EXPERIMENTAL STUDY ON VISCOUS NEWTONIAN DROPLET IMPACTS ON DRY OR PRE-WETTED MESHES
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摘要: 液滴撞击网面现象广泛存在于自然界和一系列应用中, 液滴撞网后会穿透破碎产生二次液滴或不破碎全部附着在网面上, 两种情况下都会残留液体在网面而形成预湿, 影响后续撞击结果, 但前人研究集中于低黏性液滴撞击干燥网面, 黏性牛顿流体液滴撞击干燥或预湿网面的演化与机理仍有待探索. 本文采用高速阴影成像技术, 研究了黏性液滴(甘油水溶液)撞击干燥和预湿网面形成液指和破碎的演化规律, 考虑了网面结构尺寸、液滴黏性及撞击前网面上预湿液膜厚度对撞击结果的影响. 实验结果表明, 液滴撞击干燥网面后形成液指的最大长度随网孔宽度降低、液滴黏性增加而减小; 液滴黏性增加、网孔宽度减小均会抑制液滴对干燥网面的完全穿透; 预湿液膜高度的增加抑制液滴对网面的完全穿透, 并使不完全穿透时形成液指的最大长度减小. 建立了考虑液滴黏性、网孔宽度和网面预湿的液滴撞击网面后不完全穿透时形成液指的最大长度预测模型, 以及出现完全穿透时的临界参数理论预测模型, 模型预测结果均与实验结果吻合良好.Abstract: Droplet impacts on meshes are ubiquitous both in nature and in a variety of applications. The impact may lead to liquid penetration through the mesh and formation of secondary droplets underneath the mesh, or spreading on the mesh and no occurrence of penetration. In either situation, liquid remains on the meshes and forms pre-wetted meshes after impacts, leading to different following impact outcomes compared with impacts on dry meshes. However, previous studies focused on impacts of low-viscosity droplets on dry meshes. The evolution and mechanism of viscous Newtonian droplets impacts on dry or pre-wetted meshes remain to be explored. In this paper, the liquid fingers and the fragmentation occurred underneath the mesh following a viscous droplet (aqueous glycerol solution) impacting a dry or pre-wetted mesh are investigated using high-speed shadow imaging technology, with special attention paid on the influence of mesh size, droplet viscosity and pre-wetted liquid film thickness on impact outcomes. It was observed that both a decrease of mesh size and an increase of droplet viscosity resulted in a decrease of maximum length of liquid finger and suppressed a complete penetration through a dry mesh, an increase of pre-wetted liquid film thickness suppressed a complete penetration and resulted in a decrease of maximum length of liquid finger. Considering the influence of mesh size, droplet viscosity, and pre-wetted liquid film thickness, theoretical models for predicting the maximum length of liquid finger when incomplete penetration occurs and for predicting the threshold parameters for complete penetration is proposed and is validated by comparisons with experimental results.
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Key words:
- viscous droplets /
- pre-wetted meshes /
- liquid fingers /
- secondary droplets
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图 2 不同黏性液滴撞击网面的演化过程
Figure 2. Evolution of droplets with various viscosities impacting meshes of the same parameters
图 3 黏性液滴撞击不同网面的演化过程
Figure 3. Evolution of viscous droplets impacting meshes of various parameters
图 4 最大液指长度随液滴黏性和网目数变化曲线
Figure 4. Variations of maximum liquid finger length as functions of droplet viscosity and mesh parameter
图 5 不同速度液滴撞击网面的演化过程
Figure 5. Evolution of droplets with various velocities impacting meshes of the same parameters
图 6 最大液指长度随液滴黏性和速度变化曲线
Figure 6. Variations of maximum liquid finger length as functions of droplet viscosity and velocity
图 7 φ1的实验值与拟合经验公式
Figure 7. Experimentally-determined values and fitted empirical formula of φ1
图 8 φ2的实验值与拟合经验公式
Figure 8. Experimentally-determined values and fitted empirical formula of φ2
图 9 不同黏性液滴撞击网面的演化过程
Figure 9. Evolution of droplets with various viscosities impacting meshes of the same parameters
图 10 黏性液滴撞击不同网面的演化过程
Figure 10. Evolution of viscous droplets impacting meshes of various parameters
图 11 完全穿透临界韦伯数随临界毛细数的变化
Figure 11. Variations of threshold We as a function of threshold Ca for completely penetration
图 13 最大液指长度随液滴黏性和预湿高度的变化
Figure 13. Variations of maximum liquid finger length as functions of droplet viscosity and prewetted height
图 14 液滴撞击前及液滴与液膜融合后流动示意图
Figure 14. Sketch of the flow configuration (a) before the impacts and (b) after coalescence of the impacting droplet with the liquid trapped in the mesh.
图 16 5.01 mPa·s液滴撞击120目网面的结果相图
Figure 16. Phase diagram illustrating the outcome of 5.01 mPa·s droplets impacting 60 mu meshes
图 17 5.01 mPa·s液滴撞击80目网面的结果相图
Figure 17. Phase diagram illustrating the outcome of 5.01 mPa·s droplets impacting 60 mu meshes
图 18 5.01 mPa·s液滴撞击60目网面的结果相图
Figure 18. Phase diagram illustrating the outcome of 5.01 mPa·s droplets impacting 60 mu meshes
图 20 80.16 mPa·s液滴撞击60目网面的结果相图
Figure 20. Phase diagram illustrating the outcome of 80.16 mPa·s droplets impacting 60 mu meshes
图 19 36.94 mPa·s液滴撞击60目网面的结果相图
Figure 19. Phase diagram illustrating the outcome of 36.94 mPa·s droplets impacting 60 mu meshes
表 1 甘油水液滴的物理属性(25°C)
Table 1. The physical properties of aqueous glycerol solutions at 25°C
ψ/wt% μ/(mPa·s) ρ/(kg·m−3) σ/(mN·m−1) 50 5.01 1113.41 68.05 64 11.54 1150.67 66.95 70 18.07 1167.41 66.85 75 27.73 1181.74 66.54 78 36.94 1190.50 66.16 80 45.37 1196.42 66.77 81.5 53.35 1200.90 65.43 83 63.16 1205.41 65.52 84 71.06 1208.43 66.20 85 80.16 1211.48 66.13 
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表 3 甘油水溶液在网面上的静态接触角
Table 3. The equilibrium contact angles of aqueous glycerol solutions droplet on the mesh
Nm θeq/(°)
(μ = 5.01 mPa·s)θeq/(°)
(μ = 80.16 mPa·s)60 125.1 128.8 80 123.0 124.2 100 119.5 124.8 120 114.8 126.7 180 125.3 119.8
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