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.

Assessment of the impact of nanosecond plasma discharge on the combustion of methane air flames

“…The resulting electron impact reactions are driven by the incident energy and the cross-sectional area. Electric field strength, gas composition, gas density and the ionization degree are the important parameters to control the distribution of electron energy deposition [4].…”

Plasma-enhanced catalysis for the upgrading of methane: a review of modelling and simulation methods

“…29,32,37,48 Other than that, a certain degree of detail for hydrocarbon ions and also or can also be found in some plasma assisted combustion studies. 68,69,80 3.2. Chemical processes.…”

Plasma assisted combustion of methane-air mixtures: Validation and reduction

“…As this is difficult to address with experiments, numerical simulations may be used to analyze the flame-discharge interaction. Most of the studies including detailed kinetics are limited to 0D simulations [7,8,9]. One dimensional case [10,11] or two-dimensional simulations [12,13] of plasma-assisted methane combustion are usually limited to simplified chemistry.…”

“…While the most important processes and their effects are highlighted, it is not straightforward to derive a complete kinetic mechanism relying solely on [24]. In [8], a detailed plasma-assisted combustion mechanism has been used to study ignition with pre-calculated electron impact reaction rates. Although computationally efficient, this strategy limits the conditions on which the detailed mechanism is valid.…”