EXPERIMENTAL INVESTIGATION OF A SUBMILLIMETER SPHERE IMPACT ON DROPLET SURFACE
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摘要: 球体撞击液面是自然界和工业过程中的普遍现象. 目前相关研究主要关注毫米级及更大尺寸球体撞击水平液面. 对于亚毫米球体撞击过程的动力学特性及液滴弯曲液面对撞击行为的影响仍需深入研究. 本研究基于高速显微数码摄像技术, 开展了不同速度亚毫米球体撞击液滴不同位置的实验研究. 撞击行为呈现振荡和浸入两种模式. 由于液滴弯曲液面的存在, 撞击现象与水平液面不同, 润湿过程中三相接触线 (three phase contact line, TPCL)固定点方位角与撞击角度线性正相关, 非轴对称的空穴在TPCL固定点较高一侧率先形成且曲率半径较大. 揭示了撞击过程中球体主导力变化和能量转化机制. 分析了撞击速度和撞击角度对撞击行为的影响, 并给出了撞击模式图. 结果表明: 冲击阶段形状阻力主导撞击行为, 球体动能损耗量与撞击速度正相关. 空穴发展阶段由表面张力主导, 球体动能转化为维持空穴形状的表面能. 振荡模式空穴长度与韦伯数We正相关, 空穴发展速度差异较小. 根据量纲分析及实验结果拟合得到临界浸入韦伯数Wecr与撞击角度α关系式
$We_{cr}^{1/2} $ = α/40.Abstract: Impact of spheres on liquid surfaces is a universal phenomenon in nature and industrial processes. However, the relevant researches mainly focused on the impact of millimeter or larger spheres on the horizontal liquid surface. Further studies on the dynamic characteristics of submillimeter sphere impact process and the influence of curved interface on impact behavior is necessary. Herein, we presented the observation on the impact of submillimeter spheres on the curved surface of droplet by using high-speed microphotography technology. Owing to the existence of curved liquid surface, the impact phenomenon is different from those after impact horizontal liquid surface. The azimuthal angle of TPCL (three phase contact line) pinned point is positive linear correlation with the impact angle during wetting process, while the non-axisymmetric cavity is first formed on the higher side of TPCL pinned point and the curvature radius is larger. The evolution of the dominant forces and the energy conversion mechanism during the impact process were revealed. The influence of impact velocity and angle α on impact behavior were analyzed, and the impact pattern diagram was provided. The results show that the form drag dominates the motion of the sphere at the slamming stage, while the kinetic energy loss of the sphere is positive correlation with spheres velocity. The surface tension dominates the process at the cavity development stage, and the kinetic energy of the sphere is transformed into the surface energy that maintains the cavity. The cavity length of oscillation mode increases with the increase of Weber number We, and the cavity development velocity is basically consistent. According to the dimensional analysis and experimental results, the relationship between critical Weber number Wecr and α is$We_{cr}^{1/2} $ = α/40 by fitting.-
Key words:
- submillimeter sphere /
- droplet /
- impact /
- three phase contact line /
- surface tension
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图 6 冲击阶段球体动能变化(
${{\bar u}_1}$ = 0.838 m/s,${{\bar u}_2}$ = 1.045 m/s,${{\bar u}_3}$ = 1.142 m/s)Figure 6. Kinetic energy variation of spheres in slamming stage (
${{\bar u}_1}$ = 0.838 m/s,${{\bar u}_2}$ = 1.045 m/s,${{\bar u}_3}$ = 1.142 m/s)
图 7 不同撞击模式的无量纲空穴长度发展过程
Figure 7. Development process of dimensionless cavitation length under different impact modes
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