Gas-phase diffusion in droplet train measurements of uptake coefficients

Experiments were conducted to study the nature of gas-phase diffusive trans port to a stream of fast moving droplets in a droplet train apparatus used for measuring gas uptake coefficients. The stream of droplets is produced b y 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 numb er (Kn) as transport to a stationary droplet, except that the droplet diame ter 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 fact or 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 m um in diameter. That is, for a given orifice the effective Kn for a train o f moving, closely spaced droplets produced at frequencies in the range simi lar to4-60 kHz, depends only on the orifice diameter and not on the diamete r of the droplets. Using this formulation, mass accommodation collision pro babilities and liquid-phase solubilities and reaction kinetics have been me asured for wide range of gases and liquids. (C) 2001 Elsevier Science Ltd. All rights reserved.