Effect of the uncertainty of kinetic and thermodynamic data on methane flame simulation results

Abstract

A method for assessing and comparing the impact of uncertainties in both kinetic and thermodynamic parameters on the predictions of combustion chemistry models has been developed. Kinetic, thermodynamic and overall uncertainty parameters are defined, which allow tracking the sources of uncertainties for a particular model result. The method was applied to premixed laminar methane-air flames using the Leeds Methane Oxidation Mechanism (K. J. Hughes et al., Int. J. Chem. Kinet., 2001, 33, 513). Heat of formation and rate coefficient data for species and elementary reactions, respectively, related to methane combustion were collected from several recent reviews and critically assessed error limits were assigned to them. Local rate coefficient sensitivities and heat of formation sensitivities were calculated for lean (φ = 0.62), stoichiometric (φ = 1.00) and rich (φ = 1.20) laminar atmospheric premixed methane-air flames. Uncertainties of flame velocity, maximum flame temperature and also the value and location of maximum concentration of radicals H, O, OH, CH and CH₂ were obtained from the sensitivities and the uncertainties of thermodynamic and chemical kinetic data. The uncertainty of the calculated flame velocity is typically 2–5 cm s⁻¹. Maximum flame temperature and concentration of H, O, and OH can be calculated accurately, while there is high uncertainty in the calculated maximum concentration of CH and CH₂. The calculations have revealed that the uncertainty of the calculated flame velocity is caused mainly by errors of the input rate coefficients. This is the case also for the calculated concentration of CH and CH₂. The uncertainty of the location of concentration maxima is also of kinetics origin and it is caused by the very same rate coefficients that affect flame velocity. Uncertainty of maximum adiabatic flame temperature and maximum concentration of H, O and OH originates mainly from errors of the input heat of formation data. In order to obtain good simulation results for methane flames, accurate heats of formation are required in particular for radicals OH, CH₂(S), CH₂, CH₂OH, HCCO and CH₂HCO. Simulation results could be improved by better knowledge of the reaction rate parameters for the reactions O₂ + H = OH + O, O₂ + H + M = HO₂ + M, CO + OH = CO₂ + H, H + CH₃(+M) = CH₄(+M), CH₃ + OH = CH₂(S) + H₂O, C₂H₂ + OH = C₂H + H₂O and C₂H₂ + CH = C₂H + CH₂. This conclusion is somewhat surprising since at least the first three reactions are among the most frequently studied ones in chemical kinetics. The calculations demonstrate that all simulation results of chemical kinetic modelling studies should be accompanied by uncertainty information (e.g. standard deviation) for the model outputs to indicate which results are well supported by the model and which ones are merely nominal values that were obtained using the selected set of input parameters.