Microscopic Characteristic And Heat Transfer Mechanisms Of Dropwise Condensing Droplets

The heat transfer of steam condensation has broad applications in the fields of petro-chemical process, refrigeneration system, power generation, spaceflight and micro-electronic devices. For the increasing severe situation of energy demand, the improvement of the performance of the heat transfer equipment also is of importance in saving energy, raw materials and protecting the environment. Dropwise condensation with a higher heat transfer coefficient has aroused great interest since last century. A deep investigation into the microscopic mechanism of dropwise condensation would help the enhancement of the process and the innovation and exploitation of novel techniques. This paper mainly focuses on the phenomena of dropwise condensation of steam at low pressures, and the clustering model of steam condensation is established to illustrate the microscopic mechanism.Superhydrophobic coating of n-octadecyl mercaptan by controlled surface oxidation and self-assembled techniques is prepared on copper substrate. The topography of the superhydrophobic surface is analyzed by scanning electron microscope and atomic force microscope. A two-tier micro-/nanostructures were constructed on the copper substrate. The contact angle is measured to determine the hydrophobicity of the coating. The contact angle in the atmosphere is 157.18°. Perfect dropwise condensation of steam is realized.The contact angle of the coating in the pure steam environment is measured at different pressures with the same surface subcooling degree. The contact angle is almost the same as the steam pressure varied, but is lower than that in the atmosphere of air. The contact angle is the property of the surface, and is relevant to the physical and chemical properties of the surface. And the advancing and receding contact angles are tested at different pressures. The contact angle hysteresis is increased a little as the pressure is decreased. The increase of the contact hysteresis makes the extent of deformation of droplets increase, which impedes the clearing up of droplets and reduces the heat transfer performance.Stable dropwise condensation is realized on the superhydrophobic surface. By the specific designed condensation block, the motion characteristics of droplets are investigated using the high speed camera. The growth rate of droplets grown by direct condensation at. different pressures and different subcooling degrees are calculated by the model in the literature. The growth rate of droplets by direct condensation is lower when the pressure or the subcooling degree is decreased. That is bad for the condensation heat transfer. High-speed camera is applied to monitor the dynamic characteristics of droplets. The growth rate of droplets by coalescence is the same at different pressures. The critical radii of droplets at different pressures are calculated by the time sequence model. As the pressure is decreased, the minimum radius of droplets is smaller, while the critical radius and the departure radius are getting larger, which means that the time of droplets' growth by direct condensation and that mainly by coalescence are getting longer. Therefore the life cycle of droplets is getting longer, that is to the disadvantage of condensation heat transfer.A physical and mathematical droplet model was proposed for condensation heat transfer process near the cooled solid surface, according to micro physical mechanism and thermodynamic characteristics in phase change process. The model based on the refined DM homogeneous nucleation model, introducing the wall conditions and making some correlations, was used to calculate the sizes and distribution of clusters, and also describe the effect of the presence of non-condensable gases on the distribution of clusters. The model presented explained quantitatively the fact of presence of small amount of non-condensable gases deteriorating condensation heat transfer performance significantly. The predictive results of the model made an agreement with the experimental results in the literature.