雷诺数和界面污染程度对气泡水动力学特性的影响

深入研究气泡界面污染程度对气泡水动力学特性的影响,对控制和改善泡状流动中气泡的运动速度和气液界面的传质和传热性能具有重要意义。为此,该文利用停滞帽模型详细研究了不同气泡雷诺数下,球形气泡界面污染程度对气泡界面参数(切向速度、压力、切应力和涡量)及其整体运动特征(尾流和阻力系数)的影响。基于停滞帽模型,通过改变帽角来定量控制气泡界面的污染程度,气泡雷诺数20≤Reb≤200,计算区域为轴对称结构。研究表明,在不同气泡雷诺数下,气泡界面污染程度对气泡水动力学单一物理参量的影响趋势是相似的,且界面参数在帽角处发生了突变;界面污染程度对气泡整体运动特征的影响与气泡雷诺数有关,气泡雷诺数越小,界面污染程度对阻力系数的影响越明显,而尾涡现象越弱。

Influence of Reynolds number and interfacial contamination degree on hydrodynamic characteristic of bubble

The bubble flow is extensively encountered in natural and industrial fields. For the majority of bubbly flows, the liquid phase is more or less polluted, which causes the change of the interfacial state of the bubble. The state of the bubble interface has the significant effect on hydrodynamic properties around the bubble. Furthermore, the changed flow field around the bubble directly influences the heat and mass transfer properties between the bubble and the liquid phases too. Therefore, it is necessary to deeply investigate the influence of interface contaminated degree on the dynamic properties of the bubble. In this paper, a bubble with the diameter of d is considered to be suspended in a rectangular region and the fluid flows around it with the velocity of U. As for the size of the region, the distances between the bubble center and the inlet, the outlet and the region wall are 5, 12 and 7 d, respectively. In view of the physical environment of the bubble, the bubble Reynolds number is not lager than 200, and thus the flow structure exhibits the two-dimensional properties. So the axisymmetric field can be used as the computational region, and thus the computational cost can drop greatly. Considering the contaminated degree of the bubble surface, the stagnant cap model is used. With this model, the interface contaminated degree is controlled artificially by changing boundary conditions (such as the interfacial velocity and the tangential stress) directly on the bubble surface. The cap angle, measuring from the rear stagnant point to the front edge of the contaminated interface, is used to describe the interface contaminated degree. In order to understand physical phenomena deeply, the computational cases as many as possible are designed. The cape angles representing the interfacial pollution degree are respectively designated as 0, 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, 165° and 180°, and the bubble Reynolds numbers are selected as 20, 40, 75, 100, 150 and 200. For the present investigation, the quadrilateral grids are used to discretize the computational field. And the non-uniform grids are used in the radial direction so as to well capture flow properties near the bubble. Thus, the closer to the bubble surface the grids are, the smaller the radial grids size is. Based on the present conditions, it is fully investigated and analyzed on the influence of interfacial contamination degree on interfacial physical parameters (such as the tangential velocity, pressure, tangential stress and vorticity) and overall motion characteristics (such as the wake and drag coefficient) under different bubble Reynolds numbers. The present investigations show that, for any bubble Reynolds number, the influence of interfacial pollution levels on the physical parameters are similar, and the interfacial physical parameters have an abrupt change near the cap angle. The influences of interface contamination levels on the overall motion characteristics of the bubble are related to the bubble Reynolds number. It seems that the smaller the bubble Reynolds number is, the drag coefficient is more sensitive to the interfacial pollution level.