Methods for Predicting Supersaturation in a Laminar Flow
Egor V. Demidov, Alexei F. Khalizov
AMS, Oral presentation, 2022
Abstract
Aerosols in the atmosphere age by several processes and condensation is one of them. Typically, condensation leads to increased size and water-solubility of particles, thus making them better Cloud Condensation Nuclei (CCN). Optical properties are also affected – coated aerosols tend to scatter light better than their fresh counterparts. Absorbing aerosols, such as soot, also show absorption enhancements after acquisition of coating. Commonly, to investigate aerosol processing by vapor condensation in laboratory, the aerosol is sent sequentially through saturator and condenser. In a warmer saturator, aerosol flow becomes saturated with vapor. In a colder condenser, as the flow cools, vapor supersaturation occurs. The magnitude of supersaturation and spatial location of supersaturated region depend on the competition between heat and mass transfer. To design experiments and rationalize experimental findings, vapor concentration and vapor supersaturation often need to be predicted to evaluate the condensation rate and resulting particle size. In some cases, a one-dimensional model only considering axial profile is sufficient for modelling vapor supersaturation in the flow and vapor condensation on aerosol particles. In other cases, a more rigorous two-dimensional model that accounts for axial and radial vapor and temperature profiles may be required. We show that based on Lewis number (Le), which is a ratio thermal diffusivity of the medium to mass diffusivity of the condensing material, the highest supersaturation can occur not only at the hot-to-cold transition, but also at the cold-to-hot transition. The cold-to-hot transition is usually significant when Lewis number is lower than one. To verify our model calculations, we conducted experiments where Polystyrene Latex nanospheres (PSL) were exposed to supersaturated vapors of different compounds – dioctyl sebacate (pvap=24·10-18 torr, Le>>1), triethylene glycol (pvap=13·10-3 torr, Le>1), and water (pvap=24 torr, Le<1). The increase in particle diameter was measured experimentally with an electrostatic particle classifier. The one-dimensional model was able to accurately predict condensation of dioctyl sebacate and triethylene glycol, but not of water vapor. The two- dimensional model agreed with experiments in all three cases.