Effect of surface contamination on interfacial mass transfer rate

Abstract

The influence of surface contamination upon the mass transfer rate of a low diffusivity gas across a flat surface is studied using direct numerical simulations. The interfacial mass transfer is driven by isotropic turbulence diffusing from below. Similar to Shen et al. (J. Fluid Mech., vol. 506, 2004, pp. 79–115) the surface contamination is modelled by relating the normal gradient of the horizontal velocities at the top to the horizontal gradients of the surfactant concentrations. A broad range of contamination levels is considered, including clean to severely contaminated conditions. The time-averaged results show a strong correlation between the gas transfer velocity and the clean surface fraction of the surface area. In the presence of surface contamination the mass transfer velocity $K_{L}$ is found to scale as a power of the Schmidt number, i.e. $Sc^{-q}$, where $q$ smoothly transitions from $q=1/2$ for clean surfaces to $q=2/3$ for very dirty interfaces. A power law $K_{L}\propto Sc^{-q}$ is proposed in which both the exponent $q$ and the constant of proportionality become functions of the clean surface fraction.