Study of splash characteristics and spreading mechanism of liquid droplets impacting walls at low temperature
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摘要: 液滴撞击低温壁面现象是导致飞机机翼结冰、电线覆冰和内燃机冷起动恶化的关键因素之一，其中液滴飞溅和铺展特性是造成上述问题的主要原因。因此，本文运用高速摄影法对正十二烷液滴撞击不同温度铝板的飞溅及铺展特性进行系统的试验研究。结果表明：随着壁面温度降低（20 ℃～−40 ℃），液滴破碎阈值明显降低，二次液滴直径与数量显著增大，附壁液膜铺展速度和最大铺展距离显著减小。本研究针对低温壁面附壁液膜快速铺展阶段的运动学特征，考虑了壁面温度对粘性力的影响，构建了新的无量纲铺展系数βT = (D/D0)/ReT0.07，建立了新的铺展模型βT = 1.76τ0.5。该模型不仅实现了对于不同入射条件下的液膜铺展过程的准确描述，还将适用范围由传统铺展模型的0.1 ≤ τ ≤ 1.0拓宽至0.1 ≤ τ ≤ 1.5，从而能够准确描述更长时间内液膜铺展规律变化。Abstract: The phenomenon of droplets impacting on the cold wall is one of the key factors leading to the deterioration of aircraft wing icing, wire icing and cold start of internal combustion engine. Droplet splashing and spreading characteristics are the main causes of the above problems. High-speed photography was used to study the splashing and spreading characteristics of n-dodecane droplets striking aluminium plates at different temperatures. The results show that as the wall temperature decreased (20 ℃ to −40 ℃) the droplet fragmentation threshold is significantly reduced, the secondary droplet diameter and number increase significantly, and the spreading speed and maximum spreading distance of the attached liquid film decrease significantly. In this study, a new dimensionless spreading coefficient (βT = (D/D0)/ReT0.07) and a new spreading model (βT = 1.76τ0.5) were constructed for the kinematic characteristics of the fast spreading phase of the liquid film attached to the low temperature wall, considering the effect of wall temperature on the viscous forces. The spreading model not only enables an accurate description of the liquid film spreading process under different insertion conditions and wall temperatures, but also widens the range of application of the model from 0.1 ≤ τ ≤ 1.0 to 0.1 ≤ τ ≤ 1.5, allowing the resulting low temperature spreading model to accurately describe changes in the spreading pattern of the liquid film over a longer period of time.
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Key words:
- droplet impact wall /
- breaking process /
- spreading model /
- high-speed photography
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图 2 液滴以3.5 m/s的入射速度撞击−10 ℃低温壁面飞溅到铺展过程
Figure 2. The droplet splashing onto the low-temperature wall at −10 ℃ at the incident velocity of 3.5 m/s
图 3 液滴以2.3 m/s撞击速度撞击不同温度壁面下液滴破碎现象
Figure 3. The droplet breaking phenomenon under the impact velocity of 2.3 m/s on the wall surface at different temperatures
图 4 不同温度壁面二次液滴特征
Figure 4. Secondary droplet characteristics on walls at different temperatures
图 9 液膜铺展因子随入射速度变化趋势
Figure 9. Trend of liquid film spreading factor with incident velocity
图 10 不同撞击条件下液滴的铺展过程
Figure 10. Droplet spreading processes under different impact conditions
图 12 不同
$ \alpha $ 取值下铺展因子的归一化比较Figure 12. Normalized comparison of spreading factors for different values of
$ \alpha $ 
图 13 不同
$ \alpha $ 取值下铺展因子离散系数Figure 13. The spread factor corresponds to the normalization coefficient under different
$ \alpha $ values
图 14 新铺展因子液膜在不同撞击条件下的归一化效果
Figure 14. Normalization-effect of new spreading factor liquid films under different impact conditions
图 15 液膜最大铺展直径随壁面温度的变化
Figure 15. Variation of the maximum spreading diameter of the liquid film with wall temperature
图 16 不同撞壁条件下的液膜最大铺展系数变化
Figure 16. Variation of the maximum spreading coefficient of the liquid film under different wall impact conditions
表 1 壁面参数（20 ℃）
Table 1. Parameters of the impact surfaces at 20 ℃
表面材料 材质 表面粗糙Ra/（μm） 导热系数λ（W/mK） 铝合金板 1050 ＜0.025 224 
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