On the size, charge and number-rate of formation of carbon particles in flames subjected to electric fields

Abstract

Previous studies have shown that carbon particles in flames, being all charged, can be manipulated by electric fields so as to control their residence time and hence size, as well as their rate of formation and all the parameters of deposition on the electrodes―mass, position and form of aggregate. To gain further insight into the fundamental processes, particularly those occurring during the very early (nucleation) stages, measurements of particle mobility and detailed size analyses are now added to those of current and mass deposition, when a variable potential is applied across seeded and unseeded flat, counter-flow diffusion flames. It is found that particles which account for the mass deposited have mobilities ranging from 10 –3 to 3 x 10 –2 cm 2 s –1 V –1 , depending on the applied potential. This allows their trajectories in a field to be calculated and also shows, when taken together with size measurements from electron micrographs, that each carries unit charge over practically the entire experimental range, the majority of the current being carried by smaller charge carriers. Among the conclusions are that both growth of carbon on flame ions and initially neutral growth followed by attachment-charging do occur. The rate of mass deposition is determined entirely by particles following the latter course because, in the presence of a field, the former have a very much shorter residence time available for growth in the pyrolysis zone― at high field strengths these times are indeed too short for growth to a measurable size. The theory of the growth of the larger, initially uncharged particles, is developed in an appendix. It is shown that they acquire charge predominantly by diffusion of ions, although thermionic emission can become important under certain conditions. All the trends recorded in this and in earlier work, including the variation of particle size, mass and number rate of collection are accounted for.