【作者】 韩路长；

【导师】 罗和安；

【作者基本信息】 湘潭大学 ， 化学工程， 2010， 博士

【摘要】 多相流动现象在工业反应器中经常遇到,其中伴随发生的流体颗粒(气泡或液滴)表面传质、破裂分散与聚并的过程对于分相的混合、分散起着决定作用。建立合理的流体颗粒传质、破裂分散和聚并过程的机理模型对于模拟、设计和优化反应器至关重要。但由于多相流动现象具有强烈的随机性、非线性,非常复杂,至今有关这一领域的许多问题仍是研究的难点和热点,尚待进一步解决。本文针对多相反应器内流粒传质、破裂分散和聚并机理研究中尚存在的问题,运用湍流理论、概率统计理论和计算流体力学的方法,对过程的本质机理进行了深入分析,建立了相应的改进或新的机理模型,为反应器的模拟、设计和优化提供了一个相对较为系统的理论模型框架。将离散颗粒模型应用于搅拌釜气液两相流的模拟,考察了气泡计算数目、气泡表面污染和曳力系数的湍流修正对模拟结果的影响,并提出了一个改进的湍流扩散模型,提高了对气泡速度场、气含率的预测能力。通过改进的离散颗粒模型和群平衡模型的耦合,可实现对流体颗粒表面传质、破裂与聚并过程的模拟。基于上述模型框架,对流体颗粒表面传质的机理进行了研究。提出了一个新的基于宽谱涡接触概念的液相传质系数模型。该模型从全波谱的范围内来考虑不同尺度湍流涡对传质的贡献,可以对较宽湍动条件范围的传质系数作出良好的预测。模型对于液膜厚度不敏感,从而为从数学上简化模型形式提供了证据。对湍流条件下流体颗粒的破裂机理进行了研究。深入分析了现有破裂标准存在的问题,并提出了一个新的适合于液滴破裂的能量密度增量标准和一个改进的适合于气泡破裂的约束条件,建立了改进的破裂模型。模型预测结果与文献报道的实验数据吻合良好。另外,从多元破裂的物理过程出发,对子尺寸分布进行了初步探索,建立了相应的多元破裂子尺寸分布理论模型。模型预测结果与实验数据具有合理的一致性,该部分工作可为多元破裂的研究提供一个基础和思路。对两流体颗粒靠近过程中形成的夹持液膜的排液减薄过程和聚并过程进行了研究。建立了新的平面膜和曲面膜的排液速率模型,并在平面膜模型的基础上将改进的两流粒靠近过程模型与液膜排液速率模型耦合起来,依据所提出的临界速度的聚并标准,运用两流体颗粒的初始内部动能与湍流涡动能的传递关系,建立了一个新的聚并速率模型。该模型不再采用聚并时间和接触时间比率服从指数分布的关联,且能体现湍流脉动对聚并过程的影响。更多还原

【Abstract】 The phenomena of multiphase flows in industry reactors have often been encountered where the accompanying behaviors of mass transfer, breakup and coalescence of fluid particles (droplets or bubbles) play a crucial role in determining the mixing and dispersion of fluid particles. It is very important for simulation, design and optimization of reactors to establish reasonable mechanisms models of mass transfer, breakup and coalescence of fluid particles. However, the studies in this field are still a challenge and the corresponding theoretical works are needed urgently since the multiphase flows are strongly random, nonlinear and very complicated.Based on the previous work, this thesis focuses on the intrinsic mechanism of mass transfer, breakup and coalescence, and established the corresponding improved or novel models through applying the turbulence, probability statistics theories and computational fluid dynamics. This work may provide a systemic model framework for the simulation of multiphase flow reactors. The contents are mainly as follows.The discrete particle model has been used to simulate the gas-liquid flows in stirred tank reactors, the effects of the number of the tracked bubbles; surface contamination and turbulence correction for the drag coefficient have been tested. An improved model of turbulent dispersion has been proposed and the prediction ability for the bubble velocity and gas holdup has been improved. The processes of mass transfer, breakup and coalescence of fluid particles can be simulated by the coupling of the discrete particle model and the PBM.Based on the above model framework, the mechanism of mass transfer of fluid particles has been studied. A novel multi-scale model for gas-liquid mass transfer based on the wide spectrum eddy contact concept has been proposed. This model considered the contribution of the eddies of different scales to the overall mass transfer coefficient and can give a good agreement with the experimental data in various turbulence conditions. It is not sensitive to the liquid film depth and can provide a proof for the simplification of the model form.The mechanism of fluid particle breakup has been studied and the issues of the existing breakup criteria have been analyzed in depth. A novel breakup criterion of surface energy density increase for droplets and an improved breakup criterion for bubble has been have been proposed. Two improved breakup models have been proposed for droplets and bubbles respectively. These models show a good agreement with the reported experimental data. In addition, the daughter size distribution for the multi-breakage has been studied from the breakup physical process. A novel distribution function has been proposed and shows a good agreement with the experimental data. This work can provide a basis and idea for the multi-breakage issue.The processes of film drainage and approach of two fluid particles have been studied. The models for parallel and curved fluid film have been developed. Based on the model of parallel fluid film, the improved approach model has been coupled with the fluid film drainage rate model. Using the criterion of critical coalescence velocity, the systemic initial interior kinetic energy of two fluid particles has been related with turbulent eddy kinetic energy, a novel model of coalescence rate has been proposed. This model does not use the exponential correlation of the coalescence time and the contact time and reflects effect of turbulent fluctuation on the coalescence.更多还原