Experimental study on primary atomization characteristics of double swirl air-blast atomizer in near field region
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摘要:
利用背光高速阴影法结合液膜跟踪识别程序对双旋流气动雾化喷嘴出口处的液膜失稳、破碎过程进行了全时序的研究。基于唯像描述对液膜破碎过程进行了分析，总结了旋流作用下液膜的偏转、周向转动、径向拍振以及自旋的特性。基于破碎过程中液体形态随时间的演化特征总结出3种双旋流作用下的典型液膜破碎模式：液袋破碎、液丝破碎及复合破碎模式。结合实验统计数据与高速图像进行定量分析发现：液丝平均长度受工况参数影响明显，气流压降较液体流量对其影响更大，进一步通过实验数据拟合出其与液体韦伯数
、 液体雷诺数间的经验公式。提出利用液体聚集特征长度偏差量度量破碎模式出现的概率。研究发现，气流压降对偏差量影响较大且呈现随压降增加而减小的规律，表明气流压降主导了液膜破碎模式的变化，而液体流量对偏差量影响较小，不影响破碎模式转变。Abstract:By using backlight high-speed shadowgraphy and liquid film tracking and recognition program, a full time series experimental study was conducted on the instability and breakup process of the liquid film at the exit of a double swirl air-blast atomizer. Based on the phenomenological description, the process of liquid film breakup was analyzed. The characteristics of liquid film deflection, circumferential rotation, radial flapping and spinning under swirling flow were summarized. Based on the time evolution characteristics of liquid structure during the breakup process, three typical breakup modes of liquid film under double swirl airflow were acquired as follow: sheet-bag breakup, ligament breakup, and hybrid breakup modes. By analyzing experimental statistical data and high-speed images, it was found that the average length of the ligaments was significantly affected by operating conditions. The impact of airflow pressure drop was greater than that of liquid flow rate. In addition, the empirical formulas between it and liquid Weber number and liquid Reynolds number were fitted through experimental data. A length deviation of film accumulation deviation was used to measure the probability of the occurrence of breakup modes. It was found that deviation was significant affected by airflow pressure drop and showed a decreasing tendency as airflow pressure drop increased. This indicated that airflow pressure drop dominated the change of liquid film breakup, while liquid flow rate had a less impact on deviation and did not affect the breakup modes.
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图 6 各气流压降条件下正视角液膜破碎时间序列（模式 A，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
Figure 6. Time series of liquid film breakup under various airflow pressure drop conditions （model A，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
图 7 正视角液袋破碎模式时间序列（模式 A，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
Figure 7. Time series of front view sheet-bag breakup （model A，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
图 8 侧视角液袋破碎时间序列（模式 B，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
Figure 8. Time series of side view sheet-bag breakup （model B，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
图 9 正视角（上）、侧视角（下）液丝破碎模式时间序列（模式 B，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
Figure 9. Time series of front view （up） and side view （down） of ligament breakup modes （model B，Δpg=2 kPa，$ {\dot{m}}_{\mathrm{l}} $=10 kg/h）
图 10 正视角碰撞、叠加现象（液丝破碎与液袋破碎叠加）时间序列（模式 A，Δpg =1 kPa，$ {\dot{m}}_{\mathrm{l}} $=18 kg/h）
Figure 10. Time series of collision and superposition phenomena （superposition of ligament breakup and sheet-bag breakup） （model A， Δpg =1 kPa，$ {\dot{m}}_{\mathrm{l}} $=18 kg/h）
图 11 正视角不同工况下液膜初始雾化（模式A）
Figure 11. Primary atomization of liquid film under different operating conditions （model A）
图 13 气流压降和液体流量对Llig的影响
Figure 13. Influence of airflow pressure drop and liquid mass flow rate on Llig
图 14 Llig实验结果与拟合结果的对比
Figure 14. Comparison of Llig between experimental results and fitting results
图 15 气流压降和液体流量对Lac的影响
Figure 15. Influence of airflow pressure drop and liquid flow rate on Lac
图 16 不同工况下偏差量C的计算示意图
Figure 16. Calculation diagram of deviation C under different working conditions
图 17 液体聚集特征长度偏差C与气流压降及液体流量的关系
Figure 17. Relationship between the deviation of liquid accumulation characteristic length C and the pressure drop and liquid flow rate of airflow
表 1 流体工质物性参数表
Table 1. Physical property of fluid medium
工质 密度
ρ/（kg/m3）动力黏度
μ/10−3 （Pa·s）表面张力
σ/（N/m）水 998 1.004 0.0728 空气 1.205 0.017 9 
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表 2 实验工况表
Table 2. Experimental condition table
气流静压降Δpg/kPa 液体流量$ {\dot{m}}_{\mathrm{l}} $/（kg/h） 液体雷诺数Rel 气流韦伯数Weg 气液动量比M 2 10，18，26，34 29.0 3.6 191，59，28，16 4 52.2 7.3 382，118，57，33 6 75.4 11.3 592，183，87，51 8 98.6 15.4 811，250，119，70
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表 3 估算液膜轴向流速表
Table 3. Estimated axial flow velocity of liquid film
液体流量$ {\dot{m}}_{\mathrm{l}} $/（kg/h） 液膜厚度δ/mm 液膜轴向速度vl/（m/s） 10 0.6 0.048 18 0.6 0.086 26 0.6 0.125 34 0.6 0.164
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表 4 高速摄像参数表
Table 4. High speed camera parameter table
气流压降/kPa 拍摄速度/（帧/s） 曝光时间/s 图幅尺寸/（像素×像素） 拍摄时长/s 2 20000 1/ 307000 320×160 0.25 4 60000 1/ 535000 320×160 0.083 6，8 60000 1/ 1000000 320×160 0.083
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