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Thermocapillary convection in evaporating thin liquid films
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posted on 21.02.2017by Lim, Elaine
The thermocapillary convection is induced along a liquid-vapor interface due to surface tension gradient which is temperature dependent. Due to the large increase in surface area relative to volume, surface tension has a dominant effect on the fluid behavior in a capillary structure. Evaporation of a thin film induces temperature gradient and hence surface tension gradient along the liquid-vapor interface, leading to Marangoni convection which is regarded as thermocapillary flow. The evaporation of a liquid thin film is governed by the transport processes of the working fluid, thus the selection of appropriate working fluid for a designated range of operating temperature is of vital importance in delineating the thermal performance of a two-phase capillary structure. The impact of thermocapillary effect on the thermal and fluid flow behaviors of an evaporating thin liquid film is investigated in this thesis. By employing the long-wave evolution model, a mathematical model based on first principles for fluid flow and heat transfer is derived for the interface shapes that govern the thickness of the evaporating thin film. The two-dimensional information of the liquid temperature which is a prerequisite for the incorporation of the thermocapillary effect can be obtained. The evaporation rate is overrated when the thermocapillary effect is neglected and the overestimate increases with increasing excess temperature.
The theoretical model is further deployed to investigate the effect of thermo-physical properties on the impact of thermocapillary effect on different types of working fluids, i.e. polar and non-polar liquids. The temperature gradients induced by different degree of thermo-physical properties are closely associated with the Marangoni number in justifying the significance of thermocapillary effect in an evaporating thin liquid film. Surface tension gradient induced by temperature gradient is usually in the negative linear variation form. The thermocapillary flow provoked by the negative surface tension gradient retards the liquid circulation and impedes evaporation process. Water portrays a unique thermocapillary characteristic where the thermocapillary flow manifests in the form of swirls along the liquid-vapor interface at a large excess temperature. By introducing the dilute aqueous solution (DAS) of long-chain alcohol, the temperature dependence surface tension can be modified into a quadratic function to analyse the thermocapillary action engendered by the positive surface tension gradient of DAS in the evaporating thin liquid film. To investigate the thermocapillary effect on the thermal and fluid flow characteristics of the pseudoplastic fluid, the Ostwald-de Waele power law model is employed. The power-law model is developed with the power-law index ranges between 0 to 1 for pseudoplastic (shear-thinning) fluids. The significance of thermocapillary effect in the pseudoplastic fluid is determined by the critical Marangoni number. Results show that the total heat transfer rate of both Newtonian and pseudoplastic fluids is overestimated with the presence of thermocapillary effect and the overestimate is comparatively pronounced in the Newtonian fluid compared to the pseudoplastic fluid. This study reveals the conditions under which the thermocapillary effect is significant and should not be neglected in the heat transfer analysis of an evaporating thin film.