Combustion is a violent chemical reaction process, which involves multiple thermochemical nonequilibrium phenomena and complex multi-scale characters. These feathers are especially notable at micro-scale due to the complexity of microscopic physical and chemical processes, therefore have become one of the important challenges in combustion research. In micro-scale combustion, the thermal and chemical non-equilibrium processes interact with each other, which means thermal non-equilibrium state influences reaction rates, and reaction process in turn changes the thermal state. It is diffcult to describe such a strong coupling process through macroscopic method, thus in order to determine the non-equilibirum process the microscopic state of molecules are required to resolve. Direct Simulation Monte Carlo (DSMC) is a well-established molecular simulation method, which can be used as the numerical tool to study micro-scale combustion. However, the microscopic thermochemical models for molecular simulation, as well as the microscopic mechanism of thermochemical coupling, requires further investigation and improvement. In addition, the scale effect of micro-scale combustion on local and overall processes can also be investigated with the help of microscopic approaches.
This paper investigates the micro-scale process of hydrogen-oxygen combustion phenomena like auto-ignition and detonation using molecular simulation method, and studies the microscopic mechanism of the thermochemical process with emphasis on the influence of thermochemical non-equilibrium and microscopic fluctuation of reactions. The main contents are as follows.
1) The microscopic collision models of chemical reactions are investigated theoretically for their reaction kinetic properties and post-reaction energy re-distribution methods. The reaction kinetic analysis illustrates the influence of vibrational favor of reactions on non-equilibrium thermochemical process from several aspects, and determines the proper microscopic reaction models for typical dissociation reaction and exchange reaction. Based on previous work, this paper first proposes the general criterions for post-reaction energy re-distribution method which can satisfy the detailed balance principle, and the criterions are applied to post-reaction energy re-distribution method for specific reaction model.
2) This paper introduces a hybrid scheme algorithm of stochastic simulation for low temperature auto-ignition process with reaction stiffness; as a result, the application range of stochastic reaction kinetics is expanded to the statistical analysis of auto-ignition process at micro-scale and meso-scale (0.1μm~10μm).
3) The hydrogen-oxygen auto-ignition problem is studied through DSMC simulation. Results show two phenomena that have not been revealed in previous macroscopic reaction kinetic calculation, including the vibrational non-eqiulibrium in the induction period and the fluctuation of ignition delay time. Detailed explanation is provided for these two phenenomena, which advances the understanding of micro-scale combustion. The vibrational non-equilibrium analysis shows that as the vibrational relaxtion time of hydrogen and oxygen is close to ignition delay time at high temperate (>1500K), the influence of vibrational favor of chain reactions on the thermal dynamic state of gas mixtures is able to accumulate, which eventually leads to the obvious decrease of vibrational temperature. The microscopic fluctuation of auto-ignition is investigated through a statistical analysis using the hybrid scheme stochastic simulation algorithm. The analysis show that due to the effect of chain reaction mechanism, the microscopic stochastic reaction events significantly influences auto-ignition process at micro and meso scale, causing the fluctuation of ignition delay time and the an increase of its averaged value for low temperature auto-ignition. A theoretical expression of the standard deviation of ignition dealy time is derived with the help of dimensional analysis, which gives the influence of parameters like temperature and volume size. The analysis suggests that microscopic fluctuation is a potential cause for the non-homogenous low temperature ignition.
4) The hydrogen-oxygen detonation problem is studied through DSMC simulation, and the result reveals the microscopic details of detonation propagation. Due to the limited simulation particle number, this paper proposes a radical initialtion strategy in order to control the detonation initiation delay problem caused by the restriction of spanwise length. The steady propagating detonation wave and the detailed 1-D structure are derived from the simulation result. Owing to the fact that post-shock gas mixture is vibrational non-equilibrium at the beginning of ignition, the reaction rates are restrained and ignition delay time is prolonged. The half reaction width increase for about 50% as a result in space, which demonstrates the influence of vibrational non-equilibrium on detailed structure of detonation.