Experimental investigations on impingement dynamics and freezing behaviors of a supercooled water droplet onto a cold substrate
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摘要: 对低温表面毫米级过冷水滴的撞击冻结耦合过程进行实验研究，综合分析了水滴撞击速度、水滴过冷度（0～10 ℃）和基板温度对水滴撞击动力学和冻结行为的影响。研究表明：当撞击速度一定时，最大铺展直径系数随着水滴过冷度的降低而减小，但和基板温度无关，提出了一个修正模型来描述最大铺展直径系数的实验结果；成核时间随基板温度降低而提前，导致最终冻结面积增大，当基板温度为−24～−28 ℃，“珊瑚状”成核点在回缩阶段的三相接触线上形成；当基板温度低于−28 ℃，“菌状”成核点在铺展阶段中出现；水滴回缩动力学和冻结耦合作用决定冻结形貌，最大铺展面积的增大促结冰形貌由“煎饼状”向“盆状”转变。Abstract: The impinging-freezing coupling process of millimeter-sized supercooled water droplets onto a cold substrate is experimentally investigated in this work. The effects of the droplet impact velocity, initial droplet temperature （−10 ℃ to 0 ℃）, and substrate temperature on the impingement dynamics and freezing behaviors of the droplet are comprehensively analyzed. The experimental results show that for a constant impact velocity, the maximum spreading diameter factor decreases with the initial droplet temperature, but it is independent of the substrate temperature. A modified universal model is proposed to describe the experimental results of the maximum spreading diameter factor. In addition, the nucleation time is advanced with the decrease of the substrate temperature, resulting in the increase of the final frozen area as the substrate temperature is −24 ℃ to −28 ℃, where ‘Coral’ nucleation appears in the contact line region during the retraction stage. Once the substrate temperature is lower than −28 ℃, ‘Fungus’ nucleation forms in thin liquid film during the spreading stage. Moreover, the freezing morphology is determined by retraction dynamics and solidification. The transition from the pancake to basin morphology is prompted due to the increase of the maximum spreading area.
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Key words:
- supercooled droplet /
- impact /
- freezing /
- nucleation
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图 3 撞击图片序列（Ⅰ：D0=2.75 mm、v0=1.67 m/s、Ts=−20 ℃； Ⅱ：D0=2.75 mm、v0=1.67m/s、Ts=25 ℃；Ⅲ：D0=2.75 mm、v0=1.67 m/s、Ts=−30 ℃）
Figure 3. Sequence images of the impact process （Ⅰ：D0=2.75 mm、v0=1.67 m/s、Ts=−20 ℃； Ⅱ：D0=2.75 mm、v0=1.67m/s、Ts=25 ℃；Ⅲ：D0=2.75 mm、v0=1.67 m/s、Ts=−30 ℃）
图 10 过冷水滴撞击动力学过程和相变过程：（a） 平衡阶段成核；（b）在回缩过程中成核；（c）在铺展过程成核
Figure 10. Coupling process of droplet dynamics and phase change. （a） The nucleus formation during the equilib- rium phase; （b） The nucleus formation during the receding stage; （c） The nucleus formation during the spreading stage.
图 4 不同基板温度铺展系数随时间演化
Figure 4. Temporal evolutions of spreading diameter factor for different substrate temperatures (D0=2.75 mm, V0=1.67 m/s, Tw=−10 ℃)
图 5 不同基板温度下高度系数随时间演化
Figure 5. Temporal evolutions of height factor at different substrate temperatures （D0=2.75 mm，V0=1.67 m/s，Tw=−10 ℃）
图 6 不同过冷度过冷水滴铺展系数随时间演化
Figure 6. Temporal evolutions of spreading diameter factor for different droplet temperatures (D0=2.75 mm, Ts=−25 ℃, V0=1.67 m/s)
图 7 基板温度和韦伯数最大铺展直径系数的影响
Figure 7. Effects of substrate temperature and impact weber number on the maximum spreading diameter factor(D0 = 2.75 mm, Tw = −7.5 ℃)
图 8 撞击速度和水滴过冷度对最大铺展直径系数的影响
Figure 8. Effects of impact velocity and droplet temperature on the maximum spreading diameter factor(D0 = 2.75 mm，Ts = −20 ℃)
图 9 WeRe−2/5为函数的最大铺展直径系数
Figure 9. The maximum spreading diameter factor as a function of WeRe−2/5
图 12 基板温度对冰珠剖面的影响
Figure 12. Effect of substrate temperatures on the ice bead profiles from a side view.
图 11 最终润湿面积比vs初始水滴温度和韦伯数
Figure 11. The final wetted area ratio vs initial droplet temperature and impact weber number
图 13 雷诺数对冰珠剖面的影响
Figure 13. Effect of Reynolds number on the ice bead profiles from a side view.
图 14 过冷水滴撞击冷基板三种不同冻结形貌相图
Figure 14. Phase diagram of three freezing morphologies for a supercooled water droplet impinging on a cold substrate.
表 1 实验参数
Table 1. Experimental parameters
参数 数值 水滴直径${D_0}$/mm 2.75±0.05 撞击速度${V_0}$/（m·s-1） 1.67、2.00、2.60 水滴温度${T_w}/ ℃$ 0、−5、−10 基板温度${T_s} / ℃$ −20、−25、−30 
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