Thermocapillary convection was investigated in pure water (H2O) and heavy water (D2O) during steady-state evaporation. By using a moveable 25.4-mum-diameter thermocouple, temperatures were measured in the vapour and liquid phases in three dimensions close to a spherical interface for H2O and D2O and to a cylindrical interface for H 2O respectively. A method was established to demonstrate the transition to thermocapillary convection by energy transport analysis at the interface. The transition is parameterized by the Marangoni number (Ma). When Ma 100, the interface was quies-cent with a uniform temperature. Thermal conduction to the interface provides sufficient energy required for evaporation. When 100 Ma 22, 000, a significant temperature gradient existed along the interface. A uniform temperature layer appeared in the liquid below the interface. The induced thermocapillary flow was verified by the visualization experiments using a 12.7-mum-diameter probe. Thermal conduction along could no longer satisfy the energy required for evaporation. By adopting the Gibbs dividing-surface approximation, the "surface-thermal capacity" is found to be a constant for the H2O and D2O experiments respectively. When Ma > 22, 000, the interfacial flow is turbulent, and the viscous dissipation becomes important. With the local evaporation flux and the measured interfacial temperatures in the liquid and vapour phases, statistical rate theory (SRT) was examined. The mean predicted vapour-phase pressure agreed with that measured for each experiment with H2O and D2O.
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