This work experimentally studies the fundamental mechanism by which the ultrasonic vibration enhances natural convection and pool boiling heat transfer. A thin platinum wire is used as both a heat source and a temperature sensor. A high speed video imaging system is employed to observe the behavior of cavitation and thermal bubbles. Experimental results show that the effects of ultrasonic vibration on flow behavior are vastly different depending on the heat transfer regime and the amount of dissolved gas. In the natural convection and subcooled boiling regimes, behavior of cavitation bubbles strongly affects the degree of heat transfer enhancement. In saturated boiling, no cavitation occurs thus the reduced thermal bubble size at departure and acoustic streaming are major factors enhancing heat transfer rate. The highest enhancement ratio is obtained in natural convection regime where no bubbles are present without ultrasonic vibration.

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