Effect of non-equilibrium plasma on two-stage ignition of n-heptane

Abstract The effect of pulsed nanosecond dielectric barrier plasma discharges on the ignition characteristics of n-heptane and air mixtures is investigated through self-consistent simulations in a plane-to-plane geometry at reduced pressures (20.3 kPa). The present work represents one of the first attempts in understanding the kinetic and thermal effects of nanosecond pulsed discharges on the two-stage ignition process of n-heptane. A plasma-fluid formulation is developed with ions and neutral species at gas temperature, and electrons in non-equilibrium. The work makes use of an optimized chemical kinetics mechanism consisting of 166 species and 611 reactions, obtained by combining a reduced n-heptane kinetic model, a non-equilibrium plasma chemistry scheme, and a NO x kinetic model. The catalytic effect from plasma-generated radicals on the first stage of the n-heptane ignition process has been identified. Production of radicals such as O, H and OH from the plasma initiates and accelerates the H abstraction of fuel molecules, and dramatically reduces the induction time of the exothermic cycle ( RH  →  R  →  RO 2  →  OROOH ) by a factor of 10. Furthermore, the plasma action on low temperature chemistry is found to be nearly independent of the equivalence ratio and more pronounced at lower temperatures (550–650 K). A staggered application of nanosecond voltage pulses (2–4 pulses at the beginning and 20–30 pulses after the first stage heat release) is shown to be optimal, resulting in a reduction of the ignition delay by approximately a factor of 2. NO production from the plasma via electron impact and quenching processes at low temperatures plays an important role in promoting chain-branching reactions and contributes to shortening the ignition delay by approximately 10%.