Rectified heat transfer in vapor bubbles

Some experimental findings on the dynamics of vapor bubbles in ultrasonic fields are reviewed. In particular, attention is drawn to a phenomenon which might be described as ultrasonically stimulated boiling. There is also a summary of certain pertinent facts about the threshold of cavitation in liquids with low foreign gas content, such as liquid nitrogen or liquid helium. It is emphasized that threshold pressure amplitudes can be less than the static pressure. It is proposed that an explanation of these observations could lie in a net positive heat transfer into a pulsating bubble. The minimum amplitude required to cause such a ‘‘rectified’’ heat transfer is calculated.