On the Bubble Behavior and Heat Transfer Mechanism in Microbubble Emission Boiling

微泡喷射沸腾（MEB）在高过冷度条件下，可以获得远超过CHF的高热流密度，但CHF不会显著增加壁面热流密度，同时伴随着大量微泡从加热表面喷射出来。这些高的热通量应该被认为是在该区域中聚结气泡的剧烈行为的结果。气泡向加热表面收缩的快速破裂引起过冷液体的强流动，如微射流，即使在高表面温度区域也能进行稳定的固液接触。这种气泡行为在表面附近产生高压。在本研究中，为了探讨表面供水和传热机理，用高速摄像机同步测量了表面附近液体的压力和温度波动，并记录了气泡的运动。实验结果表明，压力和温度波动与气泡运动，特别是气泡的运动有密切的关系。压力波动的波形是由第一个大峰和随后的较小峰组成的一组峰的重复。当气泡开始破裂时，压力急剧增加。压力波动中的尖锐的正的大峰值对应于气泡的破裂，并且在短的延迟之后观察到温度波动中的负峰值，指示液体流到表面上。温度和压力脉动功率谱的峰值频率以及气泡破裂频率是一致的，并且与热流密度成简单的关系，在较高的热流密度下，频率较高。

High heat flux far exceeding CHF can be achieved in microbubble emission boiling (MEB) without a marked increase in the wall superheat from CHF under high liquid subcooling condition, accompanying a lot of microbubbles emitted from the heating surface. Those high heat flux should be considered a result of violent behavior of coalescent bubbles in that regime. A rapid collapse of a bubble which shrinks toward the heating surface induces a strong flow of subcooled liquid like a micro-jet to make steady solid-liquid contact even in the high surface temperature region. This bubble behavior generates a moment of high pressure in the vicinity of the surface. In this study, a measurement of the pressure and the temperature fluctuation in the liquid near the surface was performed synchronously with a recording of the bubble motion by a high speed video camera to look into the mechanism of the water supply to the surface and therefore the heat transfer.The experimental results reveals that there exists a close relation among the pressure and the temperature fluctuations and the bubble motion especially the collapsing process of it. The waveform of a pressure fluctuation is a repetition of a group of peaks which consists of a first large peak and following smaller peaks. The pressure steeply increases when a bubble begins to collapse. A sharp positive large peak in the pressure fluctuation corresponds to the collapse of a bubble, and after a short delay negative peak in the temperature fluctuation is observed, indicating a liquid flow onto the surface. The peak frequencies of the power spectrums of the temperature and the pressure fluctuation as well as the bubble collapse frequency are coincident with each other and are expressed by a simple relation against heat flux, showing a higher frequency at a higher heat flux.