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基于Darcy-Stokes耦合模型的多孔介质颗粒悬浮液等效黏性系数计算

THE EFFECTIVE VISCOSITY OF A SUSPENSION OF POROUS PARTICLES BASED ON THE DARCY-STOKES COUPLING MODEL

  • 摘要: 颗粒悬浮液广泛存在于自然界和工程应用领域, 其黏性特征对其流动行为有着重要的影响. 本文基于Darcy-Stokes耦合模型, 给出了低浓度多孔介质颗粒悬浮液的等效黏性系数的计算公式. 首先求解了一个辅助问题, 即低雷诺数条件下线性分布的流场中多孔介质球引起的扰动问题. 自由流区域采用Stokes方程, 多孔介质球内部区域采用Darcy方程, 界面上则采用质量守恒条件、法向力平衡条件以及Beavers-Joseph条件或Beavers-Joseph-Saffman条件, 使用待定系数法推导了自由流和多孔介质区域的流场的解析表达式. 其次, 依据流场解析解计算了外边界上由多孔介质球引起的附加热耗散率, 确定了低浓度条件下特性黏度与达西数Da和Beavers-Joseph系数\alpha _\rmBJ的定量关系, 结果发现: 特性黏度随着\alpha _\rmBJ增加而增加, 且\alpha _\rmBJ越大, 特性黏度增加的幅度也越小; 当10^ - 6 \leqslant Da \leqslant 10^ - 4时, 特性黏度接近于2.5, 与经典的爱因斯坦黏性公式相符. 当10^ - 4 \leqslant Da \leqslant 10^ - 1时, 特性黏度快速下降, 因而等效黏性系数更加接近于流体本身的黏性. 最后将本模型计算结果, 与Darcy-Brinkman模型结合界面剪切应力跳跃条件计算得到的结果进行对比, 结果发现当Beavers-Joseph系数和界面应力跳跃系数之和为1时, 两类模型在低达西数条件下的结果是几乎是一致的.

     

    Abstract: Particle suspensions exist widely in nature and engineering applications, and their viscous characteristics have an important influence on their flow behavior. Based on the Darcy-Stokes coupling model, the analytical formulas of effective viscosity of dilute suspensions containing porous particles are derived in this paper. Firstly, an auxiliary problem is solved, that is, the disturbance caused by porous media spheres in the flow field with linear distribution under the condition of low Reynolds number. The fluid flows in the free-flow domain and porous medium are governed by the Stokes equation and Darcy’ law, respectively. The mass conservation law, the balance of normal forces, and the Beavers-Joseph (-Saffman) interface condition are used at the fluid–porous interface. An analytical solution for the present coupled free-flow and porous-medium system is derived by using the undetermined coefficient method. Then the additional heat dissipation rate caused by the porous media particle is calculated. Intrinsic viscosity of the porous media suspension under the condition of low concentration is determined as a function of the Darcy number and the Beavers-Joseph coefficient, which is based on the additional heat dissipation rate under the condition of low concentration. It is found that the intrinsic viscosity increases with increasing the Beavers-Joseph coefficient, and the larger the Beavers-Joseph coefficient is, the slower the increase of the intrinsic viscosity. When Darcy number is in the range of 10^ - 6 to 10^ - 4, the intrinsic viscosity is close to 2.5, which was consistent with the classical Einstein viscosity formula. When Darcy number is in the range of 10^ - 4 to 10^ - 1, the intrinsic viscosity decreases rapidly, so the effective viscosity coefficient of porous media suspension is closer to the viscosity of the based fluid. At last, the present effective viscosity formula is compared with that obtained by the Darcy-Brinkman equation coupling with the shear stress jump condition. It can be found that the effective viscosities obtained two different models agree well with each other in the low Darcy number regime when the sum of the Beavers-Joseph coefficient and the shear stress jump coefficient is unity.

     

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