拉伸流变对粘弹性流体射流破裂机理的影响研究

Gelled propellant is of great importance in the field of aerospace propulsion application. However, the current studies almost ignore the effect of extensional rheology on atomization. It is of both great practical and theoretical interest to investigate the role of extensional rheology in the breakup process of viscoelastic liquid jet. It is planned to study the stability and breakup behavior of viscoelastic liquid jet in this project theoretically, numerically and experimentally. To make the problem simple, the jet breakup process will be divided into several stages. For the disturbance growing stage and liquid ligament formation stage, the linear and weakly nonlinear stability analysis will be performed, and the energy budget method will also be used to discuss the effect of extensional rheology. For the strong nonlinear stage of ligament approaching to breakup, the dynamics equations of lubrication theory will be numerically solved to obtain the interface displacement and disturbance waveform. The experiments will be conducted to measure the disturbance growth rate to validate the stability analysis results. The jet breakup process will be shot using high-speed camera and the disturbance wave form will be extracted to verify the weakly nonlinear analysis and numerical results. A characteristic dimension of liquid particle will be defined to represent the atomization quality, which helps to establish the relation between the extensional rheological parameters and characteristic dimension. This project aims at fully understanding the atomization mechanism for gelled propellants, to provide theoretical basis and direction for practical applications.

凝胶推进剂在航天动力装置中有重要应用，然而现有雾化机理研究未考虑拉伸流变的影响，开展拉伸流变在粘弹性射流破裂过程中所起作用的研究，具有重大的应用背景和学术价值。本项目采用理论、数值模拟和实验相结合的手段，对考虑拉伸流变的粘弹性射流稳定性进行研究。将射流破裂过程分为不同发展阶段：对于扰动初始发展阶段和液丝形成阶段，开展线性和弱非线性稳定性分析，并结合能量分析方法探讨拉伸流动的影响机制；对液丝接近断裂的强非线性阶段，采用润滑理论动力学方程进行数值模拟，得到空间模式下的界面位移、扰动波形等，研究拉伸对液丝断裂过程的影响。开展实验研究测量射流扰动增长率并验证稳定性分析结果；拍摄射流破碎过程，提取扰动波形，验证弱非线性分析和数值模拟结果；定义液体微团特征尺寸来表征雾化效果，建立拉伸流变参数与特征尺寸之间的关系。通过本项目的研究，力争对凝胶推进剂雾化机理进行全面认识，为实际应用提供理论依据和指导。

本研究通过综合运用理论研究、数值模拟和实验验证相等多种手段，对考虑拉伸流变的非牛顿射流开展研究，对液体射流或液膜线性失稳以及后续的非线性破裂的全过程进行了明确描述。建立了考虑未松弛拉应力的圆柱射流、复合液膜和撞击射流等多种工况下的失稳破碎模型，对表面扰动波的发展进行了理论和数值求解，明确了拉伸流动在粘弹性射流和液膜破碎过程中发挥的作用；对长液丝拉伸破碎过程进行了二维直接数值模拟，分析了不同拉应力等参数对液丝变形的影响规律，其结果可与线性稳定性分析和一维有限差分结果较好地吻合；开展了液丝拉伸断裂实验，通过对实验结果中液丝长度、最小直径等数据信息的综合处理，完成了对理论计算结果的验证。研究结果表明：在射流失稳方面，拉应力的增加可以促进稳定性，且拉应力的大小可以使液体弹性表现出双重作用：当拉应力较小时，液体弹性的增加使应力松弛时间增加，减小了应力张量，使粘性耗散减小，促进液膜的不稳定；而拉应力较大时，弹性数的增加会使应力张量增大，导致粘性耗散增加，起到促进稳定的作用；在液丝断裂方面，德博拉数De的增大会导致对应的增长率增加，促进了液丝的失稳和断裂，而较大De数下“串珠”结构的振荡则延缓了细丝半径的减小，这又不利于液丝的断裂；实验研究方面，在液滴滴落过程中，当距离断裂的时间较远时液丝的最小直径下降得较慢，当临近断裂时液丝的最小直径下降变快，并在一定距离后呈直线下降，自相似理论可以很好地解释射流轮廓随时间的变化。本项目开展拉伸流变在粘弹性射流破裂过程中所起作用的研究，为凝胶推进剂雾化机理提供了更深层次的认识，为实际应用提供了更加准确的理论依据和指导。