硬球—拟颗粒—软球耦合模拟及其化工应用

发布时间：2018-08-17 19:57

【摘要】：纳微流动与传递过程广泛存在于化学工程领域,诸如多相介质的界面、催化剂颗粒中的多级孔道和微化工系统等。随着对化工工艺和设备精准设计与调控要求的不断提高,认识这些过程的机理变得越发重要。在纳微尺度,对动态过程的全面实验研究还存在诸多困难,而传统的连续介质模拟方法随着系统特征努森数的增大逐渐失效。在该尺度下,流体更多地表现出分子的离散性质,因此各种离散粒子方法的应用近年来受到广泛重视,但其计算速度与精度之间的矛盾一直没有很好地解决。为此,本论文通过不同离散粒子方法的耦合建立了一套能够从微观水平高效准确地模拟纳微尺度或其他高努森数条件下气体流动、扩散和反应过程的模型和算法框架。论文的主要内容如下：绪论部分分析了软球、硬球、拟颗粒和直接模拟蒙特卡洛等典型的离散粒子模拟方法及其各自的优缺点以及前人在耦合不同模型方面的工作,据此提出了本论文的研究思路。即：以严格的软球模型(或其组合)模拟稠密或接近界面的复杂过程,而以简化的硬球模型(或其组合)模拟稀薄和远离壁面的条件下气体的流动与扩散过程,而以结合两者优势的拟颗粒模拟提供其过渡以及硬球并行模拟中局部的近似。第二章首先改进了事件驱动的硬球模拟与时间驱动的拟颗粒模拟的耦合与并行方法,严格确定了硬球与拟颗粒物性间的转换关系,并在管内流动和扩散模拟中验证了其正确性,表明了该耦合能有效克服硬球模拟扩展性差和拟颗粒模拟对稀薄气体效率低的问题,实现高效准确的大规模并行模拟。该方法还成功应用于气体在纳微孔道内的非平衡扩散以及在复杂多孔介质内的扩散研究,表明了其在微化工过程和催化剂开发等方面实际应用的可行性。另外,应用该方法还初步开展了高超声速流动的模拟,说明了其在航天航空等其他领域的潜在应用价值。第三章从算法改进和并行优化两方面深入研究了软球模拟方法,创新提出了关于粒子搜索的多壳层邻居列表算法。该算法通过对粒子跨越各壳层可能性的简单预估,有效提高了搜索效率。在此基础上,发展了针对软球模拟的多线程和向量化并行、众核与多核处理器协同、计算／通信／存取重叠等方法,建立了一套高效的大规模并行程序。基于上述工作,第四章最终实现了软球、硬球和拟颗粒模拟三者的耦合。论文导出了软球与拟颗粒模型间的参数转换关系,提出了采用不同模型的区域间通用的连接模式,在保持软球区域外部有拟颗粒过渡层的条件下,可构建任意复杂的界面。论文还为耦合模拟建立了简单几何体和固定粒子等多种边界条件,提高了方法的实用性。论文还通过经典的管流模拟等验证了该方法的正确性。第五章应用上述耦合模拟方法通过简单的概念模型研究了气固界面反应中界面结构对扩散和反应过程的影响。研究发现,在给定的反应条件下,由于扩散和反应过程在不同条件下相互影响的不同方式,界面结构存在最有利于整体反应速率的特定形状参数。通过建立更真实与细致的反应物及界面模型,该耦合方法有望提供诸如催化剂孔道结构设计等方面的机理分析与优化工具。第六章概括了论文的主要结论和创新点,并展望了后续工作。
[Abstract]:Nano-micro flow and transfer processes exist widely in chemical engineering, such as the interface of multiphase media, multistage channels in catalyst particles, and micro-chemical systems. With the increasing requirements for precise design and control of chemical processes and equipment, it becomes more and more important to understand the mechanism of these processes. At nano-micro scale, dynamic processes are becoming more and more important. There are still many difficulties in the comprehensive experimental study, but the traditional continuum simulation method gradually fails with the increase of the system characteristic Knudsen number. At this scale, fluids show more discrete properties of molecules, so the application of various discrete particle methods has been widely valued in recent years, but there is a contradiction between the calculation speed and accuracy. The main contents of this paper are as follows: In the introduction part, the soft sphere and the hard sphere are analyzed. Some typical discrete particle simulation methods, such as sphere, quasi-particle and direct simulation Monte Carlo, and their respective advantages and disadvantages, as well as the previous work on coupling different models, are proposed in this paper. That is, the rigid soft sphere model (or its combination) is used to simulate the complex process of dense or close to the interface, while the hard sphere is simplified. The model (or its combination) simulates the gas flow and diffusion processes in thin and far-from-the-wall conditions, and provides the transition and local approximation in the hard-sphere parallel simulation by combining the advantages of both quasi-particle simulation. In Chapter 2, the coupling and parallel methods of event-driven hard-sphere simulation and time-driven quasi-particle simulation are improved. The conversion relationship between hard sphere and pseudo-particulate matter is strictly determined, and its correctness is verified in the simulation of flow and diffusion in pipe. The results show that the coupling can effectively overcome the problems of poor expansibility of hard sphere simulation and low efficiency of pseudo-particulate simulation for rarefied gas, and realize efficient and accurate large-scale parallel simulation. The non-equilibrium diffusion in nano-microporous channels and the diffusion in complex porous media show the feasibility of its practical application in micro-chemical process and catalyst development. In the third chapter, the soft sphere simulation method is deeply studied from two aspects of algorithm improvement and parallel optimization, and the multi-shell neighbor list algorithm about particle search is innovatively put forward.The algorithm effectively improves the search efficiency by simply predicting the possibility of particles crossing each shell.On this basis, the multi-threading and direction of soft sphere simulation are developed. Based on the above work, Chapter 4 finally realizes the coupling of soft-sphere, hard-sphere and quasi-granular simulation. The paper derives the parameter transformation relationship between soft-sphere and quasi-granular model, and proposes to adopt the method. A general connection model between different regions can be used to construct arbitrarily complex interfaces while maintaining the quasi-particle transition layer outside the soft sphere. Simple geometry and fixed particle boundary conditions are also established for the coupling simulation, which improves the practicability of the method. In the fifth chapter, the effect of interface structure on diffusion and reaction process in gas-solid interface reaction is studied by using the coupling simulation method. It is found that the interface structure exists most under given reaction conditions due to the different ways in which diffusion and reaction process interact under different conditions. By establishing a more realistic and detailed model of reactants and interfaces, the coupling method is expected to provide a mechanism analysis and optimization tool, such as the design of catalyst pore structure.
【学位授予单位】：中国科学院研究生院(过程工程研究所)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TQ021

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