高黏度牛顿流体和剪切变稀非牛顿流体中单气泡的运动和传质
文献类型：学位论文
| 作者 | 张丽 |
| 学位类别 | 博士 |
| 答辩日期 | 2009-01-09 |
| 授予单位 | 中国科学院过程工程研究所 |
| 授予地点 | 过程工程研究所 |
| 导师 | 毛在砂 |
| 关键词 | 单气泡 曳力系数 非牛顿流体 传质 level set方法 |
| 其他题名 | Motion and Mass Transfer of a Single Bubble in Highly Viscous Newtonian Liquid and Shear-thinning Non-Newtonian Liquid |
| 学位专业 | 化学工程 |
| 中文摘要 | 化学工业中的各类泥浆和悬浮液、油漆、涂料、颜料等均属于高黏度非牛顿流体。非牛顿流体中气泡的生成、相互作用和聚并行为在诸如废水处理、石油加工、发酵、聚合物脱挥、有机合成、沸腾、塑料泡沫加工以及环境保护等过程中发挥重要作用。由于受到表面张力、黏性力、惯性力和浮力的共同作用，加之非牛顿流体流动固有的复杂性，非牛顿流体中的气泡往往会表现出异常的流体力学行为，从而影响气液两相间的传质、传热和化学反应。因此，研究非牛顿流体中的气泡行为具有重要意义。本文主要从数值模拟和实验两个方面对气泡在高黏度牛顿流体和剪切变稀非牛顿流体中的运动和传质进行研究。 论文采用CCD照相技术实验测量了单个气泡在高黏度牛顿液体中从加速到稳态的上升运动过程。由受力分析推导计算曳力系数，并通过因次分析引入加速度数 ，获得考虑附加质量力和Basset力的情况下非稳态曳力系数和雷诺数之间的无因次关联式。当 的取值为零时，关联式也可用于气泡稳态上升过程。由此建立气泡从加速到稳态上升时的曳力系数关联式，关联结果与实验数据基本吻合。实验测定了气泡在非牛顿流体中的运动过程，通过修改雷诺数的表达形式，将剪切变稀和黏弹性非牛顿流体的稳态曳力系数用一个统一的公式表示。在牛顿流体研究的基础上，提出气泡在非牛顿流体中运动的非稳态曳力系数公式。 数值模拟方面，本文用level set方法追踪气液界面。采用控制容积法离散流体运动控制方程组，SIMPLE算法求解离散后的差分方程组。数值模拟了气泡在高黏度流体中运动的终端速度和形状，数值计算结果和实验结果非常吻合，验证了数值方法的可靠性。同时，选择Carreau模型表征非牛顿流体的剪切变稀特性，将此方法扩展到非牛顿流体的模拟，通过对气泡在非牛顿流体中运动进行数值模拟，分析了非牛顿流体物性、气泡大小等对气泡运动的影响，气泡形状变化以及气泡周围流场的黏度变化，并通过剪切速率变化图解释了剪切速率与连续相黏度的变化关系。将模拟结果与非牛顿流体实验结果进行比较，两者吻合较好，分析模拟结果得到了关于非牛顿液体中单个气泡运动的新认识。 传质实验方面，利用恒压和CCD照相法，实验测定了单个二氧化碳气泡在非牛顿流体CMC水溶液中的传质速率。在二氧化碳气泡上升初期，二氧化碳气泡的体积和表面积随着时间的增加快速下降，气泡运动的初始阶段传质系数随时间快速下降；在气泡溶解的后期，气泡的体积和表面积下降速度逐渐减小，传质系数几乎可以认为是个定值。随着针头直径逐渐减小，气泡的体积和表面积也是逐渐减小的。相同的针头情况下，稳态时1% CMC溶液的传质系数大于0.5% CMC水溶液，说明气泡在非牛顿流体中的传质系数随着非牛顿流体假塑性程度的增大而增大，即随着幂律指数的减小而增大。验证了气泡在非牛顿流体中的传质系数随着非牛顿流体假塑性程度的增大而增大的理论分析规律 |
| 英文摘要 | Bubble behavior in non-Newtonian fluid plays a key role in such diverse fields as waste water treatment, petroleum processing, fermentation, polymer devolatilization, composites processing, boiling, plastic foam processing and environment remediation. Because of the complexity of non-Newtonian fluids and mobility of bubble surface, the hydrodynamics of bubbles in non-Newtonian liquids shows significant different behavior compared to Newtonian liquids, which influences in turn mass transfer and chemical reaction in gas-liquid systems. Therefore, it is necessary to explore the hydrodynamics behavior occurring on the bubble scale. For this purpose, the motion of a bubble freely rising through highly viscous Newtonian fluids and non-Newtonian fluids was investigated experimentally and computationally. The unsteady motion of a single bubble rising freely in a quiescent liquid of high viscosity was measured using a CCD (charge coupled device) camera. The total drag coefficient was calculated from the accelerating motion way to the steady motion with the added mass force and history force included. In virtue of dimensional analysis, the total drag coefficient of a single bubble was correlated as a function of the acceleration number, Archimedes number and Reynolds number based on the equivalent bubble diameter. The proposed correlation represented very well the experimental data of the total drag force in a wide range covering both unsteady accelerating motion and steady motion. The combined added mass and history force coefficient accounting for the accelerating effect on a single bubble was evaluated and correlated. This method was extended to non-Newtonian fluids. The motion of single rising bubbles released from a fixed nozzle in a quiescent non-Newtonian liquid was investigated experimentally. By modifying the definition of the Reynolds number, a general correlation was proposed to predict the steady drag coefficient in both shear-thinning and viscoelastic non-Newtonian fluids. Based on the Newtonian fluid analysis, a proposed empirical correlation enabled the prediction of total drag coefficient from the accelerating motion to the steady motion with the added mass force and history force included for a bubble rising in non-Newtonian liquids in a wide range of conditions. A level set approach was applied for simulating the rising of air bubbles with real density and viscosity in a quiescent Newtonian viscous liquid driven by buoyancy. The control volume formulation with the SIMPLE algorithm incorporated was used to solve the governing equations on a staggered Eulerian grid. The predicted bubble shape and terminal velocity were in good agreement with the experimental results. The shear-thinning liquids were modeled using the Carreau model, and the motion of a rising gas bubble in shear-thinning liquids depending on Carreau model parameters and bubble sizes were simulated. The local effects of shear-thinning on bubble motion, which are hard to be acquired from an experimental approach, were clearly revealed by numerical simulation. It was shown the local changes of apparent viscosity around a bubble depended largely on the bubble shape and the non-Newtonian properties of the surrounding liquid. The shear-rate distribution was presented to explain directly the change of viscosity around the bubble. The much higher viscosity area corresponded to the lower shear-rate area behind the bubble. The predicted terminal velocity and Reynolds number were compared with the experimental data, and good agreement was observed. For mass transfer experiments, experiment under ambient pressure videoed with a CCD camera was conducted for measuring the mass transfer rate of rising carbon dioxide bubbles in non-Newtonian liquids. This technique also recorded simultaneously determines the bubble shape, area and rising velocity at the instant of measurement. The mass transfer coefficients were high when a bubble was initially released, but it dropped off rapidly with bubble rising, which corresponds to the rapid initial decrease in bubble volume and then a gradual tapering off. As power law index decreases, the liquid becomes more shear-thinning. This decrease in viscosity gives rise to larger velocity gradients near the bubble interface thereby facilitating convective mass transfer and the mass transfer coefficient increases with the decreasing power law index. |
| 语种 | 中文 |
| 公开日期 | 2013-09-13 |
| 页码 | 181 |
| 源URL | [http://ir.ipe.ac.cn/handle/122111/1126]  |
| 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 | 张丽. 高黏度牛顿流体和剪切变稀非牛顿流体中单气泡的运动和传质[D]. 过程工程研究所. 中国科学院过程工程研究所. 2009. |
入库方式： OAI收割
来源：过程工程研究所
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