Deformation behavior of a liquid droplet impinging on hot metallic surfaces above the Leidenfrost temperature

利用大量双曝光照片，根据统计方法研究了液滴撞击热表面的变形过程。当液滴撞击低韦伯数的热表面时，液滴在表面扩散，达到最大直径，反冲，最后从表面反弹。对于较大的韦伯数，我们发现液滴在反冲过程中分裂成小的部分。随着韦伯数的进一步增加，液滴在扩散过程中发生破碎。同时研究了不同表面材料对液滴变形过程的影响。在低韦伯数范围内，液滴从表面反弹而不解体，这类表面物质的影响很难被认识到。然而，研究发现，液滴在变形过程中是否破碎成小块的临界韦伯数随表面材料的种类而变化。对液滴的变形行为进行了数值分析。控制方程系为轴对称系统中不可压缩粘性流体流动的Navier-Stokes方程。考虑了粘度、重力和表面张力的影响。用有限差分法对控制方程组进行了求解。分析了液滴在室温下撞击冷表面和在莱顿弗罗斯特温度以上撞击热表面时的变形过程。将数值计算结果与实验数据进行了比较，验证了数学模型的有效性。结果表明，数值计算结果与实验数据吻合较好。同时，从理论上阐明了液滴回弹是由于表面张力的影响而发生的。

The deformation process of a droplet impacting a hot surface has been investigated by many double exposure photographs on the basis of statistical procedure. When a droplet impacts a hot surface with low Weber numbers, the droplet spreads on the surface, reaches a maximum diameter, recoils and finally rebounds from the surface. For larger Weber numbers, we have found that the droplet breaks up into small parts in the recoiling process. Wiht increasing the Weber number further, the droplet shatters in the spreading process. Also, we have investigated the effect of kind of surface material on the droplet deformation process. The effect of kind of surface material has been hardly recognized in the range of low Weber numbers such that the droplet rebounds from the surface without disintegration. However, it has been found that the critical Weber number whether or not the droplet break up into small parts during the deformation is changeable depending upon the kind of surface material.The deformation behavior of droplet has been analyzed numerically. The system of governing equation is the Navier-Stokes equations for the incompressible viscous fluid flow in the axisymmetric system. The effects of viscosity, gravity and surface tension have been taken into account. The system of governing equations has been solved by the finite differencing method. The deformation process of droplet impinging on a cold surface at room temperature as well as a hot surface above the Leidenfrost temperature has been analyzed. The numerical results have been compared with the experimental data to verify the validation of mathematical model. As a result, the numerical results have been found to agree well with the experimental data. Also, it has been clarified theoretically that the droplet rebounding occurs owing to the effect of surface tension.