Coagulation and carbonization processes in slightly sooting premixed flames

UV absorption spectra in the 200-400 run range and transient thermocouple measurements are used to characterize the process of nanoparticle formation and their carbonization to soot particles in slightly sooting premixed flames of ethylene. The UV absorption technique allows a quantitative determination of the concentration of both soot and nanoparticles. In the examined flame, the total particulate is initially present as nanoparticles, but even in the sooting region, where the yellow luminosity is dominant, soot particles represent less than 50% of the total mass of particulate. This technique can be easily used for the determination of soot and nanoparticles at the exhaust of real combustion devices and in urban polluted areas. Time-resolved thermocouple measurements are used to follow the variation along the flame axis of the emissivity of the particulate deposited by thermophoresis on the thermocouple junction. These values are used for the evaluation of an apparent rate of carbonization, which has been compared with the coagulation rate of the particles evaluated by scattering and extinction measurements. Both processes, carbonization and coagulation, show a second-order kinetic with a constant of the order of 5 x 10(12) cm(3)/s. This technique has shown to be a very simple method for detecting nanoparticles, which can be applied also in diffusion flames and other practical systems. In addition, it furnishes emissivity values, which might be useful for a more realistic assessment of the radiative heat transfer of slightly luminous flames.