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Aerodynamic noise control technology of electronic device cooling fan based on duct acoustic mode cutoff

SUN Zonghan;TIAN Jie;ZHANG Xiaopu;OUYANG Hua;School of Mechanical Engineering, Shanghai Jiao Tong University;Engineering Research Center of Gas Turbine and Civil Aero Engine, Ministry of Education;  
An experimental study on the aerodynamic noise and noise reduction method of a variable-speed axial flow fan for electronic device heat dissipation is conducted. The far-field radiation noise at different rotational speed is measured by a microphone array uniformly distributed along the circumference in the contour plane of the fan axis. The logarithmic relationship between the total sound pressure level(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 discrete tonal noise is the dominant noise. Based on the duct mode cutoff in duct acoustic theory, the effects on the aerodynamic noise of installing a circular short duct at the inlet and outlet of the fan are studied, pointing out that the short ducts with different lengths on the different sides have different effects on the far-field noise. At rated fan speed, installing a 2 cm duct at the inlet can reduce the average total SPL by 4.1 dB(A) at the far field of 1 m, which is significant. Mode measurement results show that in this case the main mode amplitude corresponding to the discrete tone is greatly reduced,and the reduction of discrete tonal noise leads to a significant reduction of the total SPL. This method provides a new way for cooling fan noise reduction.
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