IMECH-IR  > 高温气体动力学国家重点实验室
氢氧预混燃烧的微尺度模拟与机理分析
英文题名Micro-scale simulation and mechanism analysis of premixed hydrogen-oxygen combustion
杨超
导师孙泉华 研究员
2018-05
学位授予单位中国科学院大学
学位授予地点北京
学位类别博士
学位专业流体力学
关键词氢氧预混燃烧 自燃 爆轰 热化学非平衡 Dsmc方法
摘要

燃烧是一种剧烈的化学反应过程,包含了多种热化学非平衡现象以及复杂的多尺度特征。由于微观物理化学过程的复杂性,这些特点在微尺度等条件下尤为明显,已成为目前燃烧研究的重要挑战之一。在微尺度燃烧中,热化学非平衡过程相互影响,即热力学非平衡状态对化学反应速率有影响,化学反应过程对系统热力学状态也有影响。这种热化学强耦合过程难以由宏观模型刻画,需要从微观尺度求解分子内能的非平衡状态来确定。直接模拟蒙特卡罗(DSMC)方法是目前比较成熟的分子运动模拟方法,可以作为微尺度燃烧的分子模拟工具。但用于分子模拟的微观热化学模型,以及微观尺度的热化学耦合机理,还亟需更多的研究和进一步的完善。此外,微尺度燃烧的尺度效应以及其对局部和整体燃烧过程的影响,也可以借助微观方法一并开展研究。
本文采用微观模拟方法,对氢氧混合物的自燃、爆轰等燃烧问题,开展了微尺度模拟和微观机理分析,重点讨论了热化学非平衡以及化学反应微观随机涨落对微尺度燃烧过程的影响。主要内容如下。
1)从反应动力学性质和反应碰撞后的能量再分配方法两个方面,对化学反应的微观碰撞模型开展了理论研究和数值评估。通过反应动力学分析,阐述了化学反应的振动倾向对热化学非平衡过程的多方面影响,并明确了适用于典型的离解反应和置换反应的微观反应模型;在前人工作的基础上,首次提出了能够满足微观细致平衡原理的化学反应能量再分配准则,并在具体反应模型的能量再分配方法中得到了应用。
2)针对低温氢氧自燃过程中的反应刚性,提出了一种混合格式的随机模拟算法,将随机反应动力学方法的应用领域拓展到了微观到介观尺度(0.1μm ~ 10μm)的自燃过程随机涨落的统计研究中。该算法克服了因链式反应过程中存在微量组分而引起的模拟效率下降等问题,并通过模型算例验证了算法的精度和效率。
3)对氢氧预混气体自燃问题开展了DSMC模拟研究,发现并首次系统地解释了预混气体在诱导阶段出现的振动非平衡现象以及点火延迟时间的随机涨落现象,促进了对微尺度燃烧过程的理解,说明了宏观反应动力学方法在微尺度燃烧中的局限性。对振动非平衡的分析表明,由于高温(>1500K)时氢气、氧气的振动松弛时间与点火延迟时间较为接近,链式反应的振动倾向对预混气体热力学状态的影响能够持续积累下来从而引起振动温度的明显降低。通过混合格式的随机模拟算法,对微尺度自燃的随机涨落开展了统计分析,结果表明:在链式反应机制的影响下,化学反应事件的微观随机性会显著影响微观到介观尺度的点火过程,导致低温自燃过程出现点火延迟时间波动、平均点火延迟延长等现象;借助于量纲分析,得到了点火延迟时间的标准差的近似理论表达式,明确了温度、系统体积等参数的定量影响。研究认为,微观随机涨落可能是低温非均匀点火现象的一种被忽视的潜在影响因素。
4)对氢氧爆轰问题开展了DSMC模拟,揭示了一维爆轰传播的微观细节。由于爆轰波后诱导区中的模拟粒子数有限,提出了初始自由基生成技术,控制了因展向尺度限制带来的起爆过程延后问题,获得了稳定传播的爆轰波以及一维爆轰波的精细结构。对模拟结果的分析表明,爆轰波后气体的点火过程在反应一开始就处于振动非平衡态,化学反应速率因为低振动温度的影响而受到抑制,明显地延长了预混气体在波后的点火延迟时间,在空间上表现为反应区半宽的增加(增幅接近50%),说明了振动非平衡对爆轰波精细结构的影响。

英文摘要

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.

语种中文
文献类型学位论文
条目标识符http://dspace.imech.ac.cn/handle/311007/73171
专题高温气体动力学国家重点实验室
空天飞行科技创新研究中心(筹)
作者单位1.中国科学院力学研究所
2.中国科学院大学工程科学学院
推荐引用方式
GB/T 7714
杨超. 氢氧预混燃烧的微尺度模拟与机理分析[D]. 北京. 中国科学院大学,2018.
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