Microscale transport in evaporating thin liquid films

Heat pipes, vapor chambers and thermosyphons are examples of two-phase devices used in electronics cooling which rely on latent heat exchange. In these devices, a wick structure forms the main unit that provides for evaporation and capillary pumping. An evaporating meniscus is fundamentally responsible for heat transport in these devices. Thin-film evaporation is the evaporation taking place in a few hundred nm region near the solid-liquid-vapor junction of meniscus and delivers heat transfer coefficients of up to 106 W/m2K. This intensive evaporation near the triple line creates a temperature gradient along the meniscus resulting in a surface tension gradient that gives rise to thermocapillary convection. Both the thin-film evaporation and the thermocapillary convection induced have been reported in the literature to play a major role in the total heat transferred. These transport mechanisms are functions of capillary structure, pore size, liquid properties and applied heat flux. But the exact nature of the relationship is not known and requires detailed experimental and theoretical investigation to establish a functional relationship. To understand this relationship, evaporation heat transfer in three different geometries - microcapillaries, microchannels and droplets - is experimentally investigated. Micro-particle image velocimetry measurements of the three-dimensional convection patterns generated near an evaporating meniscus in unheated, horizontally oriented capillary tubes are presented. The relative influence of buoyancy and thermocapillarity on the flow is studied. The local mass transport in a 100 - 400 μm region near the contact line of a water droplet of radius 1810 μm on an unheated glass substrate is experimentally quantified to show the efficiency of contact line heat transfer. Microscale infrared thermography is used to demonstrate the cooling effect due to thin-film evaporation in a meniscus in a channel. A detailed numerical model is developed that describes heat and mass transfer from a meniscus into an air-ambient. Evaporation at the interface is modeled using kinetic theory, while vapor transport in air is computed by solving the complete species transport equation. The numerical results obtained show satisfactory agreement with experimental data. It is shown that the limiting resistance in the domain is the evaporative resistance at the interface and that the thickness of the film does not play a significant role in determining the heat and mass transfer. The present generalized model may easily be extended to other geometries and hence may be used in the design of two-phase cooling devices. Contrary to the general understanding that thin-film evaporation and thermocapillary convection are the dominant modes of heat transfer in water-based systems, this work shows that it is not true