EXPERIMENTAL INVESTIGATION ON DYNAMIC CHARACTERISTICS OF LIQUID NITROGEN SINGLE BUBBLE IN THE FREE FIELD
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摘要: 本文专门设计搭建了低温介质空泡演化实验测试平台, 对液氮单空泡非定常演化过程和动力学特性开展了实验研究. 实验中利用电火花瞬态放电激发液氮汽化形成单空泡, 通过高速摄影系统对单空泡的瞬态特征进行了精细化捕捉. 为了进一步揭示低温介质独特的物理性质以及强热力学效应对单空泡演化过程的影响机制, 对比分析了在相同环境压力下, 77.41 K液氮和298.36 K水单空泡的演化过程和动力学特性. 基于实验得到空泡半径与界面速度等定量数据, 阐明了液氮单空泡球形与非球形演化阶段的非定常特性. 研究结果表明: (1) 在相同输入电压下, 液氮单空泡的整体尺寸比常温水更小, 当输入电压为400 V时, 液氮空泡的最大半径约为常温水空泡的0.69倍; 同时, 液氮单空泡经历了膨胀阶段−收缩阶段−振荡阶段以及上升阶段的演化过程. (2)液氮空泡的收缩过程主要由相界面的热传导主导, 没有明显的塌陷现象, 收缩阶段液氮空泡的最小收缩半径约为常温水的5.5倍. (3)在液氮空泡振荡初期, 空泡相界面传热增强, Rayleigh-Taylor不稳定与热力学效应共同引起了空泡界面的表面粗化效应; 在整个振荡阶段, 空泡界面附近存在破碎的小泡. 当输入电压较高时, 空泡底部的小泡数量显著增多. (4)由于液氮空泡浮力系数较大, 液氮空泡在演化后期空泡整体向上迁移显著, 液氮空泡底部收缩更快产生凹陷, 促使空泡变为环状.Abstract: The objective of this paper is to investigate the transient evolution and dynamic characteristic of liquid nitrogen single bubble. In the experiment, electric spark transient discharge (EDM) was used to stimulate the evaporation of liquid nitrogen to form a single bubble, and the evolution process of the single bubble was captured by a high-speed camera with high resolution. In order to further reveal the unique physical properties of low-temperature media and the strong thermodynamic effects on the evolution of the single bubble, the unsteady evolution process and dynamic characteristics of single bubble in liquid nitrogen at 77.41 K and water at 298.36 K under the same ambient pressure were analyzed. And quantitative data such as the radius of bubble and interfacial velocity were obtained experimentally to elucidate the unsteady characteristics of the spherical and non-spherical evolution of liquid nitrogen single bubble. The results show that (1) the size of a single bubble in liquid nitrogen is smaller than that of ambient water at the same input voltage. The maximum radius of the liquid nitrogen bubble is about 0.69 times that of the ambient water bubble, when the input voltage is 400. The evolution of a single bubble in liquid nitrogen experiences an expansion stage, a contraction stage, an oscillation stage, and a up phase, respectively. (2) The shrinkage stage of liquid nitrogen vacuoles is mainly dominated by the heat conduction at the phase interface, and there is no obvious collapse phenomenon. The minimum radius of liquid nitrogen bubble is about 5.5 times bigger than that of the ambient water bubble during the shrinkage stage. (3) The heat transfer at the phase interface is enhanced during the early stage of the oscillation stage, the surface roughening effects is amplified over the bubble surface resulting from Rayleigh-Taylor instability coupled with the thermal effects. And small broken bubbles exist near the bubble surface during the oscillation stage. When the input voltage is higher, the number of small bubbles at the bottom of the vacuole increases significantly. (4) Due to the large buoyancy coefficient of the liquid nitrogen bubble, the overall upward migration of liquid nitrogen bubble is significant in the late stage of liquid nitrogen. The bottom of the liquid nitrogen vacuole shrinks more quickly to create a depression, driving the vacuole to into a ring shape.
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Key words:
- liquid nitrogen /
- single bubble /
- dynamic characteristics /
- experimental observation
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图 1 低温介质单空泡演化观测实验平台总体示意图(左)和实物图(右) (1.实验罐 2.真空隔热层 3.真空泵 4.高速相机5.电火花空泡发生器6. LED灯 7.柔光布 8.真空罐 9.电脑 10.低温电极)
Figure 1. Schematic (left) and physical (right) picture of cryogenic single bubble test rig (1. test tank 2. vacuum insulation chamber 3. vacuum pump 4. high speed camera 5. bubble generator 6. LED lamp 7. frosted glass 8. vacuum tank 9. computer 10. vacuum electrodes)
图 2 不同放电电压下液氮和常温水空泡最大半径对比图
Figure 2. Comparison of the maximum radius between liquid nitrogen and normal temperature water at different discharge voltages
图 3 自由场中常温水和液氮空泡瞬态特征的演化过程(水: T = 298.36 K, pair = 118.325 kPa; 液氮: T = 298.36 K, pair = 118.325 kPa)
Figure 3. Unsteady evolution of ambient water and liquid nitrogen single bubble in free field (water: T = 298.36 K, pair = 118.325 kPa; liquid nitrogen: T = 298.36 K, pair = 118.325 kPa)
图 4 自由场中常温水和液氮空泡半径的变化曲线
Figure 4. Single bubble radius curves of ambient water and liquid nitrogen in free field
图 5 自由场中常温水和液氮单空泡球形演化阶段空泡半径、速度以及加速度的变化曲线
Figure 5. Radius, velocity, and acceleration curve of single bubble in ambient water and liquid nitrogen during spherical evolution in free field
图 6 自由场中液氮空泡收缩过程典型的空泡形态
Figure 6. Typical shape during liquid nitrogen bubble shrinking stage
图 7 自由场中常温水和液氮单空泡振荡阶段空泡无量纲半径变化曲线
Figure 7. Dimensionless radius curve of single bubble in ambient water and liquid nitrogen during oscillation stage in free field
图 8 液氮单空泡振荡初期空泡半径变化曲线
Figure 8. Radius curve of liquid nitrogen bubble at the beginning of oscillation stage
图 9 液氮单空泡振荡初期t6 ~ t15时刻分别对应的瞬态实验图像
Figure 9. Transient images of liquid nitrogen bubble corresponding to moments t6 ~ t15 at the beginning of oscillation stage
图 10 不同输入电压下液氮空泡在振荡阶段典型的空泡形态
Figure 10. Typical shape of liquid nitrogen bubble in the oscillation stage at different input voltages
图 11 液氮单空泡上升阶段空泡半径的变化曲线
Figure 11. Radius curve of liquid nitrogen single bubble during up stage
图 12 液氮单空泡上升阶段t16~t31时刻分别对应的瞬态实验图像
Figure 12. Transient images of liquid nitrogen bubble corresponding to moments t16~t31 at the beginning of up stage
图 13 空泡图像沿选定直线Line1上的灰度分布得到的时空处理结果
Figure 13. Temporal-spatial processing results obtained from the grayscale distribution along the selected Line1 on the single bubble image
图 14 自由场中常温水和液氮单空泡沿特定直线的灰度值时空分布云图
Figure 14. Temporal-spatial processing results obtained from the grayscale distribution along the selected line for single bubble of ambient water and liquid nitrogen in free field
表 1 输入电压400 V时实验参数与实验结果
Table 1. Experimental conditions and results when the input voltage is 400 V
Case Temperature
Tl/KPressure
pair/kPaVapor pressure
pv/kPaDensity
ρ/(kg·m−3)Maximum radius
Rm/mmVertical distance
h/mmwater 298.36 118.325 3.21 996.95 13.76 100 liquid
nitrogen77.41 118.325 101.98 806.38 9.48 100 
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