单气泡生长的高速激光干涉可视化研究

为研究过冷沸腾中气泡的生成机理并为壁面沸腾模型的建立提供数据支撑，本研究采用透光的氧化铟锡（ITO）薄膜作为加热材料，使用激光干涉法和高速相机对池沸腾中单气泡的生长过程进行同步测量，获取了气泡底部微液层的结构和变化过程，同时得到气泡干斑直径、气泡直径等相关参数。研究结果表明，气泡生长过程可分为底部存在微液层的生长阶段和底部完全蒸干后的脱离阶段两部分，两阶段的时长大致相当。在生长阶段，干斑直径和接触直径逐渐增大；在脱离阶段，接触直径逐渐减小。在气泡稳定生长时期，接触直径与气泡直径的比值约为0.6。实验结果验证了微液层边缘存在弯曲结构。本研究获得的气泡微液层演变数据可为气泡生长模型提供实验支撑。

High-speed Visualization of Single Bubble Growth Using Laser Interferometry

Boiling is a typical phenomenon in the field of heat and mass transfer, which has important influence on industrial safety and energy efficiency. However, boiling phenomenon is extremely complex and it is difficult to describe its mechanism. High-precision experimental measurement is the key to obtain boiling mechanism in order to obtain accurate growth model. Based on the accurate measurement, an accurate mathematical model can be established to provide support for the development of wall boiling model. In this study, the transparent ITO film was utilized as the heating material, laser interferometry and high-speed camera were used to measure the growth process of single bubble in subcooled pool boiling synchronically. The structure and change process of the microlayer at the bottom of the bubble and other related parameters were obtained by laser interferometry. Macro parameters of bubble such as growth time，waiting time and shape evolution were obtained by the side camera. The bubble dynamics during the growth process can be obtained by comparing the side and bottom images. The result shows that the bubble growth process can be divided into two stages: bubble growth period and bubble departure period. During the growth period, there exists a microlayer at the bottom of bubble and it expands rapidly. The evaporation of microlayer provides energy for bubble growth and keeps the volume of bubble growing. At this time, the shape of bubble is hemispherical. When the microlayer evaporates, the bubble enters the departure period. At this stage, the volume of the bubble is still growing because the surrounding superheat layer is providing energy to the bubble. The shape gradually transits from hemispherical to spherical, and finally develops into inverted water drop type. The time of the two periods is approximately equal. Quantitative analysis of relevant parameters shows that the thickness of microlayer increases with the increase of radius. The maximum thickness of microlayer increases first and then decreases with the growth of bubbles. It is because that the bubble expands faster in the early stage. In this period, both dry spot diameter and contact diameter increased simultaneously. In the departure period, contact diameter gradually decreases. When the bubble is stable, the ratio of contact diameter to bubble diameter is about 0.6. At the same time, it is also observed in the experiment that when entering the later stage of microlayer evaporation, the shape of the microlayer is no longer a standard Newton ring. The deformation of the interference fringe indicates that the edge of the microlayer is bent.