Mechanisms Of The Thin Film Evaporation In The Evaporator

Due to a high heat transfer coefficient, the thin film evaporation has a broad prospect and development space in ship refrigeration industry. A model based on the augmented Young-Laplace equation and the Clausius-Clapeyron equation was developed to describe the extended evaporating meniscus in a micro channel. The effects of the adsorbed film thickness, channel height and temperature-dependent thennophysical properties of the fluid are included in the model at wall superheats up to 50 K. when a liquid wets a superheated solid surface, the extended meniscus is typically divided into the non-evaporating region (adsorbed region), evaporating thin film region and the intrinsic meniscus region. Of these, the thin film region is characterized by high heat transfer rates because of the very low thermal resistance across the liquid film, it has a high heat transfer coefficient. The liquid flow is coupled with the vapor flow to obtain the mass transport across the liquid-vapor interface. The results show that the constant thermo-physical property model greatly overestimates the liquid pressure difference and the total thin film heat transfer rate at higher superheats compared with the variable thermophysical property model. The adsorbed film thickness, which is controlled by the disjoining pressure limit, reaches a minimum near about 20 K superheat for water. The maximum film curvature and liquid pressure difference then decrease at superheats larger than 20 K. The apparent contact angle, disjoining pressure gradient and the local maximum heat flux increase by increasing the superheat. Increasing superheats tend to enhance the fluid flow and heat transfer in the thin film evaporation region. The Hamaker constant A has effect on the thermophysical properties of thin film evaporation. Increasing wall temperature also tend to enhance the fluid flow and heat transfer in the thin film evaporation region. A better understanding of the heat transport mechanisms in the thin film region will directly help the designs of highly efficient evaporator and advanced thermal management systems. The numerical investigation presented in this report can be used as a reference to design a better thin-film evaporator working in an optimizing temperature range.