INFLUENCE OF INTERNAL CIRCULATION ON ABSORPTION OF ATMOSPHERIC CO2 AND SO2 IN RAINDROP

Motion of dispersed liquid drops in an immiscible fluid can cause circulatory motion within the drop due to shear forces at the interface. This internal circulation can enhance the rate of mass transfer through the interface. The hydrodynamics associated with different drop sizes are investigated and the strength of internal circulation is observed to increase with increase in drop size. The influence of internal circulation on rate of mass transfer is investigated numerically for various combinations of Reynolds number and Schmidt number using a computational fluid dynamics (CFD) simulation tool, ANSYS FLUENT. Mass transfer within the fluid drop is observed to consist of three regimes, initial and final diffusion-dominated phases with a convection-dominated phase in between. Removal of CO2 and SO2 present in the atmosphere by absorption in falling raindrops under isothermal conditions is also studied numerically. The influence of internal circulation on rate of absorption was studied by considering water drops of 1 mm and 0.5 mm falling in air. Mass transfer resistance in air is neglected, and interface concentration is obtained by using Henry's law. The evolution of mass fraction is observed to be similar for both CO2 and SO2. Absorption of SO2 is slower than that of CO2 in spite of the higher interface concentration of CO2. The reduction in time taken for absorption by halving the drop size is slightly more than what is expected, which may be due to the reduced internal circulation associated with smaller drops.