In the current transition period toward carbon neutrality, there are many efforts on the combination between internal combustion engine technologies and their fuel compositions to lower the carbon footprint of automobiles. Given the diversification of fuel sources, it will become increasingly important to understand the combustion chemistry of hydrocarbon compounds. This article shows the development of detailed chemical kinetic model for the auto-ignition of diethylether using quantum chemical and chemical kinetics calculations. High and low-temperature reaction pathways of R, ROO, QOOH, and OOQOOH isomers, which are critical species for the ignition under engine-relevant conditions, were investigated. Geometries, vibrational frequencies, and energies of reactants, products, and transition states were calculated according to the procedure of the CBS-QB3 method. The high-pressure limiting rate constants were estimated by using a conventional transition state theory and variational transition state theory. It was found, in comparison with saturated hydrocarbons, that some low-temperature oxidation reactions are influenced by the oxygen atom in the ether group. The developed model was validated against published ignition delay times measured in a shock tube. It was confirmed that the model well reproduces the experimental data in the range of at 625-1100 K and 20-40 bar.
This article demonstrates a detailed-modeling method for liquid-phase reactions using quantum chemical calculations combined with polarizable continuum models (QM/PCMs), and the decomposition mechanism of hydrazine nitrate in nitric acid solutions within this framework. The major difference between liquid- and gas-phase reactions is the presence of numerous solvent molecules surrounding reactants and products. The effects of solvent molecules on liquid-phase reactions can be classified as static and dynamic. The former is an effect that changes the potential energy of a solute molecule, while the latter is a relaxation or diffusion effect caused by collisions between solute and solvent molecules. Some of these contributions can be modeled using QM/PCMs; however, addressing all of them is currently a challenging task.
Aluminum is a material that combusts easily and is used as a fuel for propellants or raw material for the combustion synthesis of aluminum oxide. Although the chemical reaction mechanism in the combustion of aluminum has been studied so far, few examples have been comprehensively examined in detail. In this feature article, an outline of the reaction mechanism in the previous studies is explained, and recent theoretical studies on the detailed reaction mechanism for the oxidation of aluminum in gas phase by the author's group are introduced. A detailed chemical kinetic model of aluminum combustion in gas-phase in the recent papers is also described briefly.
Spray combustion includes fundamental physical processes in a wide range of scales. A spray is initially formed after liquid fuel is injected from a nozzle where atomization occurs due to the interaction between the injected liquid and the ambient gas. Intense research efforts to identify the atomization mechanisms have been carried out both experimentally and numerically including the ISS (International Space Station) experiment on droplet generation and direct numerical simulation. The turbulent atomization mechanisms of turbulent resonance and surface Rayleigh-Taylor instability have been modeled in a new turbulent atomization model. The subsequent evaporation occurs in several modes depending on the number density of droplets. In the dense spray region, the triple-layer structure is found and the droplet group mode shifts from sheath to group mode as the spray goes downstream where mixing of vapor and air occurs globally. Ignition finally leads to combustion in the downstream. In recent spray combustion modeling, the flame models are mostly based on the flamelet assumption and recently direct chemical kinetics are becoming to be included as well. These simulations can predict flame lift-off, local extinction, wall heat transfer, etc. As some novel fuels such as bio-fuels, e-fuels and ammonia are also expected to be utilized in engines, spray combustion research will expand its region further for a sustainable future.
Particle combustion of solid carbon in the quiescent environment has been overviewed by use of accomplishment in the aerothermochemical analyses, in which not only the surface C-O2 and C-CO2 reactions but also gas-phase CO-O2 reaction is taken into account, until use has been made of an examination related to the critical size of the particle activation and/or extinction. By virtue of the generalized species-enthalpy coupling functions, close coupling of those reactions has been elucidated. It has been identified that the combustion behavior in the three limiting situations, such as the Frozen, Flame-sheet, and Flame-attached modes, can analytically be described. Explicit combustion-rate expressions by use of the transfer number in terms of the natural logarithmic term, just like that for droplet combustion, have further been obtained for the combustion behavior in the limiting situations, albeit approximate. In addition, by examining establishment of the CO-flame over a carbon particle, it has been confirmed that the combustion rate can fairly be represented by the expression in the Frozen mode when the particle diameter is 100 µm or less. Using this confirmation, combustion response in the transient situation has further been examined, from which an existence of the critical size for the particle activation has even been derived, by use of the asymptotics. Furthermore, an attempt has been conducted to obtain the critical size for the particle extinction, presenting the maximum and the minimum as particle temperature rises. It is suggested that those particles larger than the maximum value can only be activated to burn and others are extinguished. In addition, those burned during combustion are extinguished when particle sizes are smaller than the minimum value. A fair degree of agreement in experimental comparisons indicates that the present formulation has captured the essential feature of the particle activation and/or extinction.