Heat transfer in a liquid under focused ultrasonic field

Abstract

The ultrasonic heat transfer enhancement has a strong dependence on the sound intensity. The higher the local sound intensity is, the better the heat transfer enhancement achieves. Therefore, a focused ultrasonic field is proposed in cavities with elliptical cross-section based on interference criterion and standing wave criterion. To design a cavity with focused ultrasonic field, sound source and heat source are purposely located at the two focus points where sound intensity could be reinforced. There is a compromise between the value of the standing wave coefficient and the time-averaged sound pressure. The profile of heat transfer coefficient is found to be spatially consistent with the simulated time-averaged sound pressure distribution. The cavities with focused ultrasonic field always present larger heat transfer coefficient of natural convection heat transfer than the ordinary ones and the cavity with standing wave coefficient k2 = 8 presents better natural convection heat transfer performance than the cavity with standing wave coefficient k2 = 6. For the sub-cooled boiling heat transfer, bubble plays an important role on the ultrasonic heat transfer enhancement effect. At larger heat flux or smaller sub-cooled temperature, bubbles gather together to form bubble cloud that presents large thermal resistance. The cavitation collapse can disperse the bubble cloud.