An experimental study of Brownian coagulation in the transition regime

Brownian coagulation of aerosols was studied in the transition regime at Knudsen numbers ranging from 1.2 to 7.4. Coagulation constants were obtained from the change in total concentration in a flowing system by using nebulized NaCl aerosols of mean mobility equivalent diameters between 0.018 and 0.105 μm in air at atmospheric pressure. Size distributions and concentrations were measured with an Electrical Aerosol Analyzer (TSI). The coagulation constants account for the polydispersity of the aerosol, diffusional losses in the coagulation tube, and particle losses in the ducts leading to the EAA. The EAA's response to particle sizes was compared to data from a transmission electron microscope and an electrical mobility classifier. Deviations of 10-15% were found in the size range between 0.03 and 0.05 μm. The experimental rate of coagulation agrees very closely with that predicted by Smoluchowski's continuum theory with the slip correction and the Fuchs correction factor included. The results do not support a correction factor recently proposed by Davies.