Physical mechanism of heat transfer from heated and cooled cylinder to water in ultrasonic standing wave field.

Abstract

It is well known that the heat transfer rate from a solid surface to a fluid is increased in an ultrasonic field, but the mechanism of enhancement of heat transfer with ultrasonics has not been elucidated sufficiently. To clarify the mechanism of heat transfer with ultrasonic irradiation, the effects of an ultrasonic standing wave of 28KHz on the free convective heat transfer from a heated or a cooled cylinder of diameter 17.5 mm to degassed water were investigated experimentally. In particular, temperature distribution near the node and the antinode of the ultrasonic standing wave were observed by the Schlieren method. The average and the local heat transfer coefficients between the node and the antinode of the standing wave from the heated cylinder to water were measured. In the case of heat transfer from the heated cylinder, the heated water near the cylinder is moved from the node to the antinode of the standing wave, whereas the cooled water near the cooled cylinder is moved from the antinode to the node. Therefore, the thermal boundary layer thickness around the cylinder is varied by movement of fluid and as a result it is considered that the local and the average heat transfer coefficient vary. An acoustic radiation pressure theory which explains the mechanism of heat transfer in an ultrasonic stnading wave field is proposed.