Evaluation of the different definitions of the convective mass transfer coefficient for water evaporation into air

In literature different definitions of the convective mass transfer coefficient are used by different authors. The definitions differ in the driving force used to describe mass transfer. In this paper, the limitations to the use of convective mass transfer coefficients related to four commonly used driving forces (vapour density, mass fraction, vapour pressure and mole fraction) are studied for evaporation of water into air. A theoretical study based on the adiabatic saturation process and a numerical CFD study of an existing evaporation experiment show that the use of convective mass transfer coefficients related to vapour densities is only allowed under isothermal conditions while convective mass transfer coefficients related to vapour pressure show a dependence on the total gas pressure. The use of mole or mass fractions as driving force results in values for the transfer coefficient which are little affected by the thermodynamic properties such as temperature, relative humidity and total pressure and are hence better suited to describe convective mass transport.