Particle activation and droplet growth processes in condensation nucleus counter—I. Theoretical background

Abstract

The heat and mass transfer, and droplet growth processes in a continuous flow condensation nucleus counter (CNC) have been studied numerically by solving the equations of convective heat and mass transfer, and the equation of vapor transfer to the growing droplets. The influence of sampling flow rate and the saturator and condenser wall temperatures have been studied. The thermophysical properties of the carrier gas have also been systematically varied to determine their influences on the performance of the CNC. Carrier gasses studied include air, argon and helium. Calculations have been made for conditions similar to those used in the commercially available CNC manufactured by TSI Inc. (model 3020). The results show that the constant supersaturation ratio isopleth in the condenser tube of the instrument has an elongated shape and the elongation is increased in the flow direction as the sampling flow rate is increased. This implies that the activation efficiency does not depend on the sampling flow rate, as long as the highest supersaturation region is within the condenser tube. A few per cent change in the condenser wall temperature or the saturator temperature does not have a great deal of effect on the performances of the counter. The droplet growth processes and its temperature variation have been predicted numerically for air, argon and helium as the carrier gas. The final droplet sizes in air and argon show a strong dependence on the initial nuclei sizes for ultra-fine aerosols, while a relatively small dependence is shown in the case of helium.