On the Heat Transfer Mechanism and Stability of High Heat Flux Microbubble Emission Boiling

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Microbubble Emission Boiling (MEB) is the regime that occurs in the transition process from nucleate to film boiling with high liquid subcooling condition, and realizes extremely high heat flux without a marked increase in the amount of wall superheat. MEB has unique aspects different from that of ordinary boiling, such as a profusion of microbubbles emitted from a heat transfer surface. This experimental investigation reveals that this phenomenon includes two regimes, i.e., the stormy and calm microbubble emission boilings. This study also shows temperature fulctuations of a heat transfer surface and pressure fluctuations in the liquid near the surface to classify MEB into the above regimes. The temperature fluctuations in either regime are smaller than those at the critical heat flux. The pressure fluctuations are larger in the stormy MEB,and smaller in the calm MEB as compared with those at the critical heat flux. The liquid subcooling and flow velocity approaching the surface are i … More mportant aspects by which to generate those regimes. A number of experiments lead to the map of MEB regimes that consists of liquid subcooling and flow velocity. An oscillatory character between the two regimes appears under several conditions.The frequency distribution of sound pressure level when MEB exists, has two peak levels. The first peak that appears from 500 [Hz] has the highest sound pressure level and is difficult to be found out in nucleate boiling region. The second peak appears from 8000 to 16000 [Hz] and this peak is not only in MEB region but also in nucleate boiling region, especially with high liquid subcooling. The intensity of the both peaks is depending on liquid subcooling and heat flux. Bubbles in MEB region collapse toward the heat transfer surface. Bubble collapse frequency and sound pressure level of MEB have similar frequency distributions. Consideration of the above results lease to a hypothesis that collapse of a bubble is an important or controlling factor of the heat transfer mechanism of MEB. Less