IONS AND CHARGED SOOT PARTICLES IN HYDROCARBON FLAMES .1. NOZZLE BEAM SAMPLING - VELOCITY, ENERGY, AND MASS ANALYSIS - TOTAL ION CONCENTRATIONS

A study of nozzle beam sampling of positive and negative ions and of charged soot particles from premixed low-pressure ethyneoxygen flames is presented. The expansion of the ion beam depended on the average ion mass as measured by a detector consisting of concentric rings. The measurement of the average velocity of large ions and charged soot particles in the sampling beam as a function of mass, together with the determination of their energy distribution, led to more realistic mass distributions and growth rate of charged particles than has been reported previously. The dependence of this mass growth on flame temperature and C O ratio indicates that surface growth by small molecules is important in the early stage of soot formation and has an apparent activation energy of 265 ± 30 kJ/mol. Remeasuring mass distributions of negative ions confirmed previous results that an appreciable fraction of the negative charge in the soot forming zone is contributed by negative polyhedral carbon ions. Total ion concentrations were about 108 cm-3 in the oxidation zone and showed characteristic profiles with double maxima, probably caused by dissociative detachment of electrons from thermally unstable negative ions of carboxylic acids. A third increase in charge density of up to more than 109 cm-3 was initiated by thermal ionization of soot particles in flames with C O > 1.05 at 2.7 kPa pressure. The dependence of maximum soot ionization on temperature and pressure was investigated. © 1990.