone array distributed along the circumference in the contour plane of the fan axis, and the far-field noise directivity and spectrum characteristics of the fan are analyzed. The logarithmic relationship between the total SPL and the rotational speed verifies that the main aerodynamic noise of the fan belongs to the dipole source noise, and the spectrum analysis indicates that the energy ratio of the discrete single tone noise is high, which means discrete tone noise is the dominant aerodynamic noise. Afterwards, the circumferential acoustic mode measurement method is used to obtain the mode distribution on the inlet and outlet sides of the cooling fan, and the influence of the inlet unevenness on the cooling fan is found based on the double-uniform sampling points high-order mode correction technique. Considering the non-axisymmetric shape of the fan inlet shroud, the traditional Tyler-Sofrin rotor-stator interaction formula is modified. By introducing the influence of inlet unevenness, the propagation characteristics of the dominant mode of the fan with the change of distance are analyzed. The validity of the modified formula is verified by measuring the inlet and outlet circumferential acoustic modes of three types of cooling fans with different numbers of rotor and stator blades. Furthermore, aiming at the influence of uneven inlet of axial flow fan on noise amplitude and circumferential acoustic mode, CFD numerical study is carried out with Fluent. The results show that uneven inlet flow significantly affects the size and distribution of unsteady pulse on rotor blades, and presents regular periodic changes with rotor blade rotation. The interference effect between rotor blades and inlet unevenness is observed by POD decomposition method, which proves that the influence of inlet unevenness on circumferential acoustic mode cannot be ignored. The discussion on aerodynamic noise circumferential mode of axial flow cooling fan can provide reference for further noise reduction measures of the cooling fan.


Study on the Influence of Inlet Asymmetry on Aerodynamic Noise of Cooling Fan

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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