鼓泡塔内近壁效应的实验研究和多相流模型的仿真验证

发布时间：2018-09-14 15:22

【摘要】：鼓泡塔反应器是一种广泛应用于化工、石油、生物和冶金工业等领域的多相流反应装置,其优点有催化剂耐久性高、传热传质优良、无移动部件、易于操作和维护。鼓泡塔虽结构简单,但其内部的局部流动、湍流结构和气液两相之间的相互作用非常复杂。为了进一步加深对鼓泡塔内流动特性的理解,本文以一种边壁空气注入式的鼓泡塔作为研究对象,运用可视化实验方法,重点分析鼓泡塔内的近壁效应,气泡形状、尺寸和液相自由表面高度;同时,嵌入自编程序修正数值计算模型,验证鼓泡塔内的近壁效应和多相流模型的准确性。本文的主要研究内容如下:1)采用多面体网格对鼓泡塔内的单相流动进行了网格无关性检验。相比于其它网格划分方法,多面体网格具有更多相邻的网格和更精确的速度求解梯度。本文运用STAR CCM+软件分别对水和空气进行了稳态和非稳态模拟。两种模拟结果均显示,为了获得合理的单相流模拟结果,所需要的最小的网格数量为50万。通过对比模拟中监控的参数和残差值发现,正交残差和均方根残差都可以作为收敛的判定标准。2)可视化研究边壁空气注入式鼓泡塔的内流特性。实验中观察到注入的气泡贴合鼓泡塔进气口下游壁面前行,近壁效应持续距离约6cm;通过高速相机捕捉到不同尺寸气泡的形状和分布情况,同时记录下了液相自由表面高度(0.84m)。3)可视化实验结果验证多相流模拟的准确性。本文分别采用了多面体网格,裁剪网格和结构网格进行了多相流模拟。模拟结果显示,多面体网格可以获得理想的液相自由表面高度,但是不能够展现近壁效应;裁剪网格和结构网格能够展现近壁效应,但其近壁效应持续的距离与实验还有较大差别,同时模拟中的气泡形状与实验的差别较大。因此,本文在裁剪网格中嵌入自编程序生成自适应网格,该网格划分法可以随着流动的发展对气泡周围的网格进行自动加密。对比分析仿真结果发现,自适应网格能够准确地捕捉气泡形状和近壁效应。但是随着流动的发展,模拟中的液相因自由表面解的不连续性所引起的离散误差(伴生涌流),最终会从鼓泡塔中溢出,导致模拟无法展示液相的自由表面。为准确求解液相自由表面高度,本文在自适应网格的基础上增加了交界面动量耗散模型优化多相流模拟。模拟结果显示液相的自由表面无溢出现象,其高度为0.833m接近实验值;但是交界面动量耗散模型减弱了气相与液相之间的动量交换,导致近壁效应持续的距离加长,精度低于采用自适应网格的模拟结果。同时,本文也证明了在自适应网格和交界面动量耗散模型基础上,运用欧拉模型的VOF模型能够很好的展现气泡形状和近壁效应。边壁空气注入式鼓泡塔的实验和仿真研究表明,本文嵌入的自适应网格加密和交界面动量耗散模型能够准确地捕捉鼓泡塔内的近壁效应、气泡形状和液体自由表面,可以为后续的仿真研究和工程应用提供参考和新的见解。
[Abstract]:Bubble column reactor is a kind of multiphase flow reactor widely used in chemical, petroleum, biological and metallurgical industries. Its advantages are high catalyst durability, excellent heat and mass transfer, no moving parts, easy operation and maintenance. Bubble column has simple structure, but its internal local flow, turbulent structure and gas-liquid two-phase interaction. In order to further understand the flow characteristics in bubbling tower, a side-wall air injection bubbling tower is used as the research object in this paper. The near-wall effect, bubble shape, size and free surface height of liquid phase in the bubbling tower are analyzed with visual experimental method. The main contents of this paper are as follows: 1) The polyhedral mesh is used to verify the grid independence of the single-phase flow in the bubbling tower. Compared with other mesh generation methods, the polyhedral mesh has more adjacent meshes and more accurate velocity solution ladder. In this paper, steady-state and unsteady-state simulations of water and air are carried out respectively by using STAR CCM+ software. Both simulation results show that the minimum number of grids needed to obtain a reasonable single-phase flow simulation results is 500,000. 2) Visualization of the internal flow characteristics of a side-wall air-injected bubble column. It was observed that the injected bubbles were placed in front of the downstream wall of the inlet of the bubble column and the near-wall effect lasted for about 6 cm. The shape and distribution of bubbles with different sizes were captured by a high-speed camera, and the free surface of the liquid phase was recorded. Height (0.84m). 3) Visualization experiment results verify the accuracy of multiphase flow simulation. Polyhedron mesh, trimmed mesh and structured mesh are used to simulate the multiphase flow. The lattice can show the near-wall effect, but the lasting distance of the near-wall effect is quite different from the experiment. At the same time, the bubble shape in the simulation is quite different from the experiment. Comparing the simulation results, it is found that the self-adaptive mesh can accurately capture the bubble shape and near-wall effect. However, with the development of the flow, the discrete error (associated inrush) caused by the discontinuity of the free surface solution will eventually spill out of the bubble column, resulting in the simulation can not display the free surface of the liquid. In order to accurately calculate the free surface height of liquid phase, an interfacial momentum dissipation model was added to optimize the multiphase flow simulation based on the adaptive mesh. At the same time, it is proved that the VOF model based on the adaptive mesh and the momentum dissipation model at the interface can display the bubble shape and near-wall effect very well. Experiment and Simulation of the air injection bubble column with side wall The results show that the adaptive mesh refinement and the momentum dissipation model of the interface embedded in this paper can accurately capture the near wall effect, bubble shape and liquid free surface in the bubble column, which can provide reference and new insights for the follow-up simulation and engineering application.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ053.5

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