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

在高液体过冷条件下，微泡发射沸腾（MEB）可以实现远超CHF的高热流密度，而CHF的壁面过热度没有明显增加，同时受热面散发出大量微泡。这些高热通量应被认为是在该状态下聚结气泡剧烈行为的结果。气泡向受热面迅速坍缩，引起像微射流一样的过冷液体的强烈流动，即使在高表面温度区域也能实现稳定的固液接触。这种气泡行为在表面附近产生了瞬间的高压。在本研究中，通过高速摄像机同步测量了表面附近液体的压力和温度波动，并记录了气泡运动，以研究表面供水和传热的机制。实验结果表明，压力和温度的波动与气泡的运动，特别是气泡的坍缩过程有着密切的关系。压力波动的波形是由第一个大峰和随后的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。在压力波动中出现一个尖锐的正大峰，对应于气泡的崩溃，在短暂的延迟后，在温度波动中出现一个负峰，表示液体流向表面。温度和压力波动功率谱的峰值频率以及气泡破裂频率与热流密度的关系是一致的，表明热流密度越大，频率越高。

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.