Effects Of Surface Characteristics On Initial Droplet Distribution And Heat Transfer Model Of Dropwise Condensation

Dropwise condensation is the best approach to enhance heat transfer by means of surface tension. However till now, it hasn't being applied widely in industry and the theoretical model concerned has not been well recognized. In order to further study the theory of dropwise condensation, the effects of surface topography of material and apparent contact angle on heat transfer in dropwise condensation are investigated in this paper based on the theoretical model of dropwise condensation established by former researchers.In the process of dropwise condensation, small droplets grow by direct condensation and large ones grow by coalescence. And the small droplets play great roles in heat transfer of dropwise condensation. With the initial dropwise condensation experiments on magnesium film, the correlative equation between condensation surface fractal dimension and nucleation site density was firstly established in this study. Then the liquid droplet population balance concept was applied to calculate the initial droplet distribution. Thus a new theory model for droplet distribution of dropwise condensation was established combined with the well-established drop-distribution proposed by Rose for large droplets.Most researchers suppose the contact angle is 90 degree when establishing heat transfer model for dropwise condensation. However, there is an intensive relationship between contact angle and surface energy. And different surfaces have different contact angles, which effects heat transfer in dropwise condensation obviously. Base on the analysis of heat transfer through a single drop and the introduction of contact angle into calculation, a model for heat transfer in dropwise condensation affected by contact angle was established. The model can also be used to study the effect of contact angle variation on heat resistances through a single drop and the heat flux of condensation surface when droplets begin to fall.The results show that the surface topography of material has a great influence on nucleation site density and heat flux in dropwise condensation. The larger the surface fractal dimension, the higher the nucleation site density and the greater the heat flux. Through the study of the effect of contact angle on heat transfer in dropwise condensation, it has been demonstrated that an optimum contact angle in dropwise condensation exists from the view of heat transfer.