Effects Of Interfacial Interaction On Condensation Heat Transfer

The condensation heat transfer process of binary mixture of steam and ethanol vapor as well as pure steam were studied experimentally with various vapor concentrations and surface subcooling at the pressure around atmospheric. With changing of the surface free energy difference through different solid surfaces or vapor component, it is found that the condensation modes altered from filmwise, to transition and finally to dropwise with respect to the operation conditions. The concept of surface-free-energy-difference is introduced to explain the mechanism of the conversion of the condensation modes and the enhancement of heat transfer due to the interfacial interaction. From this viewpoint, the condensation characteristic curves of binary mixture of steam and ethanol vapor can be explained reasonably. Based on the data of the present paper and those reported in literatures, the conversion criterion of the condensation modes is proposed. The condensation state is filmwise for the surface-free-energy-difference less than 14±1 mJ.m^-2 and dropwise for more than 21±1 mJ.m^-2 and transition for between the above two. From the experimental results, it can be simply concluded that the interfacial interaction affects the moving of the condensates and and thus affects the heat transfer.By studying the moving of the condensates with the effect of the interfacial interaction, the departure droplets, the rivulets, and the droplet size distribution were analyzed. Firstly, this paper deals with the relationship of the departing diameter of a liquid drop and the shape of contact line on a vertical solid plate in the gravitational field. The experimental photoes showed that the contact line is not circular or elliptical, but in an irregular shape. The combination of two ellipses is proposed to model the shape of the contact line. Simultaneously, equations for the departing diameter of droplet are derived in terms of the force balance between gravity and surface tension. For a specified liquid and substrate, the departure diameter is related to the equilibrium, advancing and receding contact angles. The effects of these factors on departing diameter have been modeled in this paper. Secondly, based on the principle of the minimum total energy (MTE), the analysis of the laminar liquid rivulet flowing downward along a vertical plate is conducted theoretically and experimentally. The contact angle hysteresis of the solid surface is taken into account in the boundary conditions for the governing equations. The mechanism and possibility of the wave occurrence at the free surface of the rivulet are discussed. In addition, the maximum liquid film thickness of the rivulet influenced by the contact angle hysteresis is experimentally measured. The analytical results and the visual photos show that the wave at the free surface...