On the Occurrence, Stability and Heat Transfer Enhancement of Microbubble Emission Boiling

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In subcooled boiling under certain conditions of subcooling and velocity of the liquid, the heat flux does not decrease with increasing of the surface temperature in transition boiling regime, but increases steeply. In this phenomenon a lot of minute bubbles are emitted from the surface accompanied by extraordinary loud sound. Therefore, we call this phenomenon microbubble emission boiling, abbreviated as MEB. The high heat flux in MEB far exceeding CHF should be considered a result of violent growing and collapsing behavior of coalescent bubbles in that region, which strongly introduces subcooled liquid to the heated surface.In this study, the liquid temperature profiles were measured at the process of growing and collapsing of bubbles separately with conditional sampling method, by using a thermocouple set at a suitable position above the center of the surface as a detecting prove of bubble motion. In the period of bubble collapse, the high temperature appeared only on the central pa … More rt of the surface. This high temperature region indicates the position of high provability for vapor phase to have existed, suggesting that the bubbles shrink and collapse toward the center on the surface. Here, we tried to get an information about the moving velocity of liquid-vapor interface in the bubble shrinking stage near the moment of collapse, which was named as the collapsing velocity, here, with a twin electrode void probe. The velocity was very high in MEB comparing with that of nucleate boiling. In MEB, the collapsing velocity increased with increasing of the heat flux, to be as much as 20 m/s at 20 MW/sq.m in the heat flux.The bubble motion can be monitored directly by an electrode void probe. The frequency of the fluctuation of the void signal indicates the frequency of the chance of liquid supply onto the surface. Therefore, the peak frequency of the power spectrum of the void signal fluctuation should be an index of heat transfer, and heat flux must be a function of the frequency. The relation between the peak frequency and heat flux was obtained for various condition of liquid subcooling and velocity. Though the data showed a wide scattering, a trend of higher peak frequency is distinguished in lower liquid subcooling. It was rearranged for excess heat flux from CHF, to get a correlation function. Less