Translated Abstract
The dropwise condensation is an efficient way for heat transfer which has a higher heat transfer coefficient than that of the film-wise condensation. The traditional dropwise
condensation of pure steam is mainly caused by the surface modification and dropwise
promoter additives, which could not maintain stably for enough time. The dropwise condensation mode formed in the Marangoni condensation of mixtures with positive system properity is caused by the physical properties of the mixtures, which could maintain stably
for a long time with excellent heat transfer characteristics. It is meaningful for the industrial application of dropwise condensation. In this paper, the experimental system of Marangoni
condensation for ethanol-water mixtures on the horizontal surface was designed and built. The condensation modes and heat transfer characteristics during the time-series and time-averaged processes for Marangoni condensation merely due to the surface tension
gradient were systematically analyzed for the further mechanism research and industrial application.
Three main condensation modes, including film-wise, film-wise with dropwise and dropwise, were observed at the start of the time-series process for Marangoni condensation of
ethanol-water mixtures. Condensation modes transformed from film-wise to dropwise gradually and the periodic variations showed up gradually and then weakened with the decrease in the vapor-to-surface temperature difference and the increase in the ethanol vapor
concentration. The increase in the vapor velocity and pressure always promoted the appearence of dropwise mode and the periodic variation. Four typical styles, including filming style, periodic-dropwise style, irregular-dropwise style and slow-growth-dropwise style were concluded during the time-series process. The periodic-dropwise style had a self-cleaning function for the condensate. The periodic time was calculated which was in the
range of 1.3 s to 7.6 s and the emperical formula of the periodic time was also obtained. The 3D distribution of the condensation styles during the time-series process was established and
the emperical formulas of the boundary vapor-to-surface temperature differences for the periodic-dropwise style were also obtained to provide theoretical guidance in the application
of the self-cleaning function to the heat exchangers.
Image processing method was used to indentify the sizes and numbers of the condensate droplets. Then the maximum droplet radius and condensate droplets coverage ratio were introduced to analyze the droplets growth quantitatively during the time-series process. The droplets mainly grew up by direct condensation at the start of time-series process, and then grew up mainly by coalescence with a maximum radius of about 7.5 mm. The droplets coverage ratio increased fast and then increased slowly until kept stable during the time-series process with a maximum of 0.8. As the ethanol vapor concentration increased,
the maximum droplet radius and droplets covarage ratio decreased gradually. The maximum droplet radius and droplets covarage ratio increased with the increase in the vapor-to-surface temperature difference, vapor velocity and pressure, while the droplets coverage ratio may
decrease due to the fast movements of the condensate droplets. The emperical formula of the droplets coverage ratio was also obtained with an error of -19.8% to 30.0%.
The experimental results showed, the condensate droplets mainly moved reversely with the vapor flow with a maximum of -81.7 mm·s-1
during the time-series process for Marangoni condensation of ethanol-water mixtures. The reverse velocity increased gradually with the increase in vapor-to-surface temperature difference and vapor pressure, and decreased
gradually with the increase in the ethanol vapor concentration and vapor velocity. Dimensionless analysis showed the reverse velocity increased with the Ma number and decreased with the
Rev number, and the experimental correlation for the droplets reverse
movements was also obtained. The reverse movement of droplets was mainly caused by the surface tension gradient on the condensing surface because of the temperature gradient due to the vapor velocity. The force analysis of a single condensate droplet was conducted and
the movement velcocities were theoretically calculated which were consistent with the experimental results.
In the case of time-series process, the heat transfer coefficient decreased fast and then kept stable in the filming style, varied periodically with high heat transfer coefficient in the
periodic-dropwise style which intensified the heat transfer process by self-cleaning function, fluctuated in the irregular-dropwise style and decreased gradually in the slow-growth-dropwise style. The heat transfer coefficient decreased monotonously with the increase of the vapor-to-surface temperature difference and droplets coverage ratio during
the time-series process. The dimensionless experimental correlation for heat transfer characteristics during the time-series process was obtained with an error of -43.5% to 37.5%. In the case of time-averaged process, the heat transfer coefficient increased with the increase
in the wiping frequency, increased first and then decreased with the increase in the vapor-to-suraface temperature difference, increased obviously with the decrease in the ethanol vapor concentration, and increased slightly with the increase in the vapor velocity
and pressure. The vapor-to-surafce temperature difference corresponding to the maximum heat transfer coefficient increased obviously with the increase in the ethanol vapor
concentration and increased slightly with the increase in the vapor velocity and pressure. Comparsion between the experimental results of the horizontal and vertical surfaces showed the strengthening effect of the gravity on the heat transfer process for Marangoni condensation decreased with the increase in the ethanol vapor concentration. The dimensionless experimental correlation for heat transfer characteristics during the
time-averaged process was obtained with an error of -28.2% to 18.0%.
Translated Keyword
[Horizontal Surface, Marangoni Condensation*, Time-averaged process, Time-series process]
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