An Improved Core Reaction Mechanism for C₀-C₄ Unsaturated Fuels and C₀-C₄ Fuel Blends

Simulation of the combustion of fuels used in transportation and energy applications requires accurate chemistry representation of the fuel. Surrogate fuels are typically used to represent liquid fuels, such as gasoline, diesel or jet fuel, where the surrogate contains a handful of components. For gaseous fuels, surrogates are effectively used as well, where methane may be used to represent natural gas, for example. An accurate chemistry model of a surrogate fuel means a detailed reaction mechanism that contains the kinetics of all the molecular components of the fuel model. Since large hydrocarbons break down to smaller molecules during combustion, the core chemistry of C₀ to C₄ carbon number is critical to all such fuel models, whether gaseous or liquid. The usual method of assessing how accurate the fuel chemistry is involves modeling of fundamental combustion experiments, where the experimental conditions are well enough defined and well enough represented by the reacting-flow model to isolate the kinetics in comparisons between predictions and data. In the work reported here, we have been focused on developing a more comprehensive and accurate core (C₀−C₄) mechanism. Recently, we revisited the core mechanism to improve predictions of the pure saturated components (J Eng. Gas Turbines Power (2012) 134; doi:10.1115/1.4004388). In the current work, we focused on combustion of unsaturated C₀−C₄ fuel components and on the blends of C₀−C₄ fuels, including saturated components. The aim has been to improve predictions for the widest range of fundamental experiments as possible, while maintaining the accuracy achieved by the existing mechanism and the previous study of saturated components. In the validation, we considered experimental measurements of ignition delay, flame speed and extinction strain rate, as well as species composition in stirred reactors, flames and flow reactors. These experiments cover a wide range of temperatures, fuel-air ratios, and pressures. As in the previous work for saturated compounds, we examined uncertainties in the core reaction mechanism; including thermochemical parameters derived from a wide variety of sources, including experimental measurements, ab initio calculations, estimation methods and systematic optimization studies. Using sensitivity analysis, reaction-path analysis, consideration of recent focused studies of individual reactions, and an enforcement of data consistency, we have identified key updates required for the core mechanism. These updates resulted in improvements to predictions of results, as validated through comparison with experiments, for all the fuels considered, while maintaining the accuracy previously reported for the saturated C₀−C₄ components. Rate constants that were modified to improve predictions for a small number of reactions remain within expected uncertainty bounds.