椭球体颗粒气固两相流中固体体积分数效应的直接数值模拟

Gas-solid flows are ubiquitous in nature and in industrial energy applications. The accurate prediction of these flows is strongly dependent on the knowledge of the interaction between particles and the fluid phase. To date, in-depth understanding of the primary interaction between the ideal spherical particle and the fluid phase, i.e., the drag force, has been obtained to some degree. However, in real-world flows, particles, with ellipsoids as the typical example, are mainly non-spherical, making the drag models for spheres inapplicable. In the recent literature, the investigation on the feature of forces on ellipsoidal particles is solely limited to the analysis of isolated particles, thus cannot reveal the influence of surrounding particles on the forces experienced by the considered particle. This project, using the particle-resolved direct numerical simulations, aims to study the forces exerted on a series of ellipsoidal particles with specific aspect ratios in random assemblies in uniform flows. The focus will be on exploring the effects of the solid volume fraction on the force characteristics. Based on the data produced in the simulations, the models for the drag force, lift force and torque will be established for ellipsoidal particles with arbitrary aspect ratios. These models cover all ranges of solid volume fractions and apply in flows with low and intermediate Reynolds numbers. The effects of orientation of particles will also be reflected in the models. Therefore, this project is not only of academic significance but also of great application value in terms of helping design and develop reactors with non-spherical particles in the field of energy engineering.

气固两相流动在自然界和能源系统中无处不在，对其的准确预测强依赖于对固体颗粒和气体相互作用的认识。至今，对理想化颗粒——球颗粒所受主要作用力——曳力的研究已经比较深入。但在实际流动中，颗粒多为以椭球体为典型代表的非球形，已有的球颗粒曳力模型并不适用。目前文献中对椭球体颗粒受力特性的研究仅局限于对孤立颗粒的受力分析，不能给出真实流动中因附近颗粒的存在而对颗粒受力产生的影响。本项目拟以解析到颗粒表面的直接数值模拟为技术手段，研究一系列具有特定轴长比的椭球体颗粒的随机排布系统在均匀来流中的受力特性，重点揭示固体体积分数对受力特性的影响规律，并基于特定轴长比椭球体颗粒的研究结果，构建中低雷诺数下任意轴长比的椭球体颗粒在任意固体体积分数下的均匀流动中的曳力、升力和转矩模型；模型还将反映颗粒指向性效应。本项目研究不仅具有良好的学术意义也同时对能源领域设计和研制含非球形颗粒的反应器具有重要的工程应用价值。

为服务于多相反应器过程放大与优化，探索并揭示特定轴长比的长椭球体和扁椭球体颗粒的随机排布系统在均匀来流中的受力特性，重点揭示固体体积分数对受力特性的影响规律。本项目采用Hermans取向因子和Beta分布对椭球颗粒系统进行定量数学表征，辅以蒙特卡洛方法，生成特定固体体积分数下具有特定平均取向及取向方差的均匀随机分布椭球颗粒群，基于此，对椭球颗粒群气固系统进行了一系列直接数值模拟，并基于所得数据，构建了适用于固体体积分数从0.1到0.4的长椭球体和扁椭球体受力模型。在低固体体积分数下，通过对不同轴长比和固体积分数的椭球颗粒在简单立方的构型中进行直接数值模拟，构建了相应的椭球颗粒受力模型，然后采用等效的方法，获得随机排布的椭球颗粒系统的受力，并系统的研究了颗粒取向对椭球受力的影响规律。.在项目进行期间，参加了国际多相流会议（ICMF2019）等学术会议，在涉及流动的国际化工主流期刊例如AIChE Journal和Chemical Engineering Science上发表文章6篇，培养博士生2名、硕士生2名，1名已毕业。.本项目构建了一套生成均匀分布椭球颗粒结构并对其进行颗粒全解析直接数值模拟的仿真系统，为以后相关研究工作奠定了坚实的基础。研究成果对椭球颗粒系统的受力模型进行了重要补充与拓展，可直接用于流态化系统和其他含颗粒系统的计算中。