The role of interfaces in heat transfer processes

Abstract

An understanding of the role of interfaces between phases is essential to true understanding of many heat transfer processes. In the cases of evaporation ion and condensation in single component systems, the evaporation or condensation coefficient represents the transmitance of the vapor-liquid interface itself or the maximal possible rate of interfacial heat as well as mass transfer. Jet tensimetry is a simple but powerful technique for the study of the resistance of the interface in the evaporation and condensation processes, with or without chemical reaction, involving the interface between phases. This technique also eliminates most of the complications of the bulk phases on both sides of the interface. Get tensimetry and other recent experimental data verify that the evaporation coeffecients of water and other common liquids are unity or nearly unity; the interface does not constitute a significant resistance to the heat transfer processes.Boiling and condensation are two processes with very high heat transfer coefficients, especially in the cases of nucleate boiling and dropwise condensation. An examination of the vapor-liquid interface under equilibrium conditions shows that there are many similarities between these two processes from the thermodynamic point of view. However, since the creation of a new phase is a problem of stability, more attention should be paid to the dynamic rate of the nucleation of the vapor bubbles and condensate droplets. The solid heating and cooling surfaces introduce more complications because they are usually not smooth and homogeneous. Vapor bubbles and condensate droplets are often generated at the position of active sites. Because most of the solid heat transfer media are hydrophilic, the nucleation theory shows that from the point of view of interfacial phenomena: 1) nucleate boiling is usual but film boiling is exceptional: and 2) film condensation is usual but dropwise condensation is exceptional.Dropwise condensation can be realized by making the solid cooling surface hydrophobic with the help of promoters. Its mechnism is complicated but relactively well understood after the experimental studies and theoretical simulation carried out by many authors. Their conclusions are discussed with emphasis on the solid-liquid-vapor interfacial properties. The condensation on the surface of an immiscible liquid is also discussed because of its similarity to dropwise condensation. The treatment of dropwise condensation problems by nucleation theory and statistical methods is also introduced.