镓铟锡液滴撞击泡沫金属表面的运动学特性研究

EXPERIMENTAL STUDY ON THE DYNAMIC CHARACTERISTICS OF GALINSTAN DROPLET IMPACTING ON THE METAL FOAM SURFACE

常温下为液态的镓铟锡合金以其优异的导热性能在具有特殊要求的传热领域有着重要的应用价值,与传统流动介质相比较大的表面张力使得其产生的流动现象必有所区别.本文研究镓铟锡所形成的液滴撞击泡沫金属表面后所产生的铺展、回缩及回弹现象.采用高速相机拍摄液滴投影轮廓随液滴运动的变化过程,并通过图像处理获得不同撞击速度、底板表面孔径下的液滴铺展系数、中心位置轮廓高度以及液滴回弹后在空中的振动特性.研究结果表明:具有较高表面张力的镓铟锡液滴的铺展系数随无量纲时间的变化在铺展初始阶段仍满足常规流体的1/2次幂关系,只在铺展后期与底板的无量纲孔径有关系;液滴的最大铺展系数在较小无量纲孔径底板大于在光滑镍板,且随底板无量纲孔径增大而逐渐减小;在回弹过程,由于底板孔隙结构的存在使得液滴回弹后在空中的振动呈现3种形态:规则的横向和纵向振动、带旋转的横向和纵向振动以及旋转振动;最后,通过对振动频率的拟合和分析,进一步拓展了传统振动频率理论公式在非规则振动过程预测中的应用.

The eutectic alloy GaInSn which is liquid at room temperature has a great importance in application where the special heat transfer requirements because of its excellent heat conductivity. However, the corresponding flow characteristics in GaInSn will naturally be different from conventional fluid due to the high surface tension. In present paper,we carry out studies on the spreading, recoiling and rebounding phenomenon after the impacting of GaInSn droplets on metal foam surface. The high-speed camera is used to capture the droplet contours projected by the backlight during the moving of droplets. Through the image process method, the spreading factor, height of droplet contour in the center line and the oscillation characteristic of droplet after rebounding are obtained. Results show that at the early stage of the droplet impact, the spreading characteristic of GaInSn droplet with high surface tension is proportional to the square root of the normalized time, which is consistent with that from conventional liquid, while relates with the non-dimensional pole size of foam surface during the following spreading process. The maximum spreading factor of GaInSn droplets spreading on small non-dimensional pole size of foam surface is larger than that on smooth nickel surface, and decreases with the increase of the non-dimensional pole size of foam surface. During the rebounding process, the shape oscillation can be divided into three modes due to the difference in pore structure of surface: the regular oscillation in horizontal direction and vertical direction, the oscillation in horizontal direction and vertical direction with rotation and the rotation oscillation. Finally, the traditional theoretical formula used to predict the oscillation frequency of droplets or bubbles has been extended to cases with irregular oscillation in droplet shape through the fitting of present experimental data and analysis.