Convective heat exchange characteristics of acoustic-induced flows over a sphere: The role of acoustic streaming

Abstract

To clarify the mechanism of the enhancement of heat transfer by sound wave. We have experimentally studied the convective heat transfer characteristics of heated copper spheres under sound waves with different frequencies and sound pressure levels. The experimental results showed that the existence of an acoustic field made strengthen the cooling process of the copper sphere to some extent, and the convective heat transfer coefficient increased nonlinearly with increasing sound pressure level. The contribution of the second-order nonlinear acoustic streaming generated by the interaction between sound waves and the heated copper sphere in the process of enhancing heat transfer was theoretically analyzed. Sound frequency determines the relative strength of acoustic streaming and oscillatory flow. When the sound frequency increases, the influence of the acoustic-induced flows on the heat transfer process of the copper sphere can be divided into four stages in turn: (1) acoustic streaming control stage, (2) acoustic streaming and oscillatory flow coordinated control stage, (3) oscillatory flow control stage, (4) stable stage. There is an optimal sound frequency to enhance heat transfer. Furthermore, the flow field characteristics of the acoustic streaming outside the sphere were numerically calculated. Calculations showed that low-frequency and high-intensity sound waves can form strong acoustic streaming motion outside the sphere. This proves that acoustic streaming in the low-frequency region is the predominant mechanism affecting heat and mass transfer.