An experimental study on Microbubble Emission Boiling in a subcooled pool: Heat transfer characteristics and visualized presentation

In view of its super high ability of heat dissipation, Microbubble Emission Boiling (MEB) was investigated under atmospheric pressure and liquid subcooling of 0-60 K with the aid of a high-speed video camera. An experimental apparatus was designed and fabricated to achieve stable MEB, by which a more detailed bubble behaviors and heat transfer characteristics during MEB were obtained. It is reaffirmed that liquid subcooling of 20 K is the minimum subcooling triggering MEB for water. The increase in liquid subcooling would raise the wall superheat and vapor generation rate for forming MEB. The heat transfer coefficient in the beginning of MEB is about 20-70 kW/m2 K, less than that at CHF (Critical Heat Flux) under the same condition. Three typical interfacial phenomena were observed in MEB, which are partial collapse on the periphery of vapor film, partial collapse on the top of vapor film and violent surface wave process without collapse. The average collapse frequency in MEB is much higher than the bubble departure frequency in nucleate boiling, greatly enhancing the heat transfer process. Finally, a dimensionless heat transfer correlation for MEB is proposed with considering the effect of the vapor film collapse, condensation, Marangoni convection and vapor generation rate.