Vibrational Nonequilibrium in the Hydrogen-Oxygen Reaction at Different Temperatures

Abstract

A theoretical model of chemical and vibrational kinetics of hydrogen oxidation is suggested based on the consistent account for the vibrational nonequilibrium of HO2 radical which forms in result of bimolecular recombination H + O2 = HO2 in the vibrationally excited state. The chain branching H + O2 = O + OH and inhibiting H + O2 + M = HO2 + M formal reactions are considered (in the terms of elementary processes) as a general multi-channel process of forming, intramolecular energy redistribution between modes, relaxation, and monomolecular decay of the comparatively long-lived vibrationally excited HO2 radical which is capable to react and exchange of energy with another components of the mixture. The model takes into account the vibrational nonequilibrium for the starting (primary) H2 and O2 molecules, as well as the most important molecular intermediates HO2, OH, O2(1 ∆), and the main reaction product H2O. The calculated results are compared with the shock tube experimental data for strongly diluted H2-O2 mixtures at 1000 < T < 2500 K, 0.5 < p < 4 atm. It is demonstrated that this approach is promising from the standpoint of reconciling the predictions of the theoretical model with experimental data obtained by different authors for various compositions and conditions using different methods. It is shown that the hydrogen-oxygen reaction proceeds in absence of vibrational equilibrium, and the vibrationally excited HO2 radical acts as a key intermediate in the principally important chain branching process. For T < 1500 K, the nature of hydrogen-oxygen reaction is especially nonequilibrium, and the vibrational nonequilibrium of HO2 radical is the essence of this process.

Some Aspects of Time-Reversal in Chemical Kinetics

“…The reason that this assumption is a good approximation is that typically the relaxation to thermal equilibrium occurs with rates much faster than the chemical reaction itself. If this assumption is no longer valid (typically the vibrational relaxation is the slowest process of thermalization), state selective reaction mechanisms have to be used for the description of the system [ 9 , 10 , 11 ].…”

“…Below this last hierarchical level, there are of course even smaller levels, governed by quantum-mechanical processes, which shall not be considered here. The hierarchy of the chemical kinetics simplifies the detailed investigation considerably because it allows a decoupling of the different levels (although it is known that in some chemically reacting systems such a decoupling can lead to large errors in the quantitative description, and, e.g., state selective reaction mechanisms have to be used for the description of macroscopic systems [ 9 , 10 , 11 ]).…”

“…The answers to these questions are very challenging because there is a hierarchy involved in the chemical reaction ranging from the microscopic level (collisions between molecules, redistribution of the internal degrees of freedom, breaking and formation of bonds) up to the macroscopic level (description of a chemical reaction by a rate coefficient, which depends on temperature and in some cases on pressure, where the states of the reactants are assumed to be in thermal equilibrium [ 11 ]).…”

Колебательная Неравновесность В Реакции Водорода С Кислородом (Обзор)

“…Смирнов При моделировании и анализе процессов воспламенения и горения в реальных условиях (различного рода реакторы или ударные волны) приходится иметь дело со сложными по составу системами при наличии большого количества процессов энергообмена и химических реакций. В таких задачах кинетические уравнения целесообразно сформулировать в рамках гидродинамического уровня описания на основе соответствующих упрощающих предположений (модовое приближение) в виде макроскопических уравнений для концентраций компонентов и средних энергий колебательных степеней свободы (колебательных мод) [3,4,8,9,[16][17][18][21][22][23][24][31][32][33][34]41]. Использование упрощенного описания ставит вопрос об обоснованности вводимых в тех или иных конкретных случаях упрощений.…”

“…Игнорирование влияния колебательной неравновесности приводит в итоге к различным формальным кинетическим схемам, которые наряду с элементарными (в одну стадию) химическими реакциями, включают " брутто-реакции", детальный механизм которых неизвестен, а их эффективные константы скорости зависят от условий (см. подробнее [17,21]). Именно эти обстоятельства являются главной причиной происхождения термина "…”