An experimental study of natural ionization in hydrocarbon flames, using the negative electrostatic probe at atmospheric pressure II. The dependence of natural ionization on flame composition

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Abstract
Ion density measurements have been made, between 32 and 96 ms from the reaction zone, in constant velocity atmospheric pressure flames of H2-O2-N2, with additions of up to 20% of the following gases:methane, ethane, propane, ethylene, propylene and acetylene. Ion density is found to depend by a power law on the flow rate of reacting oxygen atoms, measured in fuel-rich flames by oxygen flow rate and in oxygen-rich flames by a parameter of the fuel. This parameter is found to differ significantly from chemical equivalence, suggesting that ion generation is associated with the formation of `over-oxidized' short-lived radicals. The paraffin and olefine additives examined show a dependence of ion density on the square of the flow rate of oxygen atoms, for wide ranges of stoichiometry, nitrogen dilution and hydrocarbon addition rate. In the case of acetylene additive this relation is found to be a cube law. The measurements also yield data on ion recombination coefficients. Maximum ion densities, in the reaction zone, of about 1011 ions/cm3 are implied, for the flames examined.