Experimental and theoretical investigations on interfacial temperature jumps during evaporation

Experimental results are summarized on investigations of positive temperature jumps at water–vapor interfaces during steady-state evaporation under low-pressure. Steady-state evaporation of water experiments were carried out to measure the interfacial properties and to obtain the evaporation rate. The interfacial vapor temperature close to the interface was always found to be higher than the interfacial liquid temperature. To study the influence of the vapor side thermal boundary conditions on the temperature jump, the evaporation chamber was heated with the help of a heating wire mesh which was mounted in the vapor side plane above the evaporating free surface. It was astounding to the authors to find that the temperature jump at the liquid–vapor interface increases linearly with the heat flux from the vapor side. The maximum temperature jump across the water–vapor interface was measured as 15.68 °C in the presence of vapor phase heating. Still higher temperature jump values can be achieved by applying higher vapor side heat fluxes close to the water–vapor interface. It was attempted to explain these unique experimental results using existing theories of evaporation. Kinetic theory of gases (KTG) predicts the temperature jumps, but the magnitude is 10–20 times smaller than the experimentally obtained temperature jumps. The linearized statistical rate theory yields the evaporation mass flux expression which is same as the KTG expression with evaporation and condensation coefficients of unity. Only non-equilibrium thermodynamics using phenomenological equations appear to predict the magnitude of the temperature jump measured in the experimental study. However, more theoretical work needs to be done to fully understand the new experimental findings reported here.