Study On The Auto-Ignition Characteristics Of Liquid Fuels Based On Chemical Kinetics Mechanisms

One of the most important parameters determining the combustion process is the auto-ignition characteristics of liquid fuels. With the increase of computational capabilities, the application of the chemical kinetics mechanisms becomes an important way to study the ignition characteristics of fuels. Based on the oxidation mechanisms established using the decoupling methodology, the ignition characteristics of fuels in various reactors under wide operating conditions were investigated.First, a new skeletal oxidation mechanism was established for the primary reference fuel (PRF) composed of n-hexadecane and iso-cetane, which consists of 46 species and 139 reactions. By using the present mechanism, the relationship between cetane number and the ignition delay in shock tubes were investigated. The cetane number of various fuels was estimated using the present PRF mechanism and a weighted least squares method. For the fuels with the cetane number beyond 15-100, the extension method was applied. The prediction of cetane number investigated in this study primarily focuses on the operating conditions of practical diesel engines, i.e., the equivalence ratio of 1.0 and the pressures from 19 to 80 atm. The mean absolute deviation of the predicted cetane number is within 3.327. Furthermore, according the cetane number of alkanes, the ignition delays in shock tubes for various fuels are reproduced by the present mechanism. The predicted results of practical fuels with complicated compositions are also accurate.Second, using the newly developed skeletal mechanism for n-dodecane and computational fluid dynamics model, the ignition delay of n-dodecane in constant volume bomb (CVB) is predicted. The variance of each operating parameter and the global variance are predicted by artificial neural network, and then the global sensitivity coefficient is calculated. The computational efficiency of global sensitivity analysis can be considerably improved using artificial neural network. Based on the present method, the influence of the boundary conditions on the of the ignition delay time was studied and the interaction effect of various boundary conditions was clarified. The results indicate that the variation of the boundary conditins determines the uncertainties of the ignition delay time of a spray in CVB. The global sensitivity coefficient of the operating parameters depends on the parameter combinations and the operating conditions, such as the nozzle diameter and the initial temperature of fuel. It is found that the initial temperature of fuel and the ambient temperature play an important role in increasing the uncertainties of the ignition delay time of a spray in CVB at the ambient temperature of 900 K, while the nozzle diameter plays a primarily inhibition role when it is combined with the initial temperature of fuel.Third, an updated n-heptane skeletal mechanism developed in this study was introduced to investigate the ignition delay characteristics of n-heptane spray in CVB and shock tubes. The critical operating conditions and chemical reactions affecting the ignition in CVB were revealed by analyzing the ignition process in detail. Meanwhile, according to the calculated sensitivity coefficients of the chemical reactions in CVB and shock tubes, the relationship of the ignition behavior between CVB and shock tubes was revealed by the similar factors of the standardized sensitivity coefficient. It is found that the negative temperature coefficient (NTC) region of fuels is the most relevant for auto-ignition in CVB. Further validation results indicate that the present analysis method and the related conclusions are nearly independent on the chemical mechanism used in the simulations and the boundary conditions.Finally, the correlation between the ignition delay in shock tubes and the ignition timing in homogeneous charge compression ignition (HCCI) engines under different operating conditions were studied using the mechanism of PRF composed of n-heptane and iso-octane. According to the similarity analysis of the sensitivity coefficient, the operating conditions which affect the similarity factor are analyzed. It is concluded that the effect of the ambient temperature is larger than that of the equivalent ratio of the fuel/air mixture and the ambient pressure. The trend of the predicted ignition delay in shock tubes is consistent with the ignition timing in HCCI engine under the operating conditions with the highest similarity factor.Overall, the cetane number of fuels as an important indicator for the fuel ignition characteristics, the influence of the boundary conditions on the ignition characteristics in CVB, and the correlation of the ignition delay in various experimental configurations (including shock tube, CVB, and HCCI engine) were systematically analyzed in this study. Thus, the relationship between the ignition characteristics with the related indicators for evaluation of the fuel reactivity and the operating conditions were analyzed, and the important reactions affecting the ignition behaviors of fuels under various reactors were revealed.