Concentration and mass distribution of charged species in sooting flames

Abstract

Total concentration and mass distribution of charged species larger than about 300 amu were measured along the centerline of a premixed sooting acetylene/oxygen flat flame at 20 mmHg. Charge concentration was determined by measuring the electric current delivered to a Faraday cage in the detection chamber of a staged molecular beam flame sampling instrument having a quenching time of about 1 μs. Charge/mass ratio distributions were measured by the incremental electrical filtration of charged species from the beam. Mass and diameter distributions were than calculated by assuming unicharged species of density 2 g/cm 3 . The observed species, which include heavy hydrocarbon ions and charged soot particles, are of positive polarity. Their total concentration at fuel equivalence ratios in the range 2.1–3.0 and cold gas velocities of 31 and 38 cm/s ranges from 10 8 to 10 12 cm −3 , exhibits a distinct peak near the onset of soot formation, increases strongly with increasing fuel equivalence ratio, and decreases with increasing cold gas velocity. The mass distribution of charged species peaks sharply at a mass which increases with increasing height above the burner or time. At a fuel equivalence ratio of 2.25 and a cold gas velocity of 31 cm/s, the peak mass and its equivalent diameter increase from 1390 amu and 13just prior to the onset of visible soot formation to 7700 amu and 23about 2 ms later. The concentration of heavy hydrocarbon ions and that of heavy hydrocarbon molecules estimated previously decrease rapidly with the onset of soot formation in a manner that correlates with the initially fast surface gorwth of soot particles. Thus the heavy hydrocarbons appear to include both soot nuclei and surface growth intermediates. The concentrations of heavy hydrocarbon ions are much larger than the peak concentrations of soot particles. Therefore ionic nucleation of soot particles is feasible for these conditions, and a tentative mechanism is described.