油滴撞击油膜层内气泡的变形与破裂过程的数值模拟

旋转工作的机械零部件和机械设备的润滑系统工作过程中普遍存在着油滴和油膜的碰撞行为,这一行为易引起气泡夹带现象.气泡将对油滴撞击油膜时的运动过程和附壁油膜层的形成质量造成不可忽视的影响.基于耦合的水平集-体积分数方法,对油滴撞击含气泡油膜的行为进行数值模拟研究,考察油膜层内气泡的变形运动过程,分析气泡大小和位置等因素对撞击过程中气泡变形特征参数的影响规律,并探讨气泡破裂的动力学机制.研究表明,随着气泡直径的增大,油滴撞击含气泡油膜后气泡会依次出现自由表面破裂、稳定变形以及油膜内部破裂等现象;直径d=20μm的气泡能较稳定地存在于油膜层内,同时该值也是气泡发生自由表面破裂和油膜内部破裂的临界值.此外,气泡所在位置同样对气泡变形历程有一定影响,气泡越接近油膜表面,其变形量越大;位于油膜底层的气泡会附着在壁面上.在自由表面破裂和油膜内部破裂过程中,气泡破裂是由气-液界面不稳定引起的,表面张力对这两种现象起重要作用;而黏性剪切力对油膜内部破裂现象也有着不可忽视的影响.

Numerical simulation of deformation and rupture process of bubble in an oil film impacted by an oil droplet

Impact of oil droplet on oil film usually takes place in the lubrication process of rotating mechanical parts and machinery which can easily lead to bubble entrainment. Bubbles have important influences on the motion process of the oil droplet impacting on the oil film and also on the formation quality of the oil film layer. An oil droplet impacting on the oil film which contains a bubble is simulated numerically based on the coupled level set and the method of determining volume fraction. The bubble deformation process in the oil film during an oil droplet impacting on the oil film is investigated by the simulation method. The influences of the bubble size and the bubble position on the bubble deformation characteristic are also analyzed. The dynamic mechanism of the bubble rupture is discussed. The numerical results show that as the oil droplet impacts on the oil film, the bubble may rupture on the free surface, presenting stable deformation, or rupture in the oil film, which is greatly influenced by the bubble size. When the bubble diameter is in a range between 10 μm and 20 μm, the bubble deformation becomes more serious with the increase of bubble diameter, and the rupture of bubble on the free surface may occur over time. When the bubble diameters are in a range between 20 μm and 30 μm, the bubble rupture occurs in a short time after the bubble has reached a maximum deformation, and there is no obvious relationship between the maximum bubble deformation and the bubble diameter. The diameter of 20 μm is a critical value for a bubble to rupture on a free surface or inside an oil film, with which a bubble can keep stable in an oil film layer. As the bubble position changes, the bubble deformation process changes correspondingly. Under the same impact conditions, bubbles at the top of the oil film are more likely to deform than those in the center of the oil film. Bubbles at the bottom of the oil film have the smallest total deformation and finally attach to the wall. The bubble rupture is caused by the instability of the gas-liquid interface and the surface tension. The viscous shear force also plays an important role when the bubble rupture takes place in the oil film.