基于分形理论的柴油机排放颗粒物生长过程研究

Particle Matter（PM）is the main pollutants in the exhaust of diesel engine. However, the understanding of the PM formation is still not enough now. The present research investigates deeply the PM formation process based on modern fractal theory, the main contents are as follows: 1. The model of primary particles nucleation is established based on the homogeneous saturated theory, by which the rate of nucleation, the critical diameter of nucleation and the distribution in spatial-temporal of primary particles are obtained. 2. The physical and mathematical model is established by using fractal theory on the dynamic evolution process of particles after the nucleation, including particles collision, coagulation, surface deposition, aggregates fragmentation etc. and the relationship between the fractal structure characteristics of particles and their growth environment such as ambient temperature, pressure, the concentration of fuel components, and fluid transport parameters and flowing state are analyzed .Based on the above, the effects of the combustion systems structure, operating conditions, fuel and the flame structure on the growth of particles are studied, by which the morphology and aggregation state of the particles are obtained. Simultaneously, the formation mechanism of the fractal structure are explored. 3. Combined with image analysis techniques, the sampling and electron microscopy analysis of the particles are carried out on engine test bench to determine the fractal structure characteristics of particles..Fractal theory being applied to the growth process of diesel PM, the research results can provide theoretical basis for the improvement of combustion and the development of advanced PM emission control technologies, and has important scientific significance on reduce the combustion source PM emissions and solve the problems of environmental pollution.

碳烟颗粒物是柴油机排放的主要污染物，目前对其形成机理的认识尚且不足，本项目基于现代分形理论对此进行深入研究，主要涉及：1.基于均相饱和理论构建颗粒物初始胚核的成核模型，获得初始胚核的成核速率、临界成核直径和时空分布；2.利用分形理论对成核后颗粒物的运动碰撞、凝聚、表面沉积、聚集体断裂等动力学演变过程建立物理数学模型，确立颗粒物的分形特征与其生长环境如环境温度、压力、组分浓度、输运参数和流态等因素之间的关系。据此研究燃烧系统的结构、工况、燃料、火焰结构对颗粒物生长的影响，获得颗粒物形貌结构、聚集状态等信息，探讨其分形结构的形成机制；3.在实验台架上对柴油机颗粒物进行抽样和电镜分析，结合图像分析技术，确定颗粒的分形结构特征。.将分形理论应用于柴油机颗粒物生长过程的研究，研究成果可为改善燃烧和研发先进的颗粒物排放控制技术提供理论依据，对降低燃烧源颗粒物排放、解决环境污染问题具有重要的科学意义。

柴油机排放的碳烟颗粒物对人体健康和环境危害极大，为充分了解其形成过程、降低其排放，本项目基于现代分形和相关理论对此进行了深入研究。碳烟生成始于成核，成核过程直接关系到碳烟排放预测的准确性，而多环芳香烃分子（PAHs）作为碳烟成核的重要前驱物，其生长机理的研究尤为重要，本项目以苯基为模型，以乙炔，乙烯基或乙烯基乙炔为加成物，获得了苯环生长到萘环的详细机理，完善了目前的PAHs生长路径。碳烟成核过程也是碳粒生长演变中最具争议的部分，国内外有关碳烟成核组分的研究都还在不断的探索和完善之中，本项目使用分子动力学方法研究了菲分子（A3）的碰撞过程，构建了两个A3分子的原子模型，证明了其成核的可能性，为碳烟成核组分的确定提供了理论依据。对于碳烟颗粒成核后的运动碰撞、凝并、表面沉积、聚集体断裂等动力学演变过程，本项目采用Fortran语言，构建了碳烟颗粒凝聚成核的分形数学模型和生长演变模型，引入了分形维数、包容半径和凝聚体体积来描述凝聚体的分形生长特征，分别基于矩方法和Monte Carlo方法求解了颗粒群平衡方程，考察了生长环境及燃料添加对碳烟颗粒生长演变的影响，获得了碳烟颗粒形貌结构、聚集状态、不同火焰结构下的尺寸分布状态等信息，进一步掌握了碳烟颗粒的生长演变及抑制其生成的相关措施。本项目还基于柴油机实验台架及相关测量设备（场扫描电镜、透射电镜和快速响应颗粒分析仪DMS500等），进行深入系统的试验研究，获得了颗粒物微观结构、形貌特征、分形维数等，及其与柴油机工况的关系，同时还获得了柴油机工况和燃料结构对颗粒物尺寸、数量和质量的影响。研究发现结合先进控制技术，如喷油策略和EGR等，含氧燃料添加可有效降低聚集态颗粒物（50nm<粒径<1000nm）数量，从而使颗粒物总质量排放降低，但却使核模态颗粒（粒径<50nm）的数量增加，而核模态小颗粒对人体健康和环境危害更大。本项目研究对于缓解能源危机和降低大气污染问题具有极其重要的理论价值和现实意义。