Abstract:

The ignition characteristics of fuels and the release of energy in combustion engines are of crucial importance to engine design and improvement. To improve the fuel combustion efficiency and to reduce the associated pollutant emission, it is necessary to develop reliable high-precision reaction mechanisms for simulating combustion. Consequently, we need to comprehensively understand the combustion mechanisms of hydrocarbon fuels, and to explore their complicated chemical reaction networks. In order to construct combustion mechanisms that can be applied to conditions over a wide temperature range, wide pressure range, and for different equivalent ratios, two detailed mechanisms for the combustion of large hydrocarbons were developed based on ReaxGen, an automatic generation program for combustion and pyrolysis mechanisms developed by LI Xiangyuan et al. Using this program, one mechanism for n-decane combustion was developed, containing 1499 species and 5713 reactions, and another was developed for n-undecane combustion, containing 1843 species and 6993 reactions. All the detailed mechanisms of the alkanes consisted of two parts, a validated core mechanism and a sub-mechanism produced by ReaxGen which worked mainly based on the rules of the reaction class. The major classes of elementary reactions considered in our detailed mechanisms for n-decane and n-undecane combustion included 10 kinds of high-temperature combustion reactions and 19 kinds of low-temperature combustion reactions. To verify the rationality and reliability of the mechanisms, ignition delay times in shock tubes and the concentration profiles of important species in a jet-stirred reactor were obtained using CHEMKIN software. The obtained calculated data were compared with the experimental data and the results of similar mechanisms at home and abroad. It was shown that the numerically predicted results of our new mechanisms were in good agreement with available experimental data in the literature. Our newly developed n-decane and n-undecane combustion mechanisms are useful for completing the combustion model of aviation kerosene. Furthermore, considering the complexity of the detailed mechanisms, the large amount of calculation and the long time required for mechanism analysis, mechanism simplification was carried out. The sampling points required for mechanism reduction were taken from simulation results near the ignition delay time with pressures ranging from 1.0 × 10⁵ Pa to 1.0 × 10⁶ Pa, equivalence ratios ranging from 0.5 to 2.0, and initial temperatures ranging from 600 K to 1400 K. The species n-C₁₀H₂₂, N₂, and O₂ were selected as the initial important species for the n-decane combustion mechanism and the species n-C₁₁H₂₄, N₂, and O₂ were selected as the initial important species for the n-undecane combustion mechanism. The predicted results of ignition delay time from the simplified mechanism for n-decane combustion (including 709 species and 2793 reactions) and simplified mechanism for n-undecane combustion (including 820 species and 3115 reactions) generated by the reduction method of Directed Relation Graph with Error Propagation (DRGEP) agreed well with the detailed mechanisms. Finally, sensitivity analysis for the ignition delay time was carried out to identify reactions that affected ignition delay times at specific temperatures, pressures and equivalence ratios. The results indicate that these mechanisms are reliable for describing the auto-ignition characteristics of n-decane and n-undecane. These mechanisms would also be helpful in computational fluid dynamics (CFD) for engine design.

Key words: ReaxGen, n-Decane and n-undecane, Combustion mechanism applied in wide temperature range, Mechanism construction, Mechanism validation