Investigation of convective heat transfer augmentation using acoustic streaming generated by ultrasonic vibrations

Abstract

An investigation of acoustic streaming induced by ultrasonic flexural vibrations is experimentally and numerically presented. The investigation includes acoustic streaming pattern, velocity, and associated heat transfer characteristics. Acoustic streaming patterns visualized using Acetone agree well with the prediction by Nyborg’s theory. Tests of streaming velocity utilizing Styrofoam showed that the acoustic streaming velocity measured prove to be two orders greater than that by Nyborg’s theory. CFD simulations also showed the same order of the velocity as the one measured. By virtue of acoustic streaming, a notable temperature drop of 40 °C was obtained in 4 min and maintained. Tests identifying major heat flow paths indicated that gaps and the vibrating beam serve as major heat flow paths. CFD simulations were conducted to observe acoustic streaming patterns and velocities in the gap. Simulation results were validated by performing heat transfer analysis based on a lump-energy method. Simulation predicted that two symmetric vortices within half wavelength, rise of air at anti-nodes, and descent at nodes as Nyborg’s theory predicts. The presence of the upper plate has no effect on the acoustic streaming patterns. However, when an upper plate shorter than the vibrating plate is used, a drastic increase in streaming velocity occurs at the edges of the upper plate due to entrainment of air, which also alters streaming pattern in the vicinity of the open end. Estimated streaming velocities from CFD simulations are found to be two orders greater than those based on Nyborg’s theory. The results of CFD simulation indicated the vortical flows induced by a ultrasonic flexural standing wave (UFSW) can be reproduced. The CFD results are experimentally validated, qualitatively through flow pattern comparisons and quantitatively by the transient temperature drop comparison. The CFD results showed that the velocity near the plates is of the order of 10–100 cm/s, which is over 100 times higher than the results from theoretical studies based on sonically induced acoustic streaming assuming inviscid flow.