Transient mass transfer and vortex bifurcation of an aerosol droplet in motion

An investigation of transient mass transfer in an aerosol droplet is performed in this study. The droplet is suddenly exposed to a polluted flow. Particular emphasis is focused on interfacial momentum and mass transport phenomena. When the aerosol droplet is under a specific Reynolds number (viz., Re_g = 120), it is discovered that there is a drastic interaction on the flow field between the gas phase and the liquid phase. Specifically, with increasing time the flow structure in the droplet proceeds from a single vertex to a double vortex and finally back to a single vortex. For the double vortex it is constructed by a primary and a secondary vortex. During the appearance of the secondary vortex, it is called a transient vortex bifurcation. Because of the variation of the flow field described above, the entire solute uptake process is divided into three subperiods, consisting of the initial period (or time lag period), the vortex bifurcation period, and the single vortex period. Furthermore, it is found that flow interaction has a dramatic influence on the interfacial mass transfer rate, notably in the vortex bifurcation period. As a whole, the distribution of the average Sherwood number decreases with time, except in the vortex bifurcation period, in which a slightly undulate profile is characterized. A correlation is given to illustrate the influence of the Reynolds number on the absorption process. In addition, when gas-liquid density and viscosity ratios vary, the transient solute uptake behavior can be interpreted by means of the droplet mass transport number.