燃烧产生自由分子区纳米颗粒的团聚-烧结核模型研究

The formation and growth processes of combustion-generated fine particles are involved in complex particle dynamic mechanisms, among which aggregation and sintering are two of the most critical mechanisms tailoring the morphology and size of particles. As known, the particle aggregation will enlarge particle while lead to irregular morphology, and the particle sintering under high temperature will "smooth" particle surface and conduce restructuring and spheroidization of irregular particles. Thus the competing aggregation-singtering mechanisms determine the surface area, characteristic diamater, internal inhomogeneous structure, and then, the functionality of nanoparticles. It is the key issue for the macroscoic simulation of the dynamic evolution processes (such as population balance modelling) to obtain the kernel models (or occurrence rates) of the two dynamic events. Gas-to-particle synthesis under high temperature is one of the most important methods of nanoparticle synthesis. In order to in-depth understand the inherent mechanisms, this project will first simulate the collision-aggregation process of nanoparticles in the free-molecular regime with respect to typical combustion environment from a mesoscopic view. Accounting for the molecular discontinuity effect from the viewpoint of nanoparticles, the continuity assumption of gas is no longer applicable, the direct simulation Monte Carlo method is used to describe the gas dynamics, and then the Brownian diffusion and particle interactions (such as van der Waals forces, electrostatic force, etc.) are considered in the DSMC-based gas-solid flows model. Using the model, the detailed insight in the collision-aggregation processes is received to explore the collision-aggregation mechanisms and kernel models of nanoparticle. Then, aiming at the process of nanoparticle synthesis via combustion, the well-designed experiments are carried out to obtain the morphology, surface area, size distribution of nanoparticles sampled from typical sintering-dominated, aggregation-sintering- competed, and sintering-dominated regions, respectively. These experimental results will help explore the rules of tailoring nanoparticles and the physical nature of competing aggregation-sintering mechansims. At last, based on the mesoscopic simulation of aggregation kerenl and well-designed experimental results, the inverse problem of population balance formulated in Monte Carlo models for nanoparticle dynamics is proposed to determine some uncertain model parameters of aggregation- sintering kernel, furthermore, analyse uncertainty propagation. All of these studies will guide the tailoring and optimization of nanomaterials synthesized by the combustion method, and also provide reference for the scientific perspective on combustion-generated particulate matter.

燃烧源细微颗粒物的生成生长过程涉及复杂颗粒动力学机理，团聚-烧结是控制颗粒物形貌和尺度的最关键机理，其速率(核)模型是对颗粒经历过程进行宏观模拟(如颗粒群平衡模拟)的关键。首先对燃烧环境中自由分子区纳米颗粒碰撞-团聚进行微观模拟，采用不依赖于流体连续性假设的直接模拟Monte Carlo方法描述气相场，考虑布朗扩散和颗粒间相互作用力(如范德华力、静电力等)，研究典型燃烧区域碰撞-团聚机理及核模型；并针对燃烧制备TiO2纳米颗粒过程，实验研究典型烧结主导区、团聚-烧结制约区、团聚主导区的颗粒形貌、表面积、尺度分布等，探究调控颗粒形貌尺度的规律和团聚-烧结竞争机制的物理内涵；最后基于团聚微观模拟和精确实验结果，利用颗粒群平衡模拟反问题方法，辨识团聚-烧结核的不确定模型参数，进行参数不确定度量化和误差传播分析。由此可指导燃烧合成纳米材料的尺度形貌调控和优化，并为燃烧源细微颗粒物的研究提供参考。

燃烧源细微颗粒物的生成生长过程涉及复杂颗粒动力学机理，团聚-烧结是控制颗粒物形貌和尺度的最关键机理，其速率(核)模型是对颗粒群动力学演变过程进行宏观模拟(颗粒群平衡模拟)的关键。本课题首先发展了高效快速的DSMC气固两相流模型，研究了火焰环境中TiO2纳米颗粒自由分子区碰撞-团聚过程及颗粒尺度形貌调控机理。主要考虑范德华力、静电力、热泳力等影响因素，探究自由分子区纳米颗粒动力学核模型。针对燃烧法制备TiO2纳米颗粒，进行适应反问题需求的实验设计和实验测量，发展快速高精度异权值Monte Carlo模型和算法，进行正问题颗粒群平衡模拟和反问题模型参数辨识，确定了多个动力学模型及参数，包括成核、团聚、烧结和相变。开发出了耦合火焰流体动力学与颗粒群动力学的CFD-PBMC高效计算方法，实现燃烧制备过程的控制、工艺的优化、产品的理性设计。分别设计出了功能化的氧载体、CO2吸收剂和催化剂应用于化学链燃烧、钙循环、光催化制氢以及催化燃烧等。