Effects of change in Frequency of Ultrasonic Vibrations on Heat Transfer

The heat transfer coefficients during natural convection of water on a downward-facing horizontal surface and that on a vertical surface were measured under the influence of ultrasonic vibration. The multi-type oscillator used in this experiment was able to irradiate in three different frequencies, 28, 45 and 100 kHz, not only continuously, but also intermittently. When ultrasonic vibrations of 28 and 45 kHz were applied to water, cavitation bubbles were generated and the heat transfer coefficient increased remarkably. When an ultrasonic vibration of 100 kHz was applied, the increase in the heat transfer coefficient was small compared to those at 28 and 45 kHz. At 100 kHz, cavitation was not observed at this low ultrasonic power. The uniformity of heat transfer, which depends on the profile of the standing wave by the ultrasonic vibration, was improved by the method of repeatedly applying multi-vibration.