【作者】 李铭迪；

【导师】 王忠；

【作者基本信息】 江苏大学 ， 动力机械及工程， 2014， 博士

【摘要】 颗粒物污染是我国最主要的大气污染之一,汽车颗粒排放已成为大气可吸入颗粒物的主要来源,严重危害人类健康。内燃机燃用清洁含氧代用燃料,不仅可以部分替代石油,同时能够降低排放污染物。论文针对乙醇、二甲醚(DME)、碳酸二甲酯(DMC)和生物柴油等含氧燃料,围绕前驱体的形成机理、中间燃烧产物形成过程、颗粒的氧化特性及结构特征等方面,采用同步辐射真空紫外光电离质谱、热重分析、扫描/透射电镜、小角X射线散射、台架试验、数值模拟等方法,测量了中间及最终燃烧产物摩尔分数的变化规律,从化学反应动力学角度,对苯、萘、菲、芘等重要前驱体物质的形成过程进行了数值模拟,探讨了燃料分子结构和含氧量对颗粒氧化特性以及颗粒微观结构、粒径分布、分形维数等结构特征参数的影响规律,提出了颗粒数的测量方法。根据碳氢燃料的燃烧特点,在总结苯环形成路径和芳香烃生长机理的基础上,构建了包含芳香烃(PAHs)生成过程的乙醇、DME、DMC和乙醇/正庚烷化学反应动力学模型,采用激波管、预混火焰和内燃机反应模型,运用生成速率和敏感性分析方法,对PAHs的形成过程进行了数值模拟。研究表明,丙炔基聚合环化反应是形成苯环的主要路径,脱氢加乙炔促进了苯环的生长,H和OH自由基消耗了生成的PAHs。对于第三体“M”参与的反应,燃用乙醇可以抑制PAHs的形成,燃用DME对PAHs的形成影响不大,燃用DMC可以促进PAHs的形成。乙醇/正庚烷缸内燃烧过程中,PAHs主要在缸内温度急剧升高的阶段形成,正庚烷和正丙基的分解反应抑制了菲和芘的生成,增加乙醇掺混比例可以减小PAHs的生成速率和循环生成量。采用同步辐射光电离质谱与光电离效率谱相结合的方法,确定了乙醇／正庚烷层流预混火焰中的最终燃烧产物以及乙炔、乙烯、丙炔基、甲醛、乙醛等中间燃烧产物,研究了不同当量比和乙醇掺混比条件下,中间及最终燃烧产物摩尔分数的变化规律。测量结果表明,H2、CO的摩尔分数随着当量比的增加而增加,随着乙醇掺混比例的增加,甲醛和乙烯酮等非常规污染物的摩尔分数下降,而乙醛的摩尔分数增加。通过分析乙醇中氧原子的迁移路径,确定了乙醇中的燃料氧一部分进入了最终燃烧产物H2O中,一部分进入了甲醛和乙醛中。结合数值模拟结果,提出掺混乙醇增加了火焰中含氧自由基的浓度,能够显著降低苯、乙炔、乙烯和丙炔基等前驱体物质摩尔分数的观点。采集乙醇/柴油、DMC/柴油和生物柴油/柴油的燃烧颗粒,采用热重分析的方法,测量了颗粒的TG和DTG曲线,确定了颗粒脱水干燥、挥发分析出和燃烧、碳烟燃烧三个失重阶段,提出采用起始失重/析出/燃烧温度、最大失重速率和对应温度、活化能等参数评价颗粒的氧化特征。试验结果表明,随着燃料含氧量的增加,碳烟的百分比减小,挥发性有机物的百分比增大。增加燃料的含氧量,挥发性有机物的起始燃烧温度TSOF2降低、碳烟的起始燃烧温度Tsoot升高、最大失重速率增加、颗粒的活化能下降。根据乙醇、DMC和生物柴油的分子结构特点,提出含氧基团促进了C-C和C-H键的断裂,是碳烟中元素碳比例减少的主要原因；此外,缩短碳链长度可以增加颗粒表面吸附的含氧基团,从而促进挥发性有机物的燃烧。在YZ4DB3柴油机上,采集了燃用乙醇／柴油的示功图,分析了燃烧过程的变化规律,探讨了燃烧过程与颗粒氧化特性之间的关系。结果表明,掺混乙醇的滞燃期延长、燃烧持续期缩短；小负荷时表现为扩散燃烧的放热率峰值升高,大负荷时表现为预混燃烧的放热率峰值升高。掺混含氧燃料通过影响燃烧过程以及中间和最终燃烧产物的形成过程,改变了颗粒特性,使得燃烧颗粒的粒径减小、有机物组分的比例增加、基本碳粒子内部无序结构减少、团聚程度提高、颗粒的活化能下降。采用扫描/透射电镜、同步辐射小角x射线散射的试验方法,考察了乙醇/柴油、DMC/柴油和生物柴油/柴油的颗粒形貌,研究了基本碳粒子的无序和内核-外壳微观结构,探讨了燃料分子结构和含氧量对颗粒粒径分布、界面厚度和分形维数等尺度参数的影响规律,提出了根据拐点和弧长个数确定团聚颗粒中单个颗粒个数的计算方法。研究表明,基本碳粒子的粒径呈高斯分布,粒径范围在14-45nm之间,平均层面间距在0.32-0.44nnm之间,微晶尺寸分布在lnm左右,弯曲度在0.8～2.0之间。增加乙醇和DMC掺混比例,基本碳粒子的平均层面间距增大、微晶尺寸减小、平均弯曲度增加；生物柴油掺混比例增加,燃烧形成基本碳粒子的氧化难度增加。颗粒的回转半径和界面厚度与燃料含氧量近似呈线性关系,燃料含氧量增加1%,颗粒的回转半径约减小2.3%,界面厚度约增加2%。掺混含氧燃料使得颗粒的表面分形维数和质量分形维数增加,颗粒表面粗糙度、不规则度和团聚重叠程度提高。更多还原

【Abstract】 Particle produced by incomplete combustion is one of the main air pollutants in China. Particles emitted by cars have become the major source of inhalable particles and have serious harm to human health. Internal combustion engine fueled with oxygenated fuels can not only partially replace oil but also reduce the emissions. The soot precursors formation mechanism, intermediate species formation process, and particle state characteristics of ethanol, dimethyl ether (DME), dimethyl carbonate (DMC), and biodiesel were studied. Methods of molecular beam mass spectrometry with tunable synchrotron photoionization, thermogravimetric analysis, scanning electron microscope, transmission electron microscopy, small angle X-ray scattering, bench test, and numerical simulation were used in this dissertation. The mole fraction profiles of major and intermediate flame species were measured and analyzed. The numerical simulation of important soot precursors, such as benzene, naphthalene, phenanthrene, and pyrene, were carried out from the perspective of chemical reaction kinetics. The effects of molecular structure and oxygen content of oxygenated fuels on the oxidation properties, microstructures, size distribution, and fractal dimension of particles were discussed. The measurement of particle number was proposed.The formation paths of benzene and the growth mechanism of polycyclic aromatic hydrocarbon (PAHs) were summarized. According to the combustion characteristics of hydrocarbon fuels, the chemical kinetics model of ethanol, DME, DMC, and ethanol/n-heptane which containing the PAHs formation process were established. The shock tube, premixed flame, and internal combustion engine reaction models were adopted, and both the rate-of-production and sensitivity analysis methods were used to simulate the formation process of PAHs. The results showed that benzene was mainly formed by polymerization of propargyl. Multiple benzene rings were formed by hydrogen abstraction and acetylene addition (HACA) reaction. H and OH radicals consumed most of the PAHs. In the reactions with the third body "M", ethanol can inhibit the PAHs formation, DME had little effect on the PAHs formation, and DMC can promote the PAHs formation. During the combustion process of ethanol/n-heptane, PAHs mainly formed during the phase of rapid increase in cylinder temperature. The decomposition reactions of n-heptane and n-propyl group can inhibit the formation of phenanthrene and pyrene. The formation rate of PAHs and the amount of PAHs during one cycle were reduced by increasing the ethanol blending ratio.Major and intermediate species of ethanol/n-heptane premixed flame, such as acetylene, ethylene, propinyl, formaldehyde, and acetaldehyde, were identified using molecular beam mass spectrometry with tunable synchrotron photoionization. The effects of ethanol blending ratio and equivalence ratio on the mole fraction profiles of major and intermediate flame species were studied. Study results showed that the mole fractions of H2and CO increased with the increase of equivalence ratio. With the increase of ethanol blending ratio, the mole fractions of formaldehyde and ketene decreased while the mole fraction of acetaldehyde increased. By analyzing the migration pathways of oxygen from ethanol, it was found that part of the oxygen formed the final combustion product H2O and part of the oxygen formed the formaldehyde and acetaldehyde. According to the simulation results, the oxygen from ethanol led to an easier production of oxygenated intermediates, compared with oxygen from the oxidizer, which can reduce the mole fractions of benzene, acetylene, ethylene, and propinyl effectively.Particles produced by ethanol/diesel, DMC/diesel, and biodiesel/diesel combustion process were collected. The thermo gravimetric (TG) and derivative thermo gravimetric (DTG) curves of those particles were measured using thermo gravimetric analysis (TGA). The weight loss of particles mainly included three stages which were water evaporation, precipitation and combustion of volatile organic compounds, and combustion of soot. The initial weight loss temperature, precipitation temperature, ignition temperature, maximum weight loss rate and corresponding temperature, and activation energy were applied to evaluate the oxidation characteristics of particles. Study results showed that with the increase of oxygen content, the percentage of soot decreased and that of volatile organic compounds increased. With the increase of oxygenated fuels blending ratio, the ignition temperature of volatile organic compounds dropped and that of soot rose, the maximum weight loss rate of soot increased, and the activation energy of particles decreased. According to the molecular structure characteristics of ethanol, DMC and biodiesel, the oxygen-containing groups played a role in promoting the break of chemical bond and that resulted in the decrease of the percentage of elemental carbon. The oxygen-containing groups adsorbed in the surface of particles were increased by shortening the carbon chain length of oxygenated fuels. And the oxidization of volatile organic compounds was promoted with the increase of oxygen-containing groups.The indicator diagram of ethanol/diesel with different ethanol ratios was performed on YZ4DB3diesel engine and the combustion process was studied. The relationship between oxidation characteristics and combustion process was analyzed. The results showed that with the increase of ethanol blending ratio, ignition delay period extended and combustion duration shortened, the peak heat release rate of diffusion combustion increased at light load, and the peak heat release rate of premixed combustion increased at heavy load. After adding oxygenated fuels, the combustion process and the formation process of flame species were changed, the characteristic of particles changed as well. The size of particles decreased, the percentage of organic matter increased, the appearance of disordered structure of elementary particle decreased, the reunion level of particles increased, and the activation energy decreased.The particle morphology of ethanol/diesel, DMC/diesel and biodiesel/diesel were studied using scanning electron microscope, transmission electron microscopy, and small angle X-ray scattering. The disordered and shell-core structures of elementary particle were analyzed. The effects of molecular structure and oxygen content of oxygenated fuels on the size distribution, interracial thickness, and fractal dimension of elementary particle were discussed. The measurement of particle number was proposed according to the number of inflection point and arc length. The size distribution of elementary particle followed the Gaussian distribution and the size was between14nm and45nm. The average fringe separation distance was between0.32nm and0.44nm. The fringe length was about1nm and the tortuosity was between0.8and2.0. With the increase of ethanol and DMC blending ratio, the average fringe separation distance of elementary particle increased, the fringe length decreased, and the average tortuosity increased. With the increase of biodiesel blending ratio, it was more difficult for the elementary particle to be oxidized. There was a linear relationship between turning radius, interracial thickness and the oxygen content. After increasing the oxygen content by1%, the turning radius decreased by about2.3%and the interracial thickness increased by2%. The surface and mass fractal dimension surface roughness, irregularity, and aggregation level of particles formed by oxygenated fuels combustion process increased.更多还原