气泡曳力系数模型分区研究

由曳力系数表征的曳力模型作为重要的相间力模型之一,被广泛应用于Euler-Euler方法和Euler-Lagrange方法下的连续相和离散相动量方程中。由于现有气泡曳力系数模型形式各异且适用范围有限,因此需要对已有模型进行充分地评价。考虑到已有曳力系数模型的适用范围和气泡变形的影响,参考各曳力系数模型采用的相关参数,建议基于Reynolds数Re和Weber数We分区选择最佳模型。将分区曳力系数模型、已有曳力模型与实验数据对比,发现分区曳力系数模型总体预测结果更符合实验测量值。将分区曳力系数模型应用至数值模拟中,可以更精确地追踪不同尺寸气泡的位置,使数值模拟结果更接近真实的物理情况。     

液滴降落过程表面曳力及其形状改变规律

在单颗粒液滴-气流体系中,采用计算流体力学方法对速度发展段的输运现象进行数值模拟。研究液滴在气流中逆流运动的形态变化规律与局部传递特征。获得在30相似文献   

球形颗粒曳力系数的直接计算

介绍了一种球形颗粒曳力系数的直接计算法。该法形式简单、结果准确、使用方便、应用范围广。     

流体沿平壁流动的曳力系数

DRAGCOEFFICIENTFLOWOVERAFLATPLATE1前言当流体流过壁面时,就壁面而言,它受到流体的曳力。曳力系数与对流传热系数和传质系数一样,是在计算传递速率时首先要确定的传递系数,它们都是雷诺数的函数,三者又可以类比。因此,可由一个已知的传递系数去预测另一个未知的传递系数。在传递过程研究中常通过动员传递中易于求得的曳力系数估算对流传热系数与传质系数。本文针对文献中存在的不足,进行透彻的分析,推导出精确计算流体沿平壁流动时平均曳力系数的公式,供设计计算以及深入研究三传类比时参考与应用。2公式推导2．三…     

大长径比细长颗粒的沉降实验和曳力系数的关联

引　言对球状颗粒的研究已取得丰硕的成果[1] ,但对其他形状颗粒的研究则较少 ,特别是细长颗粒 .形状的特殊性使细长颗粒的曳力系数与球状颗粒明显不同 ,仅有的文献[2～ 4] 将研究重点放在静止液体中颗粒沉降时的取向和终端沉降速度 ,且一般采用球形度表示曳力系数 ,颗粒长径比     

颗粒流体两相流基本方程中压差力和曳力系数的计算

本文总结了颗粒流体两相流基本方程的压差力表达方式和推导颗粒流体系统曳力系数计算公式的力平衡原理，提出了一种分析压差力的简单方法，阐明了各种压差力表达方式间的关系及对应的两相流基本方程的曳力系数的计算方法     

液固循环流化床的研究(Ⅱ)表观滑落速度及曳力系数

在本文(Ⅰ)报实验研究的基础上,进一步探讨了液固循环流化床中的两相流动特性.液固两相表观滑落速度的实测结果表明,实验结果与广义流态化方法的预测值间存在较大的差异.表观曳力系数的研究表明,实验回归的表观曳力系数与文献中的传统预测值间也存在较大的差异,进一步体现出由于状态参数的径向不均匀性带来的循环流态化与散式流态化间的差别.     

颗粒团绕流曳力系数的LBM计算

采用格子Boltzmann方法(Lattice Boltzmann method, LBM) 中的LBGK(Lattice Bhatnagar- Gross-Krook)模型和二阶精度的曲线边界条件处理方法对二维颗粒团绕流现象进行了数值模拟,并同时使用动量交换法计算了两种颗粒团构型中不同颗粒的曳力系数。结果表明：颗粒团曳力系数与颗粒聚团的构型有着密切联系,颗粒聚团的形成将导致颗粒团曳力系数大幅度减小。除颗粒团构型因素外,颗粒间距和流动Reynolds数也是导致颗粒团曳力系数发生改变的主要因素。当颗粒聚团存在时,颗粒团中不同颗粒的受力有较大差异,若忽略颗粒聚团效应,则颗粒团曳力系数的计算必然将产生偏差。     

非稳态载粒多相流曳力研究进展

介绍不稳定曳力的理论研究现状,并形成了初步的理论体系。实验研究方面,利用激波管和高速摄像法进行了曳力系数可视化研究,但仍需要更全面、更精确的数据补充;在实验研究的基础上又进行了模拟研究,有助于揭示非稳态多相流的瞬时微观特性,但由于模型本身的局限性,仍无法准确描述非稳态流场和颗粒的复杂耦合作用。最后指出DNS模型修正和可视化实验技术将是后续研究需要突破的方向。     

A note on the drag coefficient of a single gas bubble in a power-law fluid

The motion of a single gas bubble is examined in the case of creeping flow where the exterior phase obeys the power-law model. An expression is developed for the drag coefficient using a perturbation method up to the second order in the parameter α = (n-1)/2. The relation obtained is compared with existing correlations and experimental data.     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015     

用单元胞模型数值模拟气泡群中气泡的运动

用数值方法模拟了气泡群中Reynolds数为50的气泡的运动,以单元胞模型体现气泡群中气泡间的相互作用。数学模型用完全的流体力学方程、以流函数-涡度为变量来描述变形气泡周围的液体流动,控制方程在贴体正交坐标系中、以有限差分法离散后数值求解。模拟结果表明,气含率对气泡阻力系数和气泡偏心率有很大的影响,与理论分析较符合。但单元胞模型预测的阻力系数与基于爬流中球形气泡的理论值相比,在数值上偏高很多。采用无剪切为外边界条件比用无涡度为外边界条件的模型预测更接近理论公式。预计单元胞模型经进一步改进后可成为处理气泡群的有效工具。     

黏性液体中单个上升气泡的流体动力学行为(英文)

The rising behavior of single bubbles has been investigated in six systems with different viscosity and Morton number(Mo) from 3.21×10-11 to 163. Bubbles with maximum equivalent diameter of up to 16 mm were investigated. The bubble Reynolds number(Re) ranged from 0.02 to 1200 covering 3 regimes in which two func-tions are obtained relating the drag coefficient,CD,with Re and Mo. It has been found that in the high Reynolds number regime the drag coefficient increases until the Reynolds number of about 1200. The classic expression of Jamialahmadi(1994) is improved and extended to high viscosity liquids. A new relationship for the aspect ratio of deformed bubbles in terms of Re,the Etvs number and Mo,applicable to a wide range of system properties,espe-cially in high viscosity liquids,is also suggested.     

Drag coefficient—reynolds number transition for gas bubbles rising steadily in viscous fluids

Recently, the author (Rodrigue, 2001) developed a generalized correlation for the motion of gas bubbles rising steadily in uncontaminated viscous Newtonian fluids of infinite extent. It is the purpose of this note to show that this model can be written in an explicit form for the drag coefficient as a function of the Reynolds and Morton numbers. This new correlation is then used to predict the position of the minimum in a graphical representation of C_D versus Re. It is shown that the model can predict quite nicely this hydrodynamic transition for viscous fluids.     

微通道内气泡和液滴自组织行为的研究进展

微流体技术良好的可控性为制备高通量的单分散性气泡或液滴提供了新的途径,气泡和液滴的流动行为因在材料领域具有较大的应用前景而受到关注。综述了近年来微通道内气泡和液滴自组织行为的研究进展。气泡或液滴自组织晶格具有周期性的流动特征,自组织行为受分散相体积分数、液滴或气泡尺寸、聚并效应和通道构型的影响。展望了气泡和液滴自组织行为研究过程中待解决的关键科学问题,为进一步的模拟和实验研究提供了参考。     

Terminal settling velocity and drag coefficient of biofilm‐coated particles at high Reynolds numbers

The drag force (F_d) on bio‐coated particles taken from two laboratory‐scale liquid–solid circulating fluidized bed bioreactors (LSCFBBR) was studied. The terminal velocities (u_t) and Reynolds numbers (Re_t) of particles observed were higher than reported in the literature. Literature equations for determining u_t were found inadequate to predict drag coefficient (C_d) in Re_t > 130. A new equation for determining F_d as an explicit function of terminal settling velocity was generated based on Archimedes numbers (Ar) of the biofilm‐coated particle. The proposed equation adequately predicted the terminal settling velocity of other literature data at lower Re_t of less than 130, with an accuracy >85%. © 2010 American Institute of Chemical Engineers AIChE J, 2010