NUMERICAL SIMULATION OF OLDROYD-B VISCOELASTIC DROPLET COLLISION
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摘要: 复杂的流变特性使凝胶推进剂的雾化过程存在一定困难, 这制约了它的发展. 聚合物胶凝剂的加入使凝胶推进剂具有黏弹性, 从而在雾化时会产生黏弹性液滴, 因此为了进一步认识凝胶推进剂的雾化机理、提高凝胶推进剂的雾化性能, 对黏弹性液滴的碰撞行为进行数值模拟研究. 针对凝胶推进剂雾化过程中出现的液滴撞击现象, 考虑流体具有的黏弹性效应, 采用流体体积法(VOF)、自适应网格细化技术(AMR)和对数构象张量方法相结合, 使用Oldroyd-B本构模型描述液滴的黏弹性, 对两个相等体积的黏弹性液滴的碰撞过程进行直接数值模拟, 主要关注黏弹性液滴的正撞过程, 研究了松弛时间、黏度比、韦伯数对液滴正撞的影响, 并对不同参数下黏弹性液滴的撞击过程进行能量计算, 另外观察了不同偏心度下的液滴碰撞行为. 通过改变撞击速度, 得到了合并和反弹的碰撞结果, 结果表明增大松弛时间有利于合并液滴的挤压和回缩程度, 并且延迟液滴的变形过程, 这与牛顿流体得到的结果不同. 增大黏度比会阻碍合并液滴的振荡行为, 碰撞的偏心程度较大时会出现拉伸旋转, 偏心度越大时拉伸距离越长, 偏心度越小时动能耗散的速率越快, 并且耗散的动能越多.Abstract: The complex rheological properties make the atomization process of gel propellant difficult, which restricts its development. The addition of polymer gelling agent makes the gel propellant viscoelastic, so that viscoelastic droplets will be generated during atomization. Therefore, in order to further understand the atomization mechanism of gel propellant and improve the atomization performance of gel propellant , to carry out numerical simulation research on the collision behavior of viscoelastic droplets. Aiming at the droplet collision phenomenon in the atomization process of gel propellant, considering the viscoelastic effect of fluid, volume of fluid (VOF), adaptive mesh refinement (AMR), and log-conformation transformation were adopted. The Oldroyd-B constitutive model was used to describe the viscoelasticity of droplets, and a direct numerical simulation of the collision process of two viscoelastic droplets of equal volume was carried out. The head-on collision process of viscoelastic droplets is mainly concerned. The effect of relaxation time, viscosity ratio, Weber number on the head-on collision behavior was studied, the energy evolution of the droplet coalescence process under different physical properties was also calculated. In addition, the droplet collision behavior under different eccentricity was observed. By changing the collision velocity, the collision results of merging and bouncing are obtained. The results show that increasing the relaxation time is beneficial to the extrusion and retraction process of the coalesced droplet, and delaying the process of the droplet deformation, this is different from the results obtained by Newtonian fluids. Increasing the viscosity ratio can hinder the oscillation behavior of the coalesced droplet. As the eccentricity of the collision increases, extension and rotation occur, and the extensional distance increases with the degree of eccentricity. When the eccentricity decreases, the kinetic energy dissipation rate increases, and more kinetic energy is dissipated.
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Key words:
- droplet collision /
- viscoelasticity /
- numerical simulation /
- VOF
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图 3 黏弹性液滴的碰撞融合过程(u = 1 m/s)
Figure 3. Collision and coalescence process of viscoelastic droplet (u = 1 m/s)
图 5 黏弹性液滴的碰撞反弹过程(u = 0.5 m/s)
Figure 5. Collision and bounce process of viscoelastic droplet (u = 0.5 m/s)
图 6 不同松弛时间λ下液滴最大宽度Dmax随时间的变化
Figure 6. The curve of droplet maximum width Dmax with time at different relaxation time λ
图 7 不同松弛时间λ下动能KE随时间的变化
Figure 7. Kinetic energy KE variation curve with time at different relaxation time λ
图 8 不同松弛时间λ下表面能SE随时间的变化
Figure 8. Surface energy SE variation curve with time at different relaxation time λ
图 9 不同黏度比β下液滴最大宽度Dmax随时间的变化
Figure 9. The curve of droplet maximum width Dmax with time at different viscosity ratio β
图 10 不同黏度比β下动能KE随时间的变化
Figure 10. Kinetic energy KE variation curve with time at different viscosity ratio β
图 11 不同黏度比β下表面能SE随时间的变化
Figure 11. Surface energy SE variation curve with time at different viscosity ratio β
图 12 不同We下液滴最大宽度Dmax随时间的变化
Figure 12. The curve of droplet maximum width Dmax with time at different Weber number
图 13 不同We下动能KE随时间的变化
Figure 13. Kinetic energy KE variation curve with time at different Weber number
图 14 不同We下表面能SE随时间的变化
Figure 14. Surface energy SE variation curve with time at different Weber number
图 15 不同碰撞因子B下液滴的碰撞过程
Figure 15. The collision process of droplet under different impact factor B
图 16 不同B下动能KE随时间的变化
Figure 16. Kinetic energy KE variation curve with time at different impact factor B
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