Insights into Constraining Rate Coefficients in Fuel Oxidation Mechanisms Using Genetic Algorithm Optimization

Abstract

Accurate fuel oxidation mechanisms can enable predictive capabilities that aid in advancing combustion technologies. High-level computational kinetics can yield reasonable rate coefficients with uncertainties, in some cases, below a factor of 2. Computed rate coefficients can be constrained further by optimizing against experimental data. Here, we explore the application of genetic algorithm (GA) optimization to constrain computed rate coefficients in complex fuel oxidation mechanisms in conjunction with temperature-dependent species mole fractions from jet-stirred reactor (JSR) measurements. Cyclohexane is a model candidate for understanding the reactivity of cyclic fuels. In this work, we optimize the rate coefficients of the most recent literature cyclohexane mechanism, which incorporates theoretically computed rate coefficients for the reaction networks stemming from the first and second O2 addition pathways, against the experimental results of two separate literature JSR studies. Optimization consistency is evaluated by carrying out three GA optimizations: fitting to the temperature-dependent species mole fractions in each JSR experiment separately and simultaneously fitting the species mole fractions in both experiments. Local sensitivity analyses are used to identify five influential low-temperature oxidation reactions for optimization. Although the three optimizations do not yield identical rate coefficients, the direction of change in all five rate coefficients is consistent among the three optimizations. Performance of the models from the three optimizations is assessed against literature ignition delay times with differences in the level of agreement observed among the different optimizations. Comparisons are made with our recent optimization work of a cyclopentane oxidation master-equation model against time-resolved species concentrations, and insights and improvements of the strategy for constraining rate coefficients using GA optimization are discussed.

Time-resolved quantification of key species and mechanistic insights in low-temperature tetrahydrofuran oxidation

“…In addition, H abstraction from THF by ˙OH to produce α-R˙ or β-R˙ also have relatively large sensitivity coefficients. In our recent work on constraining mechanism parameters through genetic algorithm optimization, we found that the modeled species concentrations in MPIMS experiments of CPT oxidation 34 and the mole fractions in JSR experiments on cyclohexane oxidation 46 were sensitive to two key reactions, (R1) and (R2).ROO˙ ↔ ˙QOOHROO˙ → HO 2 + co-productFor the present study, the HO 2 co-product in (R2) is DHF, as shown in Scheme 1. For THF, the sensitivity analysis finds (R1) to have a large sensitivity coefficient but not (R2).…”