关键词:油滴;气泡;油膜壁面;气泡破裂;数值模拟 Abstract Hollow oil droplets are easily formed by the high velocity air turbulence in the process of oil-gas lubrication. The micro bubble has an important influence on the movement process and oil film formation quality when an oil droplet impacting on the wall. The coupled level set and volume of fluid (CLSVOF) method is adopted to simulate the impact of a hollow droplet on the oil film wall. The dynamic mechanism of bubble rupture is investigated by investigating the deformation and movement of bubbles when the hollow droplets are impacted on the wall of the oil film. And the influence of bubble size, collision velocity and liquid viscosity on the characteristic parameters of bubble deformation in the process of bubble wall collision is also analyzed. The results reveal that the bubbles will deform and break up to form film droplet after the hollow droplets impact the wall of the oil film. The change of pressure and velocity gradient inside and outside the bubble is the main cause of bubble rupture. The bubble size has a great influence on the bubble rupture mode, single-point rupture occurs when the bubble is small, larger bubbles are more likely to cause multiple ruptures. The difference of force between different sizes of bubbles is larger, and there is no obvious correlation between the size of the bubble and the moment of rupture. The velocity of the collision and the viscosity of the liquid have a certain influence on the deformation, rupture and rupture time of the bubble. The larger the collision velocity, the greater the kinetic energy of the oils droplet, and the more likely the bubble deformation and rupture. When the viscosity of the liquid increases, the bubble deformation is promoted at the early stage of the movement of the oil droplet, and the rupture behavior of the bubble can be delayed in the later period of the movement.
为验证水平集--体积分数方法计算含气泡液滴与壁面碰撞行为的可行性,将数值模拟结果与文献[11]中含气泡甘油液滴撞击固体壁面的实验结果进行对比. 数值计算采取二维轴对称模型,甘油液滴的直径为5.25 mm,内部气泡直径为4.389 mm,碰撞速度为5.94 m/s. 计算区域取20 mm #x00D7;10 mm,当网格尺寸达到0.033 mm时,可满足计算精度. 控制方程通过有限体积法进行离散,压力速度耦合采用PISO方法,压力求解使用 交错压力方法,气 液界面插值选择几何重构方法,动量求解采用二阶迎风格式. 图1给出了含气泡甘油液滴撞壁的实验结果和模拟结果,其中图1(a)为运动形态对比,图1(b)为中心射流高度对比. 从图1中可以看到,含气泡甘油液滴撞击固体壁面行为的数值模拟结果和文献提供的实验结果较吻合. 可见水平集--体积分数方法能较准确模拟含气泡液滴撞击壁面这一动态行为. 显示原图|下载原图ZIP|生成PPT 图1实验和模拟结果对比 -->Fig. 1Comparison of experiment and simulation -->
1.3 参数选择及数值模型建立
本文以润滑油液滴作为研究对象,对含气泡的润滑油液滴与油膜壁面碰撞行为进行研究. 油--气润滑系统工作时,油滴颗粒直径一般为10 100 m;考虑润滑过程中的实际情况,采用的相关参数如表1所示. Table 1 表1 表1计算时采用的相关参数 Table 1Parameters used in the calculation
为精确捕捉气 液界面的变化情况,同时提高计算效率,在对计算域进行网格划分时,先将整个计算区域整体划分,再对气泡及其周围区域进行局部加密. 图3给出了5种不同的计算网格,同时对比了不同网格下的中心油膜高度随时间的变化规律. 其中,各曲线所代表的数值为加密后气泡所占的网格数. 图3中小图为含气泡液滴进入油膜后气泡及周围区域的网格分布情况. 从图3中可以发现,网格尺寸不同,中心油膜高度的变化过程相似. 而网格尺寸对中心油膜高度最小值出现的时间有显著影响. 总体看来,随着气泡所占的网格数量的增加,油膜中心高度最小值出现的越晚. 可见图3尚无法验证网格的无关性. 显示原图|下载原图ZIP|生成PPT 图3不同网格下的中心油膜高度对比 -->Fig. 3Comparison of central film height in different grids -->
为对这一现象进行研究,图4给出了气泡所占网格数为594和气泡所占网格数为3 844条件下,气泡及周围区域的分布图. 图中白色区域为空气,蓝色区域为油膜,其余颜色区域为气 液界面,气泡体积大小相等. 图4(a)对应的运动时间为19 s,图4(b)对应的运动时间为45 s,这两个时间点分别对应图3中中心油膜高度达到最小值的前3 s时刻. 中心油膜高度达到最小值是指气泡发生破裂,油膜出现短暂凹陷的过程. 含气泡油滴撞击油膜的运动过程将在下节进行详细说明. 显示原图|下载原图ZIP|生成PPT 图4两种不同网格下的气泡周围区域分布图 -->Fig. 4The regional distribution map of bubble around two different grids -->
(1)通过与实验结果比较,验证了水平集--体积分数方法在求解含气泡甘油液滴撞击固体壁面的可行性,并以此为依据,建立了含气泡油滴撞击油膜壁面的数值模型. (2)含气泡油滴撞击油膜壁面后气泡会发生变形,并在“颈部”破裂形成膜液滴;气泡破裂是由毛细波的传播造成气泡内外压力和速度梯度变化导致的. (3)随着气泡直径增大,气泡在运动过程中的变形量也相应增大,并随时间变化发生破裂,而破裂发生时刻和气泡直径之间没有明显线性关系. 其原因是不同直径气泡之间受力差异明显. 当气泡直径较大时,气泡上侧油膜出现多处断裂,使气泡提前破裂. (4)随着油滴碰撞速度增大,气泡和油膜动能增大,使气泡变形量增大并将气泡破裂发生时刻提前. 而当碰撞速度较小时,气泡变形能不足以克服油膜内黏滞阻力作用,气泡能维持较好的球形,并稳定存在于油膜中. (5)随着液体黏度增大,在油滴撞壁运动前期的气泡内部黏性切应力增大并拖拽气泡,使气泡变形量增大;而在运动后期,随着液体黏度增大,油膜内黏滞阻力增大并阻碍气泡变形,使气泡破裂发生时刻延迟. The authors have declared that no competing interests exist.
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