Abstract
Small axial flow fans are frequently used as air coolers or ventilators, and their aeroacoustic mechanism is similar to those found in larger machinery like aeroengines. Currently, there is a growing interest in adopting contra-rotation due to its inherent aerodynamic advantages. However, acoustically, contra-rotating (CR) fans radiate additional interaction noise, which decreases the sound quality and may cause extra annoyance. In this work, a sample CR fan is designed using simple radial equilibrium theory supplemented by three-dimensional steady-flow simulations, and its aeroacoustic performance is compared with that of two fans in a series. In general, the designed CR fan can produce a larger volumetric flow rate as well as a higher total pressure rise at lower rotational speeds compared with the series design. An acoustic comparison of specific noise levels shows that, in the flow rate range from the design point to higher flow rates, the CR fan is quieter. However, when the CR fan stalls in the lower flow rate region, its specific noise level is higher than that of the fan series. In addition, from the noise spectrum, the tonal noise power of the CR fan is less than that of the fan series.
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