【作者】 张引弟；

【导师】 周怀春；

【作者基本信息】 华中科技大学 ， 热能工程， 2011， 博士

【摘要】 现代社会约88%的能源由化石燃料的燃烧所产生,燃料的燃烧也引起了大气污染和能源危机等社会问题及环境问题。乙烯是化石燃料的一种重要成分,也是高阶碳氢燃料高温分解过程中产生的一种中间产物,研究其燃烧化学动力学对理解更高阶的碳氢燃料高温分解过程有帮助。另一方面,乙烯较之于其它低碳碳氢燃料更易产生烟黑这一重要的污染物。因此本文选用乙烯的氧化作为研究烟黑生成的平台。烟黑是化石燃料燃烧排放的主要颗粒物,其排放不但会降低燃料发热利用率,也会对人体会造成极大的危害。然而,相对单一的动力学模拟研究,烟黑在实际形成过程中涉及到热力学、流体力学、传质传热及化学反应动力学等输运的知识,是耦合质量能量交换的复杂物理化学过程,对烟黑详细形成过程的理解和有效模拟有待进一步研究。为此,本文基于化学反应动力学模拟研究,尝试简化既定系统的化学反应机理,进而发展化学反应动力学与计算流体力学(CFD)、辐射传热及烟黑模型的耦合模拟研究。本文系统地综述了烟黑形成国内外研究现状,重点阐述烟黑生成机理的模型研究。详细介绍了碳氢燃料化学反应动力学模型的简化方法,重点阐述了当前较成熟的乙烯氧化反应机理类型。在此基础上,本文对乙烯氧化过程中烟黑及中间体前驱物的生成行为进行了较系统的模拟研究。本文首先基于动力学模拟软件CHEMKIN-PRO及高级功能(API),采用两种反应机理对低压和富燃条件下,一维预混的C2H4/02/Ar火焰中烟黑前驱物的生成特征进行了模拟分析和验证。结果表明,对单环芳烃苯及大分子多环芳烃(PAH)中间体的预测,本文选用的两种机理优劣不明显。此外,对前人实验系统用1个全混反应器(PSR)和2个柱塞流反应器(PFR)的组合反应器来建模和模拟研究。模拟工作中,首先采用经典的乙烯燃烧化学动力学模型(Wang-Frenklach气相机理模型)对大气压下、富燃的C2H4/O2/N2火焰中,芘及以下小分子中间体的形成进行了模拟和验证。在综合考虑模拟和实验误差的基础上,通过比较模拟和实验结果改进了本文选用的气相模型,在原模型基础上添加了小分子PAH-PAH的缩合反应,与脱氢-C2H2-加成(HACA)生长路径共同来描述苯到大分子PAHs的生长机理。在此基础上,本文进而采用改进后的气相模型耦合表面化学的方法,应用粒子跟踪特征程序来预测烟黑的转化行为特征。通过比较预测与实验结果表明,本文的建模合理,改进的模型(HACA+PAH-PAH缩合生长机理)能够有效的预测烟黑前驱物及体积浓度分布。烟黑前驱物从苯生长到大分子PAH的过程是HACA与PAH-PAH缩合两个生长机理共同作用的结果。基于详细的化学机理模拟多维燃烧时,由于反应过程包括成千上万的组分和基元反应而使计算无能为力。为了发展计算燃烧学,有必要对详细的化学反应机理进行简化。为此,本文基于GRI-Mech 3.0,在常压、化学计量比条件下的一维预混火焰中,采用敏感性分析、反应路径分析(RPA)结合准稳态假设(QSSA)和计算机帮助简化机理(CARM)程序包,发展了一个19组分20步的简化动力学模型。进而分别在PSR反应器中分析测试和在一维预混火焰中验证。结果表明,简化的模型能够在一个较大当量比范围内(0.01-2.5)合理的预测乙烯在空气中燃烧的各特征量。在对二维扩散火焰的研究中,本文模拟研究了同向C2H4/O2/N2扩散火焰的温度和烟黑体积浓度分布。数值模拟工作采用简化的气相化学模型和一个2-D火焰源代码通过CHEMKINⅡ耦合复杂的热传输特性,一个简单的二方程烟黑模型用来预测烟黑生成。基于上述模拟计算与原程序包(GRI-Mech3.0的耦合)的计算时间进行了比较。结果表明,在同等的计算精度下,简化的模型与2-D火焰源代码耦合后相比原程序包可节省52.5%的计算时间,这为与多维燃烧的耦合计算提供了思路。此外,辐射图像处理技术和解耦的重建方法同时用来测量与模拟同等条件下火焰的温度和烟黑的体积浓度,通过比较预测结果和测量结果表明,两者吻合良好。作为简化模型与燃烧耦合的应用,本文采用该简化模型耦合上述的2-D火焰源代码,探索了伴流空气流速和微重力对火焰结构及烟黑生成特性的影响。结果表明,在正常重力情况下,随伴流气体速度的减小,火焰的外形结构变化不大,而烟黑的总生成量逐渐增加。随重力加速度从1g减小到Og,火焰外形结构由细长形变为微圆形且径向变宽；比较1g和Og工况下的预测结果表明,混合流场最大值骤减；火焰最高温度略微减小,高温区变短；烟黑体积浓度剧增,可见火焰高度略微增加；CO和一些未提供组分的浓度分布沿径向变宽。在1g和0.5g工况下扩散火焰由浮力控制,在Og时火焰由径向动量控制；温度和烟黑的体积浓度都分布在焰舌下部的一个环形区域。更多还原

【Abstract】 Combustion of fossil fuels provides around 88% of total energy supply for modern society, and meanwhile causes many environmental problems and social problems such as air pollution and energy crisis. Ethylene itself is major components of fossil fuels, and is an important intermediate product in the oxidation of higher-order hydrocarbons. Therefore investigation on the chemical kinetics of ethylene will help us understand the combustion behaviors of higher-order hydrocarbons and predict key parameters of practical combustion processes. On the other hand, ethylene has greater sooting tendencies than other low-order hydrocarbons, making its combustion an ideal system to study soot formation mechanism. Soot is the main particulate matter produced by burned fossil fuel, and it represents unrealized chemical energy of fuel. The emission of soot by combustion processes can reduce utilization rate of heat output of fuel, and tiny particulate matter (PM2.5) can cause serious harm to human health. Otherwise, soot formation process is a complicate physicochemical process, which involves an exchange of mass energy, such as thermodynamics, fluid dynamics, heat and mass transfer and chemical reaction kinetics. It is an interesting topic in understanding soot formation in detail and effective simulation. So, based on study of simulation for chemical reaction kinetics, the effort is to simplify detail mechanism for a specific combustion system of hydrocarbons. Another goal is to develop computational combustion for practical ways to numerically simulate flame environments, such as chemical kinetics coupled with CFD, radiant heat transfer and soot kinetics.In this dissertation, the present situation of investigations about soot formation and oxidation at home and abroad was systematically summarized. The mechanism models of soot formation and influences in soot formation process were emphatically elaborated. The simplification methods for chemical reaction kinetics models of hydrocarbons and characteristics for different reaction models were presented in detail. The fairly mature mechanisms of ethylene oxidation were were emphatically elaborated. On this basis, the characteristics of soot formation and intermediates for ethylene oxidation were systemly investigated by simulation. Firstly, bssed on CHEMKIN-PRO and advanced functions (AIP), the formations of soot precursors for ethylene flame is investigated by kinetics modeling. In the kinetic modeling work, different pressure laminar premixed ethylene/oxygen/argon flames at broad ranged of equivalence ratio (stoichiometric and rich flames) were computational studied using different gas mechanisms for formation of soot precursors, and regularly results are given. Otherwise, a reaction system for jet stired ractor/plug flow reactor (JSR/PFR) was modeled by using one perfectly stirred reactor (PSR) and two PFR and investigated by simulation. In the simulation, the gas model (Wang-Frenklach mechanism), which is used by this paper, was optimized to satisfiedly predict intermediates and soot volume fractions for ethylene oxidation using Particle Tracking Feature. The results show that modeling is reasonable in this paper, and the growth process from benzene to PAHs is caused by H-abstraction-C2H2-addition (HACA) mechanism combining PAH-PAH radical recombination and addition reactions.Nevertheless, detailed chemical kinetics simulation of hydrocarbon combustion within multidimensional turbulent reacting flows is computationally prohibitive. Therefore, it is necessary to develop a reduced mechanism with a minimum number of species and reactions. The reduced mechanism (19 species and 20 reactions) is obtained from the full ethylene mechanism (GRI-Mech 3.0) by using sensitivity and reaction path diagram analysises (RPA), quasi steady state assumption (QSSA) and Computer Assisted Reduction Mechanism (CARM) software in this paper. And the calculation results are in good agreement with the existing detail mechanism and literatures.For investigation of two-dimension diffusion flame, a combined computational and experimental investigation that examines temperature and soot volume fraction in coflow ethylene-air diffusion flames was presented. A numerical simulation was conducted by using a reduced gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete ordinates method. The results show that it saves 52.5 percent calculation time by comparing the computation time using reduced mechanism and detail menchanism. Otherwise, an image processing technique and decoupled reconstruction method were used to simultaneously measure the distributions of temperature and soot volume fraction. The results show that the simulation results are identical to the measurement results.As an application attempt, the reduced mechanism coupled with 2-D flame code by using CHEMKIN II to investigate the effects of coflow velocity and gravity on flame structure and soot formation in diffusion flames. The results show that the coflow velocities produce little influence on the distribution of temperature and average velocity in normal gravity. However, the enhancement soot formation is observed when the coflow air velocity is decreased. The gravity has a rather significant effect on the flame structure and soot formation such as flame shape, mixture velocity, temperature, species fraction and soot. The visible flame height in general increase with the gravity from 1g decreased to Og. The peak flame temperature decreases with decreasing gravity level. The peak soot volume fraction increases with decreasing the gravity level. Comparing the calculated results from 1g to Og, the flame becomes wider along radial direction, the high temperature zone becomes shorter, the mixture velocity has a sharp decrease, the soot volume fraction has a sharp increase, CO and unprovided species mole fraction distribution becomes wider along radial direction. At normal and half gravity, the flame is buoyancy controlled and the axial velocity is largely independent of the coflow air velocity. At microgravity (0g), the flame is momentum controlled. The temperature and soot volume fraction in each case occur in the annular region.更多还原