Analysis of the evaporation coefficients of water, heavy water, and methanol in a high vacuum environment

Abstract

The evaporation coefficient of liquids is an important factor for many practical situations but has been poorly understood due to the complexity of the experiments and uncertainty in the measurements. Indeed, the lack of current knowledge about the nature of the evaporation coefficient prevents accurate confirmation of its value. We have determined the evaporation coefficients of three liquids, water, heavy water, and methanol, by fitting Knudsen's evaporation rate equation to past measurements of the temperature of droplet trains and liquid jets evaporating in a high-vacuumed environment. Analysis of available experimental data suggests the value of 1 for the evaporation coefficient (αe) of water, and 0.725 for that of heavy water. A small-jet methanol study predicts αe≈1; however, analysis of experimental data from a large-jet methanol study was an exception—indicating coefficients well over unity (1.5<αe<9). These exceed the suggested maximum theoretical value of unity. We found that considering αe=1 for water, and most of the test fluids, is a fair approximation in a high-vacuumed environment that presents minimal kinetic barriers to evaporation. These results confirm that water should not be considered an anomalous liquid with respect to evaporation. This study provides insight that will help improve the design and development of the many processes in which evaporation plays a key role.