Plasma decay in hydrocarbons and hydrocarbon- and H2O-containing mixtures excited by high-voltage nanosecond discharge at elevated gas temperatures

Abstract

Plasma decay after a high-voltage nanosecond discharge was experimentally and numerically studied in pure hydrocarbons (C2H6 and C3H8), and H2O:N2 and C3H8:O2 mixtures for pressures in the range 2–4 Torr and gas temperatures from 300 to 600 K. In a stoichiometric C3H8:O2 mixture, plasma decay was also studied in a repetitively pulsed discharge for varing numbers of discharge pulses (varying degrees of fuel oxidation). The rate of plasma decay was determined from the temporal evolution of electron density measured using the microwave interferometer. It was observed that gas heating to 600 K leads to a decrease in the rate of plasma decay in all cases. The effect of heating on plasma decay was most profound in the H2O:N2 mixture (after a single discharge pulse) and in the C3H8:O2 mixture for high degrees of fuel oxidation. A kinetic scheme was developed to numerically simulate the plasma decay in hydrocarbons and combustible mixtures. Numerical analysis showed that, under the conditions studied, plasma decay was controlled by dissociative electron recombination with simple molecular and cluster ions. Gas heating led to a decrease in the rate of the electron-ion recombination and the rate of conversion of molecular ions to cluster ions. As a result, the gas temperature increase caused a decrease in the fraction of cluster ions for which the recombination coefficients are an order of magnitude higher than the recombination coefficients for molecular ions. The influence of gas heating on the decrease of the amount of cluster ions was more important when the ion composition was dominated by hydrated H3O+(H2O)k ions. The rates of the formation of these ions are extremely sensitive to any variations in gas temperature. As a result, in agreement with our observations, gas heating led to an anomalous decrease in the rates of plasma decay in the H2O:N2 mixture, as well as in the C3H8:O2 mixture when H2O molecules were produced due to fuel oxidation.