Abstract:

The effect of macroscopic temperature gradient on Marangoni condensation of ethanol-water vapor mixture was investigated experimentally in a wide range of concentration(0—50%)at different velocities (2, 4, and 6 m·s^-1)and pressures (31.2, 47.4, and 84.5 kPa)with shot condensation modes. An oblique copper block was employed to create surface temperature gradient. Six condensation modes were observed in the experiments, which are smooth film, bulk-drop, drop-streak, drop, wavy-drop and streak. Not every condensation mode appeared in each process and some condensation behavior was not distinct. For low condensation (0.5% and 1%)and high condensation (50%)smooth film was observed under the condition of large vapor-to-surface temperature difference. Drop mode occurred easily at high concentration ( >20% )while bulk-drop was a regular condensation mode at the concentration of 0.5%, 1% and 2%. Drop-streak and streak could occur almost at all concentrations with low vapor-to-surface temperature difference ( <3 K).In the wide range of vapor-to-surface temperature difference, bulk-drop was a principal condensation mode. Condensation modes could vary with some factors, such as vapor-to-surface temperature difference, ethanol mass fraction, vapor velocity, and vapor pressure, especially the former two. The heat transfer coefficients depended on the condensation behavior. In general, for low ethanol concentration (0.5% and 1%), the heat transfer coefficients could keep a large value over a relative wide range of vapor-to-surface temperature difference. The maximal heat transfer coefficients corresponded to the dropwise condensation mode and small heat transfer coefficients to the filmwise or rivulet modes. In addition, the condensate drops became smaller and denser with the increase of vapor pressure or velocity. The effect of vapor pressure on condensation behavior was less than that of vapor velocity. Meanwhile, the movement of drop or streak biasing from left to right could be observed in the experiments. For the condensation modes on flat plate and oblique block, it was found that the condensation modes on oblique block was more irregular, and condensation behavior could not be simply classified into certain mode, such as drop, streak, ring like, filmwise and film with fluctuate, since each mode may play a part in the condensation or condensation behavior is an intermediate transition state in various condensation modes.