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

在高液过冷度条件下，微泡发射沸腾(MEB)可以获得远远超过CHF的高热流密度，而不会显著增加CHF的壁面过热度，并伴随着从受热面排放的大量微泡。这些高热流密度应该被认为是该区域内聚结气泡剧烈行为的结果。向加热表面收缩的气泡的快速破裂会引起强烈的过冷液体流动，即使在表面温度较高的区域，也会像微喷一样产生稳定的固液接触。这种气泡行为在表面附近产生了一个高压时刻。通过对液面附近液体的压力和温度脉动的测量，同时用高速摄像机记录气泡的运动，研究了液面补水和换热的机理。实验结果表明，压力和温度的波动与气泡的运动，特别是气泡的破裂过程有着密切的关系。压力波动的波形是一组峰的重复，这组峰由第一个大峰和随后的较小峰组成。当气泡开始破裂时，压力急剧增加。压力波动的大的正峰值对应于气泡的破裂，在短暂的延迟后，温度波动的负峰值被观察到，表明有液体流到表面。温度、压力脉动功率谱的峰值频率和气泡崩塌频率的峰值频率是一致的，并与热流密度呈简单的关系，热流密度越高，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.