document: This study proposes a new framework of a numerical modelling of the gas exchange between air and water across their interface, and subsequent chemical reaction in water based on an extended two-compartment model. The major purpose of this study is to provide a fundamental concept for modelling physicochemical processes of the gas exchange, followed by the chemical reaction in water. Demonstrating fundamental data and knowledge on the important environmental transport phenomena, especially the effects of the Schmidt number and the chemical reaction rate on the gas exchange mechanisms across the interface have also been attempted. The gas exchange processes are separated into two physicochemical substeps, the first is the gas–liquid equilibrium between the two phases, and the second is the chemical reaction in the water phase. A first-order, irreversible chemical reaction of the gaseous material after its uptake into the water phase is assumed here to simplify interactions of the chemical reactions and turbulent transport phenomena in water. While a traditional two-compartment model assumes uniform concentration of a material in each compartment, the present two-compartment model uses a computational fluid dynamics (CFD) technique in the water compartment to evaluate temporal development of three-dimensional profiles of the velocity and concentration fields. A direct numerical simulation (DNS) approach is used to evaluate profiles of fluid velocities and concentrations in water, and several important turbulence statistics have been evaluated without using turbulent closures, and subgrid-scale models. We assume that a fluid flow in the water phase is a well-developed turbulent water layer of a low Reynolds number, and the Schmidt number is varied from 1 to 8 to observe the effects of the molecular diffusion of the gas in sub-interface water on the gas exchange rate at the interface. Six degrees of the nondimensional chemical reaction rate are used to find the effect of the chemical reaction rate on the gas exchange mechanisms. Extrapolations of the gas exchange rates and the related transport phenomena toward larger Schmidt number and the faster chemical reaction rate will also be examined to predict the gas exchange processes of the actual gases of Sc∼O(102) based on results from the present numerical experiments.
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