Numerical simulation of microwave-enhanced methane-air flames I: Modeling and one-dimensional premixed laminar flames

摘要

This study considers the numerical modeling of sub-breakdown microwave-enhanced combustion in one-dimensional laminar methane-air flames over a range of stoichiometries at standard temperature and pressure. The effect of uniform microwave fields on the laminar flame speed, post-flame temperature and composition is of primary interest here.A model for the non-thermal plasma generated by the microwave field in such flames is constructed based on fitting of electron properties and electron impact reaction rates to pre-computed solutions of the Boltzmann equation, combined with ambipolar diffusion transport for charged species. This model is used with a selection of existing and novel methane-air kinetics schemes to compute laminar flame profiles at a range of microwave field strengths and varying stoichiometry. The trends for laminar flame strength and temperature, peak-and outlet species fractions are compared over this parameter space. While all schemes demonstrate increasing flame speed and temperature with higher field strength and richer flames, significant variations in the form and magnitude of this increase are observed for the vari-ous kinetics schemes considered.The laminar flame-speed increase at sub-breakdown electric field strengths is found to be due pri-marily to electron heating of the gases in both the pre-heat region and across the flame. Analysis of the electron energy transfer shows that the majority of the energy at moderate field strengths is transferred via excitement of vibrational states of nitrogen, followed by water and carbon monoxide. In the pre-heat regime, vibrational excitement of methane is also seen to play an important role for fuel-rich flames. Particular attention is paid to the modification of the flame speed due to the relaxation time of the vi-brational N 2 states, which is demonstrated to result in a moderate reduction of the flame speed for lean and stoichiometric flames.A driving concern here is the development of models suitable for use within large-scale three-dimensional numerical simulations and the knowledge gained in comparing the existing schemes is used to derive a range of simplified mechanisms more suited for this purpose. The ability of these reduced models to capture the flame enhancement is demonstrated over a range of flame stoichiometries and sub-breakdown field strengths.(c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.