The entire liquid-film dryout process in a vertical two-phase annular flow is characterized experimentally, from inception to completion. Experiments are conducted using saturated R245fa at high vapor qualities in a heated rectangular channel with a hydraulic diameter of 18 mm and an aspect ratio of 1/3. The walls of the test section are made of glass coated with fluorine-doped tin oxide (FTO). Heat fluxes up to 50 kW/m${}^2$ are generated at the inner surface of the window by passing an electrical current through the FTO coating. Instantaneous pressure and temperature in the test section, temperature on the outer wall of the test section, liquid-film thickness, and high-speed videos are recorded simultaneously during the dryout events. In addition, the state (wet or dry) of the heated surface is measured using a non-invasive laser reflectance technique at high sampling rate (2000 Hz) and over long periods of time ($>$ 1000 s). The laser reflectance measurement is used to calculate the time-averaged dry fraction, $f_{dry}$, which is the fraction of time that the wall is dry during intermittent cycles of dryout and rewet. Data show that cyclic dryout starts before the critical heat flux (CHF) is reached. The dryout heat flux (DHF), which marks the onset of dryout, is typically 90% of the CHF, except at very high quality ($x>0.95$), where it can be as low as 50% of CHF. For all the investigated mass fluxes, CHF, where the heat transfer coefficient peaks, occurs consistently at $f_{dry}\approx$ 0.05. Further insight into the liquid-film behavior at the onset of dryout is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. The rewetting process is driven by disturbance waves. In the wake of disturbance waves, the liquid film is almost stationary. Calculations of the characteristic time it takes for this stationary film to evaporate predict well the characteristic time during dryout events measured with the laser reflectance method.

从开始到完成，垂直两相环形流中的整个液膜干燥过程通过实验表征。在水力直径为 18 毫米、纵横比为 1/3 的加热矩形通道中，使用高蒸汽质量的饱和 R245fa 进行实验。测试部分的壁由涂有掺氟氧化锡 (FTO) 的玻璃制成。热通量高达 50 kW/m${}^2$通过使电流通过 FTO 涂层，在窗口的内表面产生。在干燥过程中同时记录测试段的瞬时压力和温度、测试段外壁的温度、液膜厚度和高速视频。此外，使用非侵入式激光反射技术以高采样率 (2000 Hz) 和长时间测量加热表面的状态（潮湿或干燥）（$>$1000 秒）。激光反射测量用于计算时间平均干分数，$F_{dr是}$，这是在干燥和再润湿的间歇循环期间壁干燥的时间分数。数据显示循环干燥在达到临界热通量 (CHF) 之前就开始了。标志着干燥开始的干燥热通量 (DHF) 通常为 CHF 的 90%，除非质量非常高（$X>0.95$)，可以低至瑞士法郎的 50%。对于所有研究的质量通量，传热系数达到峰值的 CHF 始终出现在$F_{dr是}\approx$0.05。通过结合对高速视频、时间分辨液膜厚度信号以及有关干燥事件持续时间和时间的统计数据的分析，可以进一步了解干燥开始时的液膜行为。再润湿过程是由扰动波驱动的。在扰动波之后，液膜几乎是静止的。计算此静止薄膜蒸发所需的特征时间可以很好地预测使用激光反射法测量的干燥事件期间的特征时间。