We report the progress made in our recent study to develop an ultra-quiet axial fan for computer cooling applications. By using a commercially-available cooling fan as the baseline, a number of acoustically tailored modifications are implemented in order to reduce the noise level of the cooling fan, which includes optimizing the rotator blades and guide vanes according to axial fan design theory, adding an intake cone in the front of the hub to guide the airflow into the axial fan smoothly, and reducing the tip clearance to lower the noise generation due to tip vortex structures. A comparison study is conducted to measure the sound pressure level (SPL) of the reformed axial fan in an anechoic chamber, in comparison to that of the prototype fan, in order to assess the effects of the modifications on the fan noise reduction. The measurement results of our preliminary study reveal that, at the same flow rate, the SPL of the reformed fan would be up to 5 dB lower than that of the prototype fan. In addition to measuring the sound pressure levels (SPLs) of the fans, a digital particle image velocimetry (PIV) system is also used to conduct detailed flow field measurements to reveal the changes of the flow characteristics and unsteady vortex structures associated with the modifications. Besides conducting “free-run” PIV measurements to determine the ensemble-averaged statistics of the flow quantities such as mean velocity, Reynolds stress, and turbulence kinetic energy (TKE) distributions at the exit of the axial fan, “phase-locked” PIV measurements are also performed to elucidate further details about evolution of the unsteady vortex structures in fan exhaust in relation to the position of the rotating fan blades. The detailed flow field measurements are correlated with the SPL measurements in order to elucidate underlying physics associated with the fan noise reduction.

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