| The heat and mass transfer and the droplet growth processes in a continuous flow condensation nucleus counter (CNC) have been studied numerically. The influence of sampling flow rate, saturator and condenser wall temperatures, and properties of carrier gases (air, argon, and helium) have been studied. The numerical results have been compared with experiments.; Calculations have been made for conditions similar to those used in a commercially available CNC. The results show that the activation efficiency does not depend on sampling flow rate, as long as the highest supersaturation region is within the condenser tube. A few percent change in the condenser wall or saturator temperatures has only a small effect on the performance of the counter. The finite counting efficiency of the instrument has been found to be the result of the non-uniform supersaturation in the condenser tube, rather than particle loss by diffusion. The final droplet size at the condenser outlet has been found to be a function of the initial size of the condensation nuclei. The final droplet size is significantly reduced when the initial nuclei size is below 10 nm. For air as a carrier gas, the final droplet size ranges from approximately 10 to 13 {dollar}mu{dollar}m. For argon, the final droplet size is about 10% smaller than that in air. However, in the case of helium, the final droplet size is about 15 {dollar}mu{dollar}m and this does not depend strongly on the initial nuclei size. The activation efficiency of the CNC for argon is similar to that for air. However, for helium the activation efficiency is higher. The final droplet size and the activation efficiency of the CNC predicted by theory are in good agreement with the experimental results in all cases.; For ultrafine aerosols smaller than approximately 30 nm, the final droplet size has been found to depend on the initial nuclei size. This phenomenon has been used as the basis for a new technique for real time ultrafine aerosol sizing and counting. |
