Gas-phase diffusion in droplet train measurements of uptake coefficients

Experiments were conducted to study the nature of gas-phase diffusive transport to a stream of fast moving droplets in a droplet train apparatus used for measuring gas uptake coefficients. The stream of droplets is produced by forcing liquid through a vibrating orifice. Experiments over a wide range of Knudsen numbers (Kn; 0.05-4.5), gas mixtures, and uptake coefficients (gamma (0); 0.01-1), show that gas-phase diffusive transport to a stream of fast moving droplets has the same functional dependence on the Knudsen number (Kn) as transport to a stationary droplet, except that the droplet diameter in the expression for Kn must be replaced by a factor measured to be 2.0 (+/- 0.1) times the diameter of the droplet generating orifice. This factor has been measured for droplet forming orifices of diameters in the range 22-70 lan producing droplets in the size range from similar to 70 to 300 mum in diameter. That is, for a given orifice the effective Kn for a train of moving, closely spaced droplets produced at frequencies in the range similar to4-60 kHz, depends only on the orifice diameter and not on the diameter of the droplets. Using this formulation, mass accommodation collision probabilities and liquid-phase solubilities and reaction kinetics have been measured for wide range of gases and liquids. (C) 2001 Elsevier Science Ltd. All rights reserved.