Heat And Mass Transfer Characteristics Of The Evaporating Meniscus In Capillary Tubes

A computational model was developed to investigate heat and mass transfer characteristics for the evaporating meniscus in capillary tubes. Effects of radius, the magnitude and distribution of heat flux on the evaporating meniscus, buoyancy force on flow structures in capillary tubes were investigated. And the characteristics of thermocapillary flow and physical mechanism of oscillatory three-dimensional flow were reported.The simulation results indicate that the evaporation on meniscus is non-uniform, and the heat flux increases along the radial direction. The non-uniform evaporation produces a non-uniform temperature distribution along the meniscus. Therefore thermocapillary flow is induced in capillary tubes. With given magnitude and distribution of heat flux on the evaporating meniscus, the flow strength gets stronger with larger radius. With given average heat flux and radius, the magnitude of the lowest temperature on meniscus is smaller as the heat flux changes faster along the radial direction. Meanwhile the location of the region with the lowest temperature moves toward the wall of the tube. In vertically oriented capillary tubes the effect of buoyancy force on thermocapillary flow is negligible when radius is between 0.1mm and 1mm. In horizontally oriented capillary tubes the effect of buoyancy force is negligible when radius is smaller than 0.5mm, and the asymmetrical flow is obtained when radius is larger than 0.5mm. And the asymmetry gets stronger with larger radius. When the average heat flux is larger, the magnitude of the lowest temperature on meniscus is smaller and the flow strength gets stronger. When the average heat flux is lager than a critical value, the steady axisymmetric flow will undergo a transition to a steady asymmetry flow. When the average heat flux still increases and gets lager than another critical value, the flow will turn to be three-dimensional oscillatory flow. The oscillatory flow is a combined result of velocity field and temperature field which causes the delay between the flow on meniscus and the recirculation flow.