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非牛顿流体搅拌槽功率和能量耗散的实验研究
Alternative TitleMeasurements of Power Consumption and Energy Dissipation of Non-Newtonian Fluids in Stirred Tanks
王旭
Subtype硕士
Thesis Advisor杨超  ;  冯鑫
2014-05
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword非牛顿流体   搅拌槽   搅拌功率   粒子成像测速   能量耗散率
Abstract搅拌是过程工业上常见的一类单元操作,广泛应用于冶金、聚合工业、石油化工、食品及发酵等过程中。在这些过程中,搅拌的对象多为非牛顿流体,即流体的粘度不是一个常数,会随剪切速率等其他因素发生变化。 随着可视化技术的发展,以粒子成像测速仪(PIV)为代表的先进流场测试技术逐渐成为研究流场的首选手段,由于PIV在瞬时即能够提供直观的全流场的流动信息,近年来已广泛应用于各种流动的测量与显示中,并且尤其适用于研究湍流、涡流等复杂的流动现象。采用PIV获得的流场信息,可以后处理获得剪切速率分布、粘度分布以及局部能量耗散率的分布。局部能量耗散率是衡量搅拌槽内流体混合情况的一个重要参数,在快速反应体系中,能量耗散率的最大点一般为最佳进料位置;能量耗散率的分布情况对于存在固相颗粒分散、气泡和液滴聚并破碎的多相体系也十分重要。 与牛顿流体相比,目前关于非牛顿流体在搅拌槽内流动情况的研究相对较少,已有研究主要集中于搅拌功率、混合时间和对应的数值模拟,关于湍流状态下的流场研究仅有几篇文献。搅拌槽的流场的研究,可以为搅拌槽内局部能量的耗散情况、搅拌桨的选型、以及搅拌槽的优化设计和放大提供重要参考。 本文在一个直径为T=120 mm的圆柱形平底搅拌槽内进行实验,搅拌桨采用D=T/3的Rushton桨,选取具有剪切变稀特性的非牛顿流体作为研究对象,同时以具有相同粘度的牛顿流体作为对比,采用轴上扭矩法测量其搅拌功率,以及二维PIV方法测量其湍流状态下的流场,并对流场数据进行编程处理得到能量耗散率,对粘性项和速度梯度项造成的区别进行了分析,也分析和比较了最大耗散率及最大耗散点的位置,综合对比了非牛顿流体和牛顿流体的搅拌功率和搅拌流场的区别。 本文主要获得了以下的研究成果: 1.非牛顿流体和牛顿流体在雷诺数为102到104的范围内功率准数曲线非常接近,Np与Re的关系可以用统一的拟合式来表示; 2.在相同的湍流状态下,非牛顿流体与牛顿流体的整体流场十分相似,平均速度的径向和轴向分量各自分别在桨叶尖端的轴向分布和桨盘高度的径向分布情况相差不大,说明了非牛顿流体与牛顿流体在平均速度上的雷诺相似性; 3.非牛顿流体和牛顿流体的径向和轴向均方根速度分别在桨叶尖端的轴向分布和桨盘高度的径向分布有一定的差别,表现为非牛顿流体的径向均方根速度低于牛顿流体,从而导致二者的湍动能分布上的区别,即牛顿流体在搅拌桨的排出流区具有更高的湍动能分布,其根本原因是二者流变性的区别; 4.PIV的流场数据经过直接数据处理,计算出牛顿流体和非牛顿流体的能量耗散率,对于非牛顿流体,分别采用Metzner关联式和考虑局部粘度的方式来计算耗散率,并与甘油溶液进行对比,得到:牛顿流体与非牛顿流体的耗散率的差别同时由粘度项和速度梯度项的差别引起;采用Metzner关联式计算得到的羧甲基纤维素钠(CMC)溶液最大耗散率的值介于甘油的最大耗散率和考虑了局部粘度计算出的最大耗散率之间,并且采用后一种方法计算的最大耗散点的位置比前者计算的最大耗散点更靠近Rushton桨的桨尖。
Other AbstractStirring is one of the most common operation unit in process industry such as biohydrometallurgy, polymer industry, petrochemical, food industry, etc.. Fluids stirred in vessels often exhibit non-Newtonian characteristics, i.e., the viscosity changes with shear rate, time, elasticity and other factors. Moreover, due to the interaction between the fast rotating impellers and the stationary baffles, these fluids in stirred vessels are usually under turbulent regime or locally transitional. Difference of rheological properties may result in the different optimal operating conditions of mixing systems. Optical techniques such as laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) have been widely adopted to examine the flow characteristics in chemical reactors. PIV is proved an effective way of measuring flow fields, because it can provide thousands of velocity vectors at the same instant in a plane. Additionally, data obtained by the PIV technique could be used to validate the numerical simulation. Therefore, PIV has enabled obtaining the shear rate distribution, viscosity distribution, and local velocity gradients and local energy dissipation rate distribution, which is crucial to the quality and efficiency of the mixing process. The point where energy dissipates the most is always the optimum feeding location in fast reaction systems. For the dispersion of bubbles or drops in multiphase systems, local ε is also an important factor to determine the bubble/drop sizes. The investigation of non-Newtonian fluids can provide useful reference for understanding of energy dissipation distribution, the selection of stirring impeller, and the design and optimization of stirred tanks. Compared with Newtonian fluids, fewer open literature reports the flow characteristics of non-Newtonian fluids in stirred tanks. Previous works on non-Newtonian fluids stirred in vessels are mostly focused on the laminar flow, especially on power consumption, mixing time and computational simulation. This work is done in a flat-bottom T=120 mm cylindrical tank stirred by a Rushton impeller with D=1/3T, and the non-Newtonian fluids with typical shear-thinning property and the Newtonian fluids with similar viscosities are selected as the objective of study. The shaft-torque method is used for studying the stirred power consumption. 2D PIV experiment is done for exploring the turbulent flow field of Newtonian and non-Newtonian fluids, mainly focusing on the discharge area of the stirred reactor. With programming work, the data of flow field is used to get the distribution of shear rate, viscosity and local energy dissipation rate. Viscosity term and velocity gradient term are compared to analyze the difference of energy dissipation rates between fluids with different rheological properties. The value of maximum energy dissipation rate and the location are also investigated. The major conclusions are as follows: 1.When Re is at the range between 102 to 104, the power consumption of Newtonian fluids and non-Newtonian fluids are very close, and the relation of Np and Re can be fitted into a correlation. 2.Under the same flow regime (Re=2000), Newtonian fluids and non-Newtonian fluids have similar flow fields, and the axial (r/R=0.44) and radial (2z/w=0) profiles of mean velocity components are very close, which reflects the ‘Reynolds number similarity’ of mean velocity components of Newtonian and non-Newtonian fluids in stirred tanks. 3.The axial (r/R=0.44) and radial (2z/w=0) profiles of RMS (root-mean-square) velocity components are different from Newtonian fluids to non-Newtonian fluids. The radial RMS velocity of non-Newtonian fluids is lower than that of Newtonian fluids, which leads to the lower distribution of kinetic energy of non-Newtonian fluids. And this might be caused by the difference of viscosity. 4.The method of direct calculation is adopted in getting energy dissipation rate of Newtonian
Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/15564
Collection研究所(批量导入)
Recommended Citation
GB/T 7714
王旭. 非牛顿流体搅拌槽功率和能量耗散的实验研究[D]. 中国科学院研究生院,2014.
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