Thermophysical phenomena of working fluids of thermocapillary convection in evaporating thin liquid films

摘要

The progressive evaporation in a thin liquid film is sustained by the circulation of its working fluid. Due to the surface tension gradient which is temperature dependent, thermocapillary flow is induced and the evaporation is impeded. We employ the long-wave evolution theory to derive a two-dimensional mathematical model based on the first principles for polar and nonpolar liquids. The thermocapillary disparities among different types of working fluid are explicitly observed and the changes entailed in the heat transfer characteristics are scrutinized. By comparing the models with and without thermocapillary effect for different types of working fluid, it can be observed that the evaporative mass and heat rates are overrated when thermocapillary effect is neglected and the overestimates increase with increasing excess temperature. For different types of working fluid under the same operating conditions, the variations of thermo-physical properties induce different temperature gradient magnitudes in the liquid phase, incurring a different degree of thermocapillary effect on the heat transfer characteristics. The significance of surface tension gradient in thermal performance of working fluid is justified. This study evinces that by neglecting the thermocapillary effect, acute errors can be incurred even at a sufficiently small excess temperature for certain types of fluid in the heat transfer analysis of an evaporating thin liquid film.