Propagation of ignition kernel in CO2-diluted, CH4/air mixtures

Abstract

Recent experiments on ignition probability in flowing CH4/air/diluent mixtures near minimum ignition energies suggested that ignition is more likely to succeed in N2-diluted mixtures compared to that in CO2-diluted mixtures for the same ignition energy. Such difficulty in igniting combustible flowing mixtures, when diluted with CO2, was observed for various fuel-lean conditions and flow velocities. A numerical study is performed for understanding the influence of diluent gas on the success of an ignition kernel in a flowing environment. Calculations for the modeled experimental rig are performed using UNICORN code and utilizing GRI V3.0 mechanism for CH4-air combustion. Small spherical regions of ignition kernels are created through providing locally heated gas mixtures consisting of small amounts H, O, and OH radicals. The advecting ignition kernels in the premixed gas mixtures are then modeled with axisymmetric assumption along the direction of motion. Specifications for the initial ignition kernel corresponding to the minimum ignition energy are obtained from a series of simulations performed through gradually reducing the temperature of the initial ignition kernel until ignition becomes unsuccessful. Simulations have predicted the experimentally observed ignition-inhibition characteristics when CO2 is present. A good agreement between the computations and measurements was obtained. Further simulations are made with pure thermal ignition kernels for elucidating the role of radicals in the ignition kernel for predicting the experimentally observed differences in the ignition probabilities. Parametric studies are performed through altering the physical and chemical properties of CO2 for identifying the mechanisms responsible for its ignition-inhibition characteristics. The most important chemical reaction that consumes H radicals and is relevant to the growth of ignition kernels is identified. The effects of CO2 on H radicals in the ignition kernel that were formed during the initial plasma-ionization phase and that were generated during the ignition/combustion phase are discussed. The chemical mechanisms that promote the ignition-inhibition characteristics of CO2 are identified.