Formation of Small and Large Positive Ions in Rich and Sooting Low-Pressure Ethylene and Acetylene Premixed Flames

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Abstract
The identity and concentration of bothsmall and large positive ions have been determined in rich and sooting ethylene and acetylene premixed flames. These ions are formed in two independent ionisation zones. The first one is the classical chemi-ionisation zone which produces the small ions (between 12 and ≃300 a.m.u.), and which importance decreases when the fuel equivalence ratio increases. The second one appears later, at the early stages of the soot formation, where larger species are formed. The size and concentration of these large ions can be explained by the direct thermoionisation of large polynuclear aromatic molecules and carbon particles less than 20 Ȧ diameter of low ionisation potentials, which concentrations increase as the flame becomes richer
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