Ammonia absorption by water droplet in a small controlled atmosphere wind tunnel

The mass transfer characteristics during ammonia absorption into a sessile water droplet in a small controlled-atmosphere wind tunnel were investigated using particle tracking velocimetry (PTV) with polystyrene microspheres. The results showed that the flow field inside the droplet transformed from Marangoni convection to Rayleigh convection in the absence of gas flow. As the gas velocity increased, the drag force-induced circulation inhibited the Marangoni convection in the initial stage of the mass transfer process and Rayleigh convection in the advanced stage. However, the suppressed Marangoni convection cells recurred at high initial ammonia concentrations. Compared with the initial ammonia concentration in the wind tunnel and droplet size, the gas velocity significantly increased the gas-phase mass transfer coefficient. Furthermore, a calculation model for ammonia absorption by a moving droplet was derived based on the Colburn analogy and the two-film theory. The gas-phase mass transfer coefficients calculated using the model coincided with the experimental results. These results will help the design of water curtains.