单个球形颗粒在拉伸和剪切流场中运动与传质的数值模拟
文献类型：学位论文
| 作者 | 张敬升 |
| 学位类别 | 博士 |
| 答辩日期 | 2011-10-24 |
| 授予单位 | 中国科学院研究生院 |
| 导师 | 杨超 |
| 关键词 | 球形颗粒 简单拉伸流动 简单剪切流动 传质 数值模拟 |
| 其他题名 | Numerical simulation of flow and mass transfer between a sphere and ambient fluid in extensional and shear flow |
| 学位专业 | 化学工程 |
| 中文摘要 | 在自然界和化工、食品、冶金等许多传统和新兴的工业中，普遍存在涉及多相悬浮分散体系的加工过程，表现出丰富的多层次、多尺度的物理和化学现象，涉及扩散、对流、脉动、流体流变性等。认识多相系统的动量、质量和热量传递规律，是多相输送、分离和反应设备设计的关键，也往往决定了工艺是否节能高效。因此，对多相体系的流动特性及传递规律进行深入研究，具有重要的现实意义。 工业过程中所涉及的多相分散体系，往往是包含着大量分散相颗粒的复杂体系。而从微观角度来讲，单个颗粒的运动和传递特性则是研究多相分散体系的必要基础。据此，本文从基本原理出发，利用数值模拟技术，并结合部分理论分析的方法，对单个球形颗粒在另一无限大、不互溶连续相流体中的运动和传质、传热过程进行了研究。其中，球形颗粒包括固体颗粒和液滴两类，连续相流体的运动形式限定为工业设备内经常涉及的简单拉伸流动和简单剪切流动。另外，考虑到传热与传质的相似性，本文以传质过程为例进行讨论，所得到的结果同样适用于传热过程。 当研究体系为单个球形颗粒处于简单拉伸流场中，并且连续相及液滴内部的流动限定为低Reynolds数下的爬流时，基于流体力学理论的流场解析解，本文数值求解了控制传质过程的对流-扩散方程。分别针对连续相传质阻力控制下的颗粒外部传质、分散相传质阻力控制下的液滴内部传质及两相传质阻力控制下的液滴内、外同时传质这三个过程，详细考察了Peclet数、液滴内外粘度比、扩散系数比、分配系数及拉伸方向等各种因素对传质Sherwood数和浓度场分布的影响规律。在前人工作的基础上，本文依据模拟结果，提出了适用于计算较大Peclet数范围内的传质Sherwood数的关联式，藉此可对体系的传质速率进行较为可靠的预测。 当研究体系为单个球形颗粒处于简单剪切流场中时，本文通过数值求解三维Navier-Stokes方程，研究了中低Reynolds数下的固体颗粒和液滴的运动情况，详细考察了惯性力作用下（非零Reynolds数）的剪切力对体系的流场及附加应力的影响，并依据模拟结果拟合得到了中低Reynolds数范围内的体系附加应力的计算式。利用计算得到的流动参数，数值求解了球形固体颗粒外部的三维稳态传质过程，考察了Reynolds数及Peclet数对传递速率和浓度场的影响，并通过边界层理论分析推导出适用于计算大Peclet数条件下的传质Sherwood数的解析表达式。本文的数值模拟结果与已有的理论分析结果具有很好的一致性，证实了本文所采用的模型方程的正确性和数值方法的可靠性。利用本文的模拟结果，可将仅适用于某些极限情况下（如Reynolds数很小或Peclet数很大）的理论结果拓展到更为符合实际生产操作的Reynolds数和Peclet数的范围。本文依据模拟结果拟合得到的传质Sherwood数及体系应力的关联式，对实际的生产操作具有重要的指导意义。 |
| 英文摘要 | Dispersed multiphase systems are used widely in the nature as well as in many traditional and new industrial processes, such as chemical engineering, food and metallurgy. The physical and chemical phenomena involved in these systems including diffusion, convection, pulsation and rheology are multi-scale. The flow and transport processes of the dispersed multiphase systems are crucial in designing chemical equipments and developing efficient technologies. In this sense, it is of great importance to study the momentum, mass and heat transfer of the dispersed multiphase systems. The practical multiphase systems are very complicated because they contain a large number of dispersed particles. In the microscopic view, however, the movement and transport behaviors of a single particle are the basis of multiphase systems. In order to study multiphase systems better, numerical simulation was carried out in this work to investigate the flow and mass/heat transfer between a single sphere and an ambient fluid which was assumed to be infinite and immiscible. Here, the sphere refers to a solid particle or a liquid drop, the continuous phase is subject to simple extensional flow or simple shear flow. In addition, only the mass transfer problem was solved in this work but the results are applicable for the heat transfer problem because of the similarity between heat transfer and mass transfer. For the case of a single sphere immersed in simple extensional flow, Reynolds number is assumed small and hence the flow field is within the creeping flow regime. By using the analytical solution of flow velocity, the convection-diffusion equation was solved for the external, internal and conjugate mass transfer problems. The effects of Peclet number, viscosity ratio, diffusivity ratio and distribution coefficient as well as the extension direction on Sherwood number and concentration field were investigated. On the basis of previous studies and present simulation works, a few accurate correlations are put forward to predict the Sherwood numbers for a larger range of Peclet numbers. For the case of a single sphere immersed in simple shear flow, the 3D Navier-Stokes equation at finite Reynolds numbers was numerically solved in order to study the fluid mechanics of the system. The inertial effect on the flow construction and extra stress was examined in detail and a few correlations were proposed for predicting the extra stress at finite Reynolds numbers. By using the numerical results of flow field, the 3D convection-diffusion equation was solved for the mass transfer outside a solid sphere. The effects of Reynolds number and Peclet number on mass transfer were studied by numerical simulations. Moreover, the boundary layer theory was also applied to derive the expression for predicting Sherwood numbers at high Peclet numbers.The good agreement between previous theories and present simulations validates the numerical scheme in this work. The theories applicable only for some limiting cases, e.g., at very small Reynolds numbers or very high Peclet numbers, are extended to the more practical ranges of Reynolds and Peclet numbers according to present simulations. The fitting expressions of Sherwood number and extra stress derived in this work can be used to direct the practical processes. |
| 语种 | 中文 |
| 公开日期 | 2013-09-24 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1757]  |
| 专题 | 过程工程研究所_研究所（批量导入） |
| 推荐引用方式 GB/T 7714 | 张敬升. 单个球形颗粒在拉伸和剪切流场中运动与传质的数值模拟[D]. 中国科学院研究生院. 2011. |
入库方式： OAI收割
来源：过程工程研究所
浏览0
下载0
收藏0
其他版本
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。