The temperature jump at water – air interface during evaporation

Abstract

The temperature profiles are measured across a liquid–gas two-layers system at normal atmospheric conditions. A thin water layer is locally heated from the bottom substrate and it evaporates from the liquid–gas interface. A micro-thermocouple with sensor thickness of less than 4μm has been specially manufactured for the accurate measurement of the temperature profiles. This micro-thermocouple is displaced with micro-steps near the interface, providing the detailed information on the temperature field. A temperature jump at the liquid–gas interface is clearly detected even for small evaporation rate. This jump is measured for heater temperature varying in the range 25–60°C at normal atmospheric conditions. The temperature jump value is found to increase with increasing the temperature difference between heater and ambient gas, and, hence, with increasing of the evaporation rate. A specific evolution of the temperature profile with increasing of the heater temperature is obtained. Depending on the ambient condition, the temperature in the gas phase near the liquid–gas interface can be higher or lower than that of the liquid. The temperature profiles with negligible temperature jump at liquid–gas interface are observed for some operating conditions. The temperature jump depends not only on evaporation rate, but also on temperature gradients in liquid and gas phases near the interface. The experimental results are found to be qualitatively in agreement with the kinetic theory and quantitatively with classical energy balance on the interface. The reported detailed data on the phase transition phenomena for relatively high heat flux are presented for the first time in the literature. However, more precise measurements of the temperature profiles at the liquid–gas interface should be done further.