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Effect Of Surface Characteristics Of Material On Initial Droplet Formation And Heat Transfer Of Dropwise Condensation

Posted on:2009-04-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:C F MuFull Text:PDF
GTID:1101360242984636Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
The mechanism of the formation of initial condensate droplets for dropwise condensation is still in suspense. To solve the problem, we must understand if the initial condensation is in nucleus or in thin film in nanometer scale, since the calculation of the thermodynamics for new phase formation indicates that the size of the initial condensate nucleus is in nano-scale. Magnesium was applied as the condensation surfaces in this study since it can react with hot water (condensate) and thus leave marks of initial condensate state on the surface. In the experiment, the subcooling and reaction time were controlled to realized the initial condensation on the surfaces, and then an electron probe microanalyzer (EPMA) and scanning electron microscope(SEM)were used to scan the variations of the chemical compositions as well as their distribution on the surfaces before and after the initial condensation. These consequences were used to deduce whether the initial condensate state is in nuclei or in film.In this paper, mechanically polishing and magnetic-control sputtering (MCS) were used firstly to prepare the magnesium surfaces. The topography and surface roughness of the magnesium surfaces were characterized with the atomic force microscope and the results show that the average roughness of the mechanically polishing surface is about 100nm, while that of the MCS surface is about 23nm. The thickness of the magnesium film plated with MCS method was also measured with the electron probe microanalyzer, and the thickness of the plated film is 23μm, which meant that silicon substrates were covered by magnesium completely. So the films of magnesium were feasible for this condensation experiment, and they can meet the requirements of surface scan by electron probe microanalyzer.In the followed experiment, the initial condensation process was realized by controlling the subcooling and reaction time, and then, EPMA and SEM were used to scan the variation of the chemical compositions on the magnesium surfaces. The results showed that the oxygen contents on the test surfaces increased with subcooling and condensation time obviously after the initial condensation, and the oxygen on the test surface distributed non-uniformly. Moreover the results from the scan of EPMA and SEM were nearly identical, which implys that the method presented in the paper is reliable. Therefore, the reaction marks on the surface are the true display of initial condensate state. Meanwhile, the reaction dynamic equation of magnesium and water was set up and used to calculate the area occupied by initial condensate, which was well agreed with the measured results of EPMA. All these consequences indicate that the initial condensate forms in the nucleus state on solid surfaces, not in the state of thin film, and the size of the initinal droplet is 3 to 10nm. This is the first investigation that the initial condensation droplets in nanometer scale were displayed and the mechanism of initial droplet formation for dropwise condensation in nano size was confirmed.Furthermore, in view of the fact that there's close relation between the geometry structure characteristic of a condensation surface and its heat transfer performance for dropwise condensation, but there is no qutantative relation yet between surface topography and nucleation site density, SEM was used again to obtain the topography photographs of magnesium surfaces with different surface characteristics prepared with distinct MCS parameters. Then fractal theory was applied to describe the irregularity and complexity of magnesium surfaces quantitatively. And the differential box-counting was used to calculate the fractal dimension of these magnesium surfaces before condensation experiment. The initial dropwise condensation on these magnesium surfaces was then achieved by controlling the subcooling and the contacting time between the steam and the magnesium surfaces. The nucleation site density can then be obtained after the oxygen element distribution was analyzed with EPMA and the image processing technology. And the quantitative relation between the nucleation site density and surface fractal dimension was thus correlated.The results show that the surface topography effects the initial dropwise condensation significantly. When the surface fractal dimensions are different, the number of nucleation sites for dropwise condensation on the surfaces is distinct. And both the theoretical and the experimental results indicated that the larger the surface fractal dimension, the greater the heat flux of dropwise condensation.Finally, in consideration of the interfacial interaction between liquid and solid, a new model for dropwise condensation heat transfer was proposed by using the population balance concept and considering the effect of surface topography and contact angle. The calculation results indicate that dropwise concdensation can be influenced considerably by surface topography and contact angle. Under the same subcooling, the greater the surface fractal dimension, the larger the nucleation site density and the higher the heat flux of dropwise condensation. Also the effect of fractal dimension on nucleation site density and heat flux becomes more obvious as subcooling increases. Meanwhile the heat transfer rate through a single droplet increases with contact angle firstly and then decreases, i.e. there is an optimum contact angle to make the heat transfer rate of the droplet maximum. The optimum contact angle is about 120°for the droplets with radius less than 1 micron, and the value almost does not vary with the increase of radius. The optimum contact angle is 119.1°for the droplet with radius 1 microns. When the droplet radius is larger than 1 micron however, the optimum contact angle decreases with the increase of droplet radius obviously. Moreover, the heat flux of dropwise condensation on an entire surface also varies obviously with contact angle when the departure of condensed drops is considered. And the optimum contact angle value becomes 87.6°in this case. Our resutls show that there exists a proper contact angle for dropwise condensation, and it is not true that the larger the contact angle, the higher the heat flux of dropwise condensation.
Keywords/Search Tags:Dropwise condensation, Initial condensate, EPMA, Magnetic-control sputtering, AFM, SEM, Surface Topography, Fractal Dimension, Nucleation Site Density, Heat transfer model, Optimum contact angle
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