Numerical simulation on circulation flow and mass transfer inside atmospheric water drops

When a water droplet moves in atmosphere with pollutant, internal circulation is formed due to surface shear stress. This enhances internal mass transfer greatly, and improves the spray droplet SO2 absorption. In this paper, the internal circulation and diffusion of SO2 in a water droplet were numerically studied. The distribution of tangential velocity at the interface and the effect of interior circulation on sulfur dioxide transfer are analyzed under different Reynolds numbers. The numerical results indicate that there are two symmetrical vortexes inside the droplet when there is a relative motion between gas and liquid phase. The distance between the vortex core and the droplet center is around 2/3Rd, and the vortex velocity increases with the Reynolds numbers. The study shows sulfur dioxide absorption by the droplet is controlled by two mechanisms, which are (1) the radial diffusion due to concentration gradient and (2) mass transport induced by internal circulation. The characteristic times of radial diffusion and vortex formation are compared. The comparison indicates that the internal circulation dominates sulfur dioxide mass transfer inside the water droplet. The internal circulation influences the sulfur dioxide mass transfer greatly with the increase of Reynolds number. On the other hand, the effect of deformation rate on mass transfer is insignificant because of the characteristic time are of the same order with the same Reynolds number.