Modeling Negative Temperature Coefficient region in methane oxidation

摘要

Standard kinetic models are essential tools for predicting and interpreting the evolution of oxidation pro cesses and obtain useful information for designing and dimensioning practical combustion facilities. Quite often a large part of the development work consists in the determination of the most suited chem ical kinetics scheme to use in numerical simulations. This step is even more critical in the case of inno vative technologies. In fact, in this case, models are required to work in extrapolative conditions, i.e. in range of parameters outside the ones for which they have been optimized. This is the case of prediction methane autoignition at atmospheric pressure, in diluted conditions, corresponding to MILD combustion conditions, where no experimental data are available. The aim of the present work is to compare the effi cacies in predicting the existence of Negative Temperature Coefficient (NTC) behavior of ignition time of methane at atmospheric pressure of several kinetic models available in the literature. Such phenomenol ogy is extensively described in the literature for high molecular weight paraffin but few experimental evi dences are reported about its occurrence in methane oxidation. Methane autoignition time in dependence of temperature, reaction pathways with rate of production, sensitivity and flow diagram analysis have been exploited in order to highlight the kinetic controlling steps of methane autoignition at different temperature ranges. It has been shown that the prevalence of either the oxidation or the recombination results in a speeding or a slowing down of the reactive process. In this reactive network, a key role is covered by the active oxidation pathway. At the same time, in dependence of working tem perature, the branching routes of H_2/O_2 reaction mechanism supply a great part of radicals needed for ignition. Thus, numerical results presented in the paper clearly show that the Negative Temperature Coef ficient region in the Arrhenius plot of methane ignition delay marks the shift from one principal reaction route to the others.