Abstract

In this work, the effects of thermal buoyancy and rotation angle on the hydrodynamics and mixed convection heat transfer characteristics over four heated identical square cylinders placed in a parallel horizontal channel are numerically studied. For the flow field, the Lattice Boltzmann method (LBM) with multiple relaxation time collision operator (MRT) is implemented and the Bhatnagar-Gross-Ktook collision operator (BGK) is adopted for the thermal field simulation. The distance between two adjacent square cylinders is fixed for a parametric study and a uniform velocity is imposed at the inlet region. Ranges of the influence parameters are defined as: α = 45°, 90°, 120°; 50 ≤ Re ≤ 150; 0 ≤ Ri ≤ 0.5. Several flow and heat transfer parameters including the time-averaged lift/drag coefficients, the global time-averaged Nusselt number and the distributions of local Nusselt number are analyzed, respectively. The vorticity and isotherm patterns are also plotted to illustrate the visual disturbance of the thermal buoyancy and the rotation angle on the flow and thermal fields. Numerical results show that the flow and thermal patterns for all arrangements become disordered and nonsystematic with the introduction of thermal buoyancy, and the thermal pattern for the downstream cylinders has an upward drift in the wake region. The time-averaged lift coefficient for each square cylinder is relatively sensitive to the influence parameters and the arrangement, and it is always less than zero when the Richardson number is not equal to zero. The global time-averaged Nusselt number is directly proportional to the Reynolds number and Richardson number, and the local Nusselt numbers on the front and rear faces for each square cylinder reach a maximum and minimum, respectively. It is also observed that the distribution of local Nusselt number for all rotation angles is intensively affected by the thermal buoyancy.

摘要

在这项工作中，数值研究了热浮力和旋转角度对放置在平行水平通道中的四个加热的相同方形圆柱体的流体动力学和混合对流传热特性的影响。对于流场，采用具有多重弛豫时间碰撞算子（MRT）的格子玻尔兹曼方法（LBM），并采用Bhatnagar-Gross-Ktook碰撞算子（BGK）进行热场模拟。两个相邻方形圆柱之间的距离在参数研究中是固定的，并且在入口区域施加均匀的速度。影响参数范围定义为：α=45°、90°、120°；50≤Re≤150；0 ≤ Ri ≤ 0.5。几个流动和传热参数，包括时间平均升力/阻力系数，分别分析了全局时间平均努塞尔数和局部努塞尔数的分布。还绘制了涡度和等温线模式，以说明热浮力和旋转角度对流动和热场的视觉干扰。数值结果表明，随着热浮力的引入，所有布置的流动和热模式变得无序和不系统，并且下游圆柱体的热模式在尾流区域具有向上漂移。每个方柱体的时均升力系数对影响参数和排列方式比较敏感，当理查森数不为零时，它总是小于零。全局时间平均努塞尔数与雷诺数和理查森数成正比，每个方柱体的前后表面的局部努塞尔数分别达到最大值和最小值。还观察到所有旋转角度的局部努塞尔数分布受热浮力的强烈影响。