Critical heat flux and the dryout of liquid film in vertical two-phase annular flow

摘要

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) ≈ 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.