Turbulent mass transfer across an air-water interface at high Schmidt numbers

Direct numerical simulation of coupled air-water flow mass transfer was carried out by employing the Eulerian and Lagrangian approaches at moderate and high Schmidt numbers from 1 to 1000. Three different flow conditions were chosen, i.e., the Reynolds numbers of gas and liquid phases, which were based on the interfacial friction velocities and depths, were set to be 150, increased to 300 only in the gas phase, and then 300 in both phases. In the liquid phase, the correlation coefficient between the scalar and the normal velocity fluctuation near the interface is found to be high (∼0.5) even at high Schmidt numbers. Quasi-streamwise vortices, whose velocity and length scales are determined by shear units, penetrate a very thin concentration boundary layer and govern the interfacial mass transfer. The resultant mass transfer rates are almost independent of the Reynolds number and inversely proportional to the square root of the Schmidt number of the liquid.