Gas-phase diffusion in droplet train measurements of uptake coefficients

Abstract

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 ( γ 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 μm producing droplets in the size range from ∼70 to 300 μm 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 ∼4– 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.