Control of soot emission of a turbulent diffusion flame by DC or AC corona discharges

The effects of DC or AC (14 kHz) corona discharges, formed between tips of opposed needle electrodes, on soot emission of a propane turbulent diffusion flame were investigated experimentally. It is shown that when a DC corona discharge (e.g., 3.6 W; 0.06% of the combustion energy released by the flame) or a discharge system composed of three AC coronas (e.g., 25.5 W in total; 0.43% of the combustion energy) is applied across the lower part of the flame, with a gap width such that the electrode tips are located outside the reaction zone, a marked reduction in soot emission is observed, without noticeable change in the shape of flame luminous region. When corona discharges are applied, increases of the density of charged species and/or charged soot particles are observed in the flame over the whole length downstream of the corona application. It is suggested that, in the case of DC corona application, additional air and inorganic charged species and electrons, produced in the air near the tip of the positive electrode, are carried into the flame mainly by corona winds, whereas in the case of the AC corona application the inorganic charged species and electrons are carried into the flame by diffusion processes. The charged species and electrons carried into the flame may influence the state of charging of incipient soot particles and also reduce the concentration of growing ions, i.e., soot precursors, which directly relate to the soot emission of the flame. In TEM photographs it was found that separate soot particles, or those forming chains in the flame, decrease in mean size with the application of corona discharges. Smaller size soot particles burn faster than larger size particles in the high-temperature oxidizing atmosphere at the flame top region. (C) 1998 by The Combustion Institute.