Modeling plasma-assisted methane–air ignition using pre-calculated electron impact reaction rates

Abstract

Development of practical combustion applications implementing plasma-assisted ignition technology for improved efficiency or fuel versatility will benefit from computationally-feasible models which include the plasma processes governing experimentally-observed combustion enhancement. A detailed chemical kinetic reaction mechanism for methane combustion with relevant plasma reactions has been compiled, including a set of electron impact cross sections for elastic and inelastic collisions with reactants, intermediate species, and products of methane combustion. In addition to electron impact reactions, the present mechanism includes reactions involving vibrationally- and electronically-excited species, dissociative recombination reactions, three-body recombination reactions, charge transfer reactions, and relaxation reactions, taken from the literature where available, and otherwise calculated using published correlations. While many past mechanisms have made assumptions limiting their use to specific regimes such as nanosecond discharges or microwave-enhanced flames, the present mechanism is generalized to include kinetics relating to both high- and low-energy excitation. The chemical kinetic mechanism is designed for use in a two-temperature chemical kinetics solver that tracks the electron temperature in addition to the gas temperature, as non-thermal plasma regimes characteristic to plasma-assisted combustion will typically have electron energies out of equilibrium with the energy of the heavier gas particles. Analysis considers the effects of initial temperature, mixture composition, electron concentration, and electric field strength on plasma ignition effectiveness. As commonly practiced, costly calculation of the Boltzmann equation at every time step is avoided by pre-calculating electron impact reaction rate coefficients using a Boltzmann equation solver. Here we evaluate the pre-calculated rates assumption, showing that ignition predictions depend on the gas composition at which the electron impact reaction rates are generated, but that induced errors are acceptable given the uncertainty in other model parameters such as impact cross sections. Finally, chemical kinetic sensitivity analysis highlights the importance of reactions governing free charge balance and nitrogen vibrational excitation when plasma effects on combustion enhancement are strong.