【作者】 韩路长；

【导师】 刘跃进；

【作者基本信息】 湘潭大学 ， 化学工程， 2005， 硕士

【摘要】 搅拌釜在造纸、化工、石化、制药、食品加工和生物化工等领域有着广泛的应用。反应器内单相和多相流体流动都很复杂,而在反应器的放大和设计中,研究者多采用一些无因次全局参数和基于全局值的经验、半经验关联式,对釜内当地流体力学特性的研究还比较欠缺。针对这种情况,本文在计算流力学理论(CFD)的基础上,对搅拌釜反应器内的局部流动和分散特性进行了初步的数值研究。在搅拌釜单相流模拟中考察了各向异性雷诺应力模型与各向同性湍流模型在速度场和湍动参数上的预报能力,应用SIMPLEC 算法求解压力校正方程,对雷诺应力模型控制方程中的对流项采用二阶中心差分格式,扩散项采用QUICK 格式进行离散。分析并比较了影响流场计算结果的各种因素,认为湍动参数对离散格式较为敏感。分析显示QUICK 格式在收敛速度和精度上表现出色。计算表明雷诺应力模型在速度场和湍动场的预报上精度更高,并得到了雷诺应力模型所预测湍动参数、涡度、旋度和剪切速率等在釜内的分布情况。应用欧拉两流体模型模拟搅拌釜气液两相流,考虑了虚拟质量力。通过考察不同曳力系数表达式对流场计算的影响,选取了合适的曳力作用模型。采用多重参考坐标法(MRF)描述挡板与搅拌桨之间的作用。对釜内不同气流量下气含率的分布进行了定性及定量描述。计算表明模拟所得气含率分布趋势与实现数据相比具有较好的一致性。应用拉格朗日粒子跟踪技术(分散相模型)研究搅拌釜中气泡的运动轨迹,对随机轨道模型进行适当改进,给出气泡的尺寸分布函数,考察了不同气流量对气泡浓度分布趋势和液相速度场的影响,计算表明拉格朗日法模拟所得气泡分布趋势与实验现象以及欧拉两流体模拟结果相比具有一定的合理性,模拟所得的临界通气数与经验关联结果较为接近。更多还原

【Abstract】 Stirred tank reactors are extensively used in a variety of industries like the paper and pulp, pharmaceutical, fine chemicals, food industry, petroleum chemical engineering, pharmacy and biochemical engineering etc. In designing such reactors, understanding the fluid dynamics is critical particularly. Untill recently, dimensionless numbers and empirical correlations have been used as tools for design, scale-up and operation of stirred tanks. Since the turbulent three-dimensional flow of single or multiphase fluid are complex in stirred tanks, these dimensionless numbers and empirical relations are clearly inadequate. Some researchers have given some simplified mathimatical models , but these models intensively depend on experimental data. To fully understand the processes occurring in stirred tanks, mathematical modeling of local hydrodynamic behavior are desired in depth. This thesis is devoted to the above tasks on the basis of computational fluid dynamics (CFD ). In accordance with the anisotropic feature of turbulent flow, an anisotropic reynolds stress model (RSM) is adopted to predict the turbulent flow field and turbulent characteristics generated by a Rushton disc turbine with the sliding mesh iterative procedure. The discretization scheme of diffusion term has an important effect on the prediction of turbulent parameters. QUICK discretization scheme shows good ability to accelerate convergence and improve precision. The predicted turbulent flow is compared with experimental data and the simulation results by standard k ? ε, RNG k ? εand SSTk ? ωturbulence models. The RSM model are found to give beter prediction than the other isotropic turbulence models. The distribution of turbulent parameters, vorticity magnitude, helicity and shear rate in the stirred tanks are simulated with the established numerical procedure. Accurate interfacial models are needed in the simulation of the two -phase flow in a gas-liquid stirred tank based on the two-fluid model. The virtual mass force is considered. In order to choose a reasonable drag force equation, investigate the effect on predicting the mean velocity of gas and liquid and the gas fraction by diferent drag coefficien -t equation. Multiple reference frames (MRF) is used to model the action between baffles and impeller. The qualitative and quantitative distribut -ion of gas fraction at different gas flow rate show good approach of the reported experimental data. In order to predict the turbulent transport characteristics of gas bubbles in stirred tanks, a discrete phase model is adopted to calculate the gas bubbles tracks. The discrete random walk model is improved suitably. For gas bubbles, a convenient representation of the gas bubbles size distribution is the Rosin-Rammler equation. The complete range of sizes is divided into an adequate number of discrete intervals, each represented by a mean diameter for which trajectory calculations are performed. Results show that the particle tracking method is very successful in simulating gas flow patterns in dispersed, loaded, and flooded conditions at a given impeller speed for Rushton turbine and down pumped pitched-blade impeller. Results of simulation have shown good agreement with photographs and the simulation results by two-fluid model.更多还原