Dynamics of sulfur dioxide absorption in a raindrop falling at terminal velocity

Sulfur dioxide absorption dynamics in a raindrop are studied numerically by means of a fully numerical simulation method (FNSM) in which a composite orthogonal grid system consisting of both gas- and liquid-phase is adopted. When a raindrop with fixed radius falls in association with terminal velocity, a recirculation bubble always accompanies the gas-phase flow field in the aft region of the drop. With regard to the drop internal flow structure it has a drastic variation with drop size. When the drop radius is small (e.g. r_s=200μm), only a single vortex motion is seen inside the drop. Under such a situation, sulfur dioxide absorbed from the interface is mainly transported from the area in front of the aft stagnation point. In contrast, as the drop is relatively large, say, r_s=500μm, it is of interest to find that a double-vortex motion, composed of a primary and a secondary vortexes, is clearly observed. As a result, the onset of SO₂ transport process occurs at the drop's surface near where the two vortexes meet. By defining a drop mass transport number, it indicates that the mass transported via internal circulation is always much faster than that by mass diffusion and the latter is highly relevant to the drop radius. Accordingly, the SO₂ transport dynamics in a raindrop is essentially determined by drop size.