Effects Of Surface Topography Of Material On Initial Dropwise Condensation

There's close relation between the geometry structure characteristic of a condensation surface and its heat transfer performance for dropwise condensation. And there are few publications about surface topography with its effect on the initial droplet formation in nanometer scale, while the related research only is restricted in micron size. In order to seek the relationship between the surface topography and the formation of initial droplet, magnesium was applied as condensation surface in this study since it can react with hot water (condensate) and thus make the chemical compositions of the surface changed, thus leave the nucleation sites on the surface.First, magnetic-control sputtering (MCS) was used to prepare the magnesium surfaces with nanometer appearance to record the reaction marks on the surface. The topography of the prepared magnesium surfaces and their roughnesses were characterized with the atomic force microscope. It is indicated that the average roughness of the surface is about 23nm. Additionally, the thickness of the magnesium film plated with MCS method was measured with the electron probe micro-analyzer. The thickness of the plated film was 21μm, which meant that the substrate surface was completely covered by magnesium. Therefore, the magnesium surfaces prepared with the MCS could be used in the followed condensation experiments. Furthermore, based on the topography photographs of magnesium surfaces using scanning electron microscope, the fractal theory was applied to describe the irregularity and complexity of magnesium surfaces quantitatively. Then the differential box-counting was used to calculate the fractal dimension of these magnesium surfaces before condensation experiment.After that, dropwise condensation experiments were carried out under nearly same condensation parameters on these surfaces with different topography prepared under different MCS parameters. The initial dropwise condensation on the magnesium surfaces can be achieved by controlling the subcooling and the contacting time between the steam and the magnesium surface. The variations of the chemical composition of the surfaces were measured with EPMA and SEM before and after condensation. The increase of the oxygen represented the quantity of condensate formed on the surfaces. The oxygen element distribution was analyzed with the image processing technology, then we could infer the nucleation site density.The results show that the surface topography effects the initial dropwise condensation significantly. When the surface fractal dimensions are different, the nucleation sites for dropwise condensation on the surfaces are distinct. A new formula was proposed based on the experiment to reflect the quantified relation between the fractal dimension of a surface and the nucleation site density.