The AramcoMech 1.3 mechanism, containing 253 species and 1542 reactions for oxidation of hydrocarbon and oxygenated C1-C2 fuels, is reduced with six direct relation graph (DRG)-related methods. The final skeletal mechanism with 81 species and 497 reactions is achieved from the intersection of the resulting skeletal mechanisms obtained with these DRG-related methods. The maximum error for the ignition delay times with this 81-species mechanism does not increase significantly compared with that obtained for the other skeletal mechanisms. This shows that the intersection of skeletal mechanisms from various mechanism reduction methods can effectively remove the redundant species. Ignition delay times of two-component mixtures with the skeletal mechanism also agree very well with those of the detailed mechanism. The skeletal mechanism has also been validated against the detailed mechanism using many other combustion characters of the involved fuels in different reactors and flames. Results from the element flux analysis demonstrate that the reaction paths for these fuels with the detailed mechanism can be reproduced accurately with the 81-species skeletal mechanism. All the important reaction paths are thus retained in the 81-species mechanism. All these results show that the skeletal mechanism is able to provide the combustion properties of C1-C2 fuels that are in good agreement with those of the detailed mechanism. The 81-species skeletal mechanism can be employed as a reaction base for developing mechanisms of other large hydrocarbon or oxygenated fuels.

Sensitivity analysis is an important tool in model validation and evaluation that has been employed extensively in the analysis of chemical kinetic models of combustion processes. The input parameters of a chemical kinetic model are always associated with some uncertainties, and the effects of these uncertainties on the predicted combustion properties can be determined through sensitivity analysis. In this work, first-and second-order global and local sensitivity coefficients of ignition delay time with respect to the scaling factor for reaction rate constants in chemical kinetic mechanisms for combustion of H2, methane, n-butane, and n-heptane are examined. In the sensitivity analysis performed here, the output of the model is taken to be natural logarithm of ignition delay time and the input parameters are the natural logarithms of the factors that scale the reaction rate constants. The output of the model is expressed as a polynomial function of the input parameters, with up to coupling between two input parameters in the present sensitivity analysis. This polynomial function is determined by varying one or two input parameters, and allows the determination of both local and global sensitivity coefficients. The order of the polynomial function in the present work is four, and the factor that scales the reaction rate constant is in the range from 1/e to e, where e is the base of the natural logarithm. A relatively small number of sample runs are required in this approach compared to the global sensitivity analysis based on the highly dimensional model representation method, which utilizes random sampling of input (RS-HDMR). In RS-HDMR, sensitivity coefficients are determined only for the rate constants of a limited number of reactions; the present approach, by contrast, affords sensitivity coefficients for a larger number of reactions. Reactions and reaction pairs with the largest sensitivity coefficients are listed for ignition delay times of four typical fuels. Global sensitivity coefficients are always positive, while local sensitivity coefficients can be either positive or negative. A negative local sensitivity coefficient indicates that the reaction promotes ignition, while a positive local sensitivity coefficient suggests that the reaction actually suppresses ignition. Our results show that important reactions or reaction pairs identified by global sensitivity analysis are usually rather similar to those based on local sensitivity analysis. This finding can probably be attributed to the fact that the values of input parameters are within a rather small range in the sensitivity analysis, and nonlinear effects for such a small range of parameters are negligible. It is possible to determine global sensitivity coefficients by varying the input parameters over a larger range using the present approach. Such analysis shows that correlation effects between an important reaction and a minor reaction can have relatively sizable second-order sensitivity coefficient in some cases. On the other hand, first-order glob...

The ignition delay times of gas-phase n-undecane/air mixtures in a heated shock tube were measured over a wide range of temperatures, from 731 to 1399 K, at pressures of approximately 2.02 × 10 5 and 10.10 × 10 5 Pa, and at equivalence ratios of 0.5, 1.0, and 2.0. This study represents the first-ever investigation of the shock tube ignition delay times of the n-undecane/air. The ignition delay times were determined by monitoring the reflected shock pressure and OH* emission at a location on the sidewall of the shock tube. The results show that the ignition delay time increases as the temperature is decreased above 910 K, then decreases with decreasing temperature between 910 and 780 K (thus exhibiting a negative temperature coefficient behavior), and finally increases again as the temperature is further reduced below 780 K. An increase in pressure was found to decrease the ignition delay time. The effect of the equivalence ratio on the ignition delay is different at the two experimental pressures, and the ignition delay is evidently highly sensitive to the equivalence ratio in the low temperature region compared with the high temperature region. The results obtained in this work are 2216 ZHANG Wei-Feng et al.: A Shock Tube Study of n-Undecane/Air Ignition Delays over a Wide Range of Temperatures No.9

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