燃烧多相流的介尺度动理学建模研究进展

Progress of mesoscale modeling and investigation of combustion multiphase flow

基于求解Navier-Stokes方程组的传统计算流体力学已经在诸多领域取得了巨大的成功,但在航空、航天、微流控等领域也遇到了新的瓶颈与挑战。其原因分为2个方面:①物理建模层面的问题;②离散格式带来的数值精度和稳定性问题。微尺度燃烧等一系列燃烧新概念的研究表明,特征更加丰富但以前知之甚少的热力学非平衡行为蕴含着大量待开发的物理功能。物理模型合理和具备相应功能是数值仿真研究的前提;物理建模层面的问题无法通过数值精度的提高来解决。本文从物理建模与复杂物理场分析角度,介绍了非平衡燃烧系统离散玻尔兹曼建模方法(DBM)的研究进展。DBM是非平衡统计物理学粗粒化建模理论在流体力学领域的具体应用之一,是相空间描述方法在离散玻尔兹曼方程形式下的进一步发展。它选取一个视角,研究系统的一组动理学性质,因而要求描述这组性质的动理学矩在模型简化中保值;以该组动理学矩的独立分量为基,构建相空间,使用该相空间和其子空间来描述系统的非平衡行为特征;研究视角和建模精度随着研究推进而调整。借助DBM可以研究反应过程中不同自由度内能之间的不平衡和相互转换等Navier-Stokes模型无法模拟的动理学过程。在内爆和外爆过程中,几何汇聚与发散效应等效于一个"外场力",在球心处系统始终处于热力学平衡态;在冯·纽曼压强峰处,系统不是偏离平衡最远,而是在平衡态附近;在冯·纽曼峰后反应区以外,Chapmann-Jouguet理论值、Zeldovich-Neumann-Doering(ZND)理论值和DBM结果相互验证;在反应区内DBM结果与ZND结果一致;在冯·纽曼压强峰前的压缩阶段,DBM模拟结果在物理上更合理。在冲击压缩过程中,相对于其他自由度,压缩波所在自由度上的内能先增加,因而这一自由度上的内能总是朝着正向偏离其平衡态值,而横向自由度上的内能总是朝着逆向偏离其平衡态值。在二流体模型视角下,反应物和产物朝着相反的方向偏离热力学平衡态。

Traditional computational fluid dynamics based on solving Navier-Stokes equations has achieved great success in many fields,but it has also encountered new bottlenecks and challenges in aerospace,microfluidic and other fields.The reasons can be divided into two aspects:(A)the problem of physical modeling;(B)the numerical accuracy and stability caused by discrete scheme.Reasonable and functional physical model is the premise of numerical simulation research.Problems at the level of physical modeling cannot be solved by improving numerical accuracy.A series of new concepts of combustion such as micro-scale combustion remind us that these more abundant but previously poorly understood characteristics of non-equilibrium behavior contains a large number of physical functions to be explored.In this paper we review the progress of Discrete Boltzmann Modeling method(DBM)for nonequilibrium combustion from the perspective of physical modeling and complex physical field analysis.DBM is one of the specific applications of coarse-grained modeling theory in non-equilibrium statistical physics in the field of fluid mechanics.It is a further development of phase space description method in the form of discrete Boltzmann equation.The methodology of DBM is to decompose a complex problem and select a perspective to study a set of kinetic properties of the system,so it is required that the kinetic moments describing this set of properties maintain their values in the model simplification.Based on the independent components of the kinetic moments,the phase space is constructed.The phase space and its subspaces are used to describe the non-equilibrium behavior of the system.The research perspective and modeling accuracy will be adjusted as the research progresses.With the help of DBM,kinetic processes such as the non-equilibrium and mutual conversion of internal energy in different degrees of freedom during the reaction process,which cannot be simulated by Navier-Stokes model,can be studied.In the process of internal and external explosions,the geometric convergent and divergent effects are equivalent to an"external field force",and the system is always in thermodynamic equilibrium state at the center of the sphere.At the von Neumann pressure peak,the system is not the furthest off equilibrium,but near equilibrium.The theoretical Chapmann-Jouguet values,the theoretical Zeldovich-Neumann-Doering(ZND)values and the DBM results are mutually verified outside the post-peak reaction region of von Neumann.The results of DBM in the reaction zone are consistent with those of ZND.In the compression stage before the von Neumann pressure peak,the DBM results are more physically reasonable.In the process of shock compression,compared with other degrees of freedom,the internal energy on the degree of freedom where the compression wave is located increases first,so the internal energy on this degree of freedom always deviates from its equilibrium state in the positive direction,while the internal energy on the transverse degree of freedom always deviates from its equilibrium state in the negative direction.From the perspective of the two-fluid model,the reactants and products deviate from thermodynamic equilibrium in opposite directions.