Comparative study of nonequilibrium plasma generation and plasma-assisted ignition for different C2 hydrocarbons

Using a shock tube with a discharge cell, ignition delay time was measured in a lean (φ = 0.5) C2H6:O2:Ar mixture and in lean (φ = 0.5) and stoichiometric C2H4:O2:Ar mixtures with a high-voltage nanosecond discharge and without it. The measured results were compared to the measurements made previously with the same setup for C2H6-, C2H5OH- and C2H2-containing mixtures. It was shown that the effect of plasma on ignition is almost the same for C2H6, C2H4 and C2H5OH-containing mixtures. The efficiency of nonequilibrium excitation is smaller for mixtures with C2H2 because of efficient production of atoms and radicals in thermally-equilibrium chemical reactions at elevated temperature. The effect of non-equilibrium plasma on C2-hydrocarbon ignition is dominated by the electron-impact production of O and H atoms and hydrocarbon radicals in the discharge and in its afterglow. The dominant mechanisms of the production of these active species are electron-impact dissociation of fuel and oxygen, and electron-impact excitation of Ar followed by dissociative quenching of excited Ar atoms in collisions with O2 and hydrocarbons. In addition, although the hydrocarbon fraction in the mixtures under consideration is small, direct electron-impact dissociation of hydrocarbons plays an important role in the stoichiometric C2H2- and C2H4-containing mixtures due to high dissociation cross sections for these hydrocarbon molecules. In the C2H5OH-containing mixtures, the rate of electron-impact dissociation is several times lower than the same process of other hydrocarbon molecules. However, this does not lead to a lower efficiency for plasma assisted ignition in ethanol-containing mixtures because in these mixtures the rate of electron-impact ionization is high due to the low ionization energy of ethanol and the high values of the corresponding ionization cross section. As a result, the specific deposited energy in the discharge is higher in C2H5OH-containing mixtures. The deposited energy increase compensates for the smaller electron-impact dissociation rate coefficient for C2H5OH molecules. The overall efficiency of plasma-assisted ignition for mixtures with C2H5OH is close to that of C2H4.