Some aspects of the “first-principle” approach to the combustion chemistry, as a tool for understanding the chemical kinetics, are introduced and discussed. The traditional quantum chemical approach based on the multi-layered structure of the combustion science is briefly described. For the future progress of the chemical kinetics of combustion, three indispensable components are discussed; (1) the spread of the quantum chemical methods for the wide-spreading areas of combustion chemistry, including heterogeneous / catalytic combustion, catalyst for exhaust processing, and etc., (2) establishment of empirical approach in combustion chemistry based on the knowledge of the organic chemistry and the refinement by theoretical investigations, and, (3) development of the lumping and reduction methods for huge mechanism of the combustion, which is growing larger and larger, not only for the application with CFD, but also to extract the essentials of the complex mechanisms. Examples of the development of empirical rules are described for several types of reactions which are governing the low-temperature oxidation of hydrocarbons related to the engine-knock problems and the new combustion technologies such as HCCI (homogeneously charge compression ignition).
Typical method of theoretical calculations for the rate constants of elementary reactions in combustion is summarized. Higher level quantum chemistry calculations were frequently applied to obtain potential energy of reactions, which was probably good agreement with experimental results. Theoretical chemical kinetics has been developed on the rate theories such as transition state theory and unimolecular reaction theory with the quasi-equilibrium conditions. Molecular properties such as rotational constants and vibrational frequencies of normal modes are also needed because all rate theories depend on the molecular statistical themodynamics, so that those properties are also obtained from quantum chemistry calculations. Essentially, accuracy of the prediction for the rate constants from theoretical calculations strongly depends on the accuracy of potential energies of the reactions and the exactness of the statistical view for the individual reaction mechanism. Some of the practical applications of the calculations are shown.
Several applications of the quantum chemical analysis to the combustion processes were presented. In the present feature article quantum chemical analysis on the product branching ratios for the reaction of sulfur dioxide with H atoms, on the reaction pathways for the reaction of sulfur dioxide with S atoms, on the possible roles of the new spin-allowed reaction for the prompt NO formation, on the high-temperature reaction products for the decomposition and oxidation of the aromatic hydrocarbons, and also on the reasons for the different reactivities between o-, m- and p-xylenes with molecular oxygen, were presented to demonstrate the important roles of these computational techniques for the analysis of the elementary reaction kinetics of combustion.
Combustion is a kinetic and multi-physical-chemical process involving many time and length scales from atomic excitation to turbulent mixing. This paper presents an overview of the recent progress of numerical modeling using detailed and reduced chemical kinetic mechanisms for multi-scale combustion problems. A particular focus of this review is to introduce two new methods, an on-grid dynamic multi-time scale (MTS) method and a path flux analysis (PFA) to increase the computation efficiency involving multi-physical chemical processes using large kinetic mechanisms. Firstly, the methodology of the on-grid dynamic MTS method using the instantaneous time scales of different reaction groups is introduced. The definition of species timescales and the algorithm to generate species groups are presented. Secondly, a concept of hybrid multi-time scale (HMTS) method to selectively remove time histories of uninterested fast modes is introduced to increase the algorithm flexibility and efficiency. The efficiency and the robustness of the MTS and HMTS methods are demonstrated by comparing with the Euler and ODE solvers for ignition and flame propagation of hydrogen, methane, and n-decane-air mixtures. Thirdly, the PFA method to generate a comprehensive reduced mechanism by using the reaction path fluxes in multiple generations is summarized. The results are compared with that of the direct relation graph (DRG) method for n-decane and n-heptane-air ignition delay time. The PFA method shows a significant improvement in the accuracy of mechanism reduction over the DRG method in a broad range of species number and physical properties. Finally, the PFA method is integrated with MTS and HMTS methods to further increase the computation efficiency of combustion with large mechanisms. The new algorithm is used to predict unsteady flame propagation of n-decane-air mixtures in a spherical chamber. Excellent computation accuracy and efficiency are demonstrated.
A survey of published papers was carried out to evaluate the technology applied to vegetable oils expect biodiesel fuel (BDF) in diesel engines. The paper describes the findings and the development of this research field. The analysis of 40 papers in Japanese and English established the following points as important with many related studies: (1) the differences in the fuel properties and the combustion characteristics of neat vegetable oils and ordinary diesel fuel, (2) the spray characteristics and the mechanisms of the spray combustion, (3) improvements of combustion by mixing with volatile fuels, (4) improvements of combustion by fuel heating, (5) improvements of combustion by water emulsification, (6) engine modifications, (7) vegetable oil hydrogenating process. For the future, the following research themes can be suggested:development of modification engine fueled with neat vegetable oils, and development of vegetable oil hydrogenating process.
Measurement of velocity profile of a fuel jet bifurcating inside a diffusion flame under acoustic excitation is conducted by means of particle tracking velocimetry. It is confirmed prior to the detailed examination that the addition of particles to the fuel gas has negligible effect on the behavior of the jet, and that the measured velocity by this method is reasonably accurate. The results indicate that the velocity profile is not altered by the acoustic forcing in the region in which shadowgraphy shows the jet going straight before meandering. This infers that the bifurcating behavior is originated from not the effect of steady streaming but that of linear instability. On the other hand, the velocity profile oscillates synchronically with the acoustic forcing in the region in which the shadowgraphy shows the jet meandering. A qualitative explanation of the bifurcation of the jet is successfully obtained by considering the probability density distribution of the oscillating fuel jet based on the interpretation of the experimental results.
The previously proposed model for the surface-flash phenomenon is modified to account for the influence of nap thickness and the dependence of reaction rate on fuel concentration. As before, the model is an extension of ordinary premixed-flame theory and considers heat loss to the surrounding environment and fiber pyrolysis. The activation-energy-asymptotics technique is applied to obtain an asymptotic analytical solution of the model. The model predicts the existence of the critical nap thickness and the critical nap density for the occurrence of surface flash. It also shows that the surface flash velocity significantly depends on the nap density but only weakly on the nap thickness. These theoretical results qualitatively agree with previous experimental observations.
Flow and mixture formation processes of a high-speed unsteady methane-jet are calculated using a large eddy simulation in order to investigate the distribution of flammable mixture in a jet. The calculations were performed for wide range of injection pressure, from low pressure injection to super-critical cases and for an impinging jet on a circular obstacle. Based on the calculation result, formation of flammable mixture was discussed and compared with the previous experimental results of spark-ignition combustion of a natural gas jet. A flammable region is formed from 40 to 80 times nozzle diameter downstream on a jet axis and stable combustion starts by spark-ignition in this region for the case of lower injection pressure. On the other hand for high pressure injection case, a flammable region forms farther downstream and a thin flammable region is distorted by turbulence, therefore, spark-ignited combustion was not observed in the experiments. For the impinging jet case, a flammable region spread quickly and widely behind the obstacle, even if for the high pressure injection case, so that stable combustion was achieved in the experiments.