The flow field surrounding the propeller of an electric fan contributes significantly to the noise emitted by low-pressure electric fans designed to cool desktop computers. Given the daily consumer use of these devices, being at commercial or residential settings, it is necessary to reduce the noise emitted by the electronic fans as they cool the central processing units. One way of attaining this is by modifying the geometry of the fan since the blades are one of the noise sources in axial fans. Fan airflow is made of longitudinal pressure fluctuations which transmit energy in the form of sound when encountered by a surface, thus the blade rotation of a computer fan disturbs the flow around the propeller and experience drag and lift forces that cause them to vibrate and radiate noise.An attempt has been made to characterize electric fan noise through modification of geometrical features such as its hub diameter, blade length, blade thickness, blade angle of attack and number of blades, using computer simulation. A commercially available computer cooling fan has been selected as the reference fan. Its rated voltage and size are 12V and (80 x 80 x 25) mm, respectively. The fan geometry has been modeled using a Computer-Aided Design (CAD) software and the fan specifications of reference such as fan size, rated voltage and rated noise emission have been assumed from two fan manufacturers providing the same size of fan and rated voltage.Computational Fluid Dynamics (CFD) and Computational Aeroacoustics (CAA) simulations have been employed to record Sound Pressure Levels (SPL), measured in Decibels (dB), at constant rotational speeds. Velocity magnitude profiles and pressure profile distributions have been generated to visualize the flow patterns and validate aerodynamic theories citing turbulent flow in the vicinity of the rotor, characterized by a vortex field, wakes and eddies in the Trailing Edge.Two constant rotational speeds have been applied to run the simulations in this study, 2,400 rpm and 4,500 rpm. The first simulation has yielded Sound Pressure Levels of 31.94 dB and 48.99 dB, respectively. These sound levels are within the range of noise emission advertised by the reference manufacturers for the same size of fan, with number of blades and rated voltage. Consistently higher sound pressure levels were observed for 4,500 rpm, as expected due to the turbulent flow associated to high rotational speeds.CFD and CAA simulations resulted in relevant noise reduction for the modifications consisting of a reduced hub diameter and increased blade thickness. For this reason, an improved fan geometry was created by superimposing both design modifications to yield a 5.02 dB noise reduction for 2,400 rpm and a 3.53 dB noise reduction for 4,500 rpm.

Characterization of Electric Fan Noise Generation Due to Blade Geometry

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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