| Due to the tight arrangement of electronic components,heat flux can be large in computer server and communication router cabinets.A large number of variable-speed axial-flow cooling fans are used in series or parallel to form a heat-dissipating unit.Forced convection heat exchange ensures normal equipment operation,but there will be a serious noise problem when multiple fans operate simultaneously at high speed.Reducing fan aerodynamic noise in a limited space is an important research direction.In this paper,a variable-speed axial flow fan for electronic device heat dissipation is taken as the research object,and the aerodynamic noise characteristics of the cooling fan are analyzed in detail by combining acoustic experiments and numerical simulation methods.A noise reduction method based on the duct mode cutoff is proposed and verified.The main research work is:(1)The far-field noise of the fan was measured by a far-field microphone array.The noise directivity and spectral characteristics were analyzed.The logarithmic relationship between the total sound pressure level and the rotational speed verifies that the main aerodynamic noise of the cooling fan belongs to the dipole source noise.The spectrum analysis shows that the discrete single-tone noise energy has a high proportion,which is the main aerodynamic noise influencing factor.(2)The mode correction technology based on the double-uniform sampling points was used to accurately identify the inlet and outlet circumferential acoustic modes of the cooling fan.Based on the structural features of the cooling fan,the effect of inlet non-uniformity was added to modify the traditional Tyler-Sofrin rotor-stator interaction mode formula.The revised formula reasonably explained the mode distribution characteristics of the fan inlet and outlet,and the rationality of the revised formula was verified by testing the cooling fans with different counts of rotor blade and strut.(3)The CFD numerical study was carried out to analyze the influence of the irregular structure of the cooling fan inlet.The uneven inlet flow significantly affected the unsteady pulses on the rotor blades,which showed periodic changes as the rotor blades rotate.Interactions between uneven inlet flow and rotor blades were observed via the POD of the flow field.(4)A noise reduction method for cooling fan based on duct mode cutoff was proposed.The experiment indicated that installing ducts of different lengths at the inlet and outlet has different effects on far-field noise.The circular short duct installed at the fan inlet had a significant noise reduction effect.The discrete single tone and the total sound pressure level decreased significantly.The method was verified by the noise reduction effect of fans with different counts of rotor blade and strut as well as parallel fans.(5)The ratio L/D of the duct length L and the diameter D defines the normalized parameter of the duct noise reduction of the imported irregular cooling fan and its applicable range.The universal applicability of the normalized parameter was verified by installing ducts on fans of different diameters. |
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