Morphological effects of leading-edge sawtooth on the vortex evolution and acoustic characteristic of an ultra-thin centrifugal fan

Abstract

Serrations on the owl wings' leading edge (LE) are considered one of the critical characteristics leading to their silent flight. Inspired by this, LE sawtooth was innovatively induced on ultra-thin centrifugal fan blades, and the morphological effects of these teeth on the vortex evolution and aeroacoustic characteristics of the fan were studied using large eddy simulation and the Ffowcs Williams–Hawkings analogy. A single-passage model was adopted to finely simulate the flow mechanism between blades with an acceptable scale. Five sawtooth schemes with relative tooth width λ/b from 7.96% to 29.84%, as well as the prototype, were calculated and analyzed. It is found that the optimal λ/b ranges from 8% to 17.05%, which reduces the overall sound pressure level (SPL) by over 1 dB without impacting the blade pressure and efficiency. These sawteeth inhibited the LE separation, shattered the leading-edge vortex (LEV) into small vortices, and consequently weakened the pressure fluctuations on the blades. However, more prominent teeth (λ/b > 23.8%) intensify the interactions between LEV and other passage vortices, changing the dominant pressure pulsations to high frequency, in turn raising the overall SPL. Too small sawteeth are challenging to process on such ultra-thin blades, so the largest sawtooth among the suggested range was considered the optimal scheme (λ/b = 17.05%) and was manufactured to measure. The results show that the SPL of the fan with LE sawtooth is 0.24–0.57 dB lower than that of the prototype under the same flow rates, even though its rotational speed is increased.