湍流条件下液滴或气泡破裂和聚并过程研究

发布时间：2018-06-01 16:12

本文选题：宽能谱 + 聚并机制　；参考：《湘潭大学》2017年博士论文

【摘要】：流体颗粒(即液滴或气泡)的破裂和聚并是化学工业中的常见现象。破裂和聚并过程决定的流体颗粒尺寸分布将对多相反应器的混合、传递、反应等性能产生重大影响。因此,深入了解流体颗粒的破裂和聚并过程并构建合理的理论模型有利于开发、设计和优化多相反应器。本文对湍流条件下液滴的破裂机理进行了深入分析,在全能谱框架下构建了低粘液滴的多元破裂机理模型。与前人不同,本文建立的破裂机理模型不受二元破裂假设和液滴尺寸落在惯性子区假设的限制。在整个能谱区域内,从物理过程出发,建立了适用于二元、三元和四元破裂的破裂频率模型和子尺寸分布模型。在机理模型中考虑了液滴的表面摆动和初始形状对碰撞频率和破裂概率的影响。根据建立的机理模型,从理论角度合理地解释了文献中存在的两种相悖的学术观点。本文还利用实验数据对所提模型进行了验证。模型的预测结果能够与不同作者在不同操作条件下测量的破裂频率和子尺寸分布吻合。另外,将所提破裂机理模型与群体平衡方程进行耦合求解,耦合模型对搅拌釜反应器内液滴尺寸分布演变给出了令人满意的预测结果。在整个能谱区域内本文对湍流主导的流体颗粒的聚并过程进行了研究,并提出了两种不同的聚并机制。基于空间角推导了两流体颗粒间的碰撞概率以及湍流涡和流体颗粒间的碰撞概率。根据碰撞概率,运用联合概率的思路对每种聚并机制构建了相应的理论模型。采用由靠近方程和液膜排液方程构成的耦合模型预测的临界聚并速度对聚并模型进行封闭。不同于前人仅考虑等尺寸湍流涡对聚并的贡献,本文建立的聚并机理模型考虑了不同尺寸湍流涡对给定尺寸流体颗粒聚并的贡献。同时,模型还考虑了湍流涡寿命时间及湍流涡和流体颗粒间的平均碰撞自由程对聚并的影响。本文也提出了一个适用于全能谱的二阶纵向结构函数。另外,模型预测出的临界聚并速度及搅拌釜内液滴尺寸分布演变均能合理的吻合实验结果。此外,利用开源流体力学软件,对两等尺寸液滴的聚并过程进行了直接数值模拟。基于数值模拟结果,对现有排液机理模型中采用的假设进行了分析。由计算结果可以发现:(1)雷诺润滑近似假设具有局限性,随意忽略轴向压力梯度对排液的影响可能会导致机理模型预测出不合理的结果;(2)轴对称液膜假设在排液前期具有合理性,但排液后期该假设会失真;(3)液膜边界处使用的恒作用力边界条件具有合理性,而恒靠近速度的边界条件与数值计算结果有较大出入。这些研究对今后构建更为合理的排液机理模型具有指导和借鉴意义。
[Abstract]:The fracture and aggregation of fluid particles (i.e. droplets or bubbles) is a common phenomenon in the chemical industry. The size distribution of fluid particles determined by the process of fracture and coalescence will have a great influence on the mixing, transfer and reaction properties of the multiphase reactor. Therefore, it is helpful to develop, design and optimize the multiphase reactor by deeply understanding the fracture and coalescence process of fluid particles and constructing a reasonable theoretical model. In this paper, the fracture mechanism of droplets under turbulent conditions is analyzed in depth, and the multivariate fracture mechanism model of low mucus droplets is constructed under the framework of omnipotent spectrum. The fracture mechanism model established in this paper is not limited by the binary fracture hypothesis and the assumption that the droplet size falls in the inertial subregion. In the whole energy spectrum region, starting from the physical process, a rupture frequency model and a sub-size distribution model suitable for binary, ternary and quaternion fracturing are established. The influence of the surface swing and initial shape of the droplet on the collision frequency and fracture probability is considered in the mechanism model. According to the established mechanism model, two contrary academic viewpoints in the literature are explained reasonably from the theoretical point of view. The proposed model is verified by experimental data. The predicted results of the model are consistent with the fracture frequency and sub-size distribution measured by different authors under different operating conditions. In addition, the proposed fracture mechanism model is coupled with the group equilibrium equation, and the coupling model gives a satisfactory prediction result for the evolution of droplet size distribution in the stirred tank reactor. In this paper, the coalescence process of turbulent dominated fluid particles is studied in the whole energy spectrum region, and two different mechanisms of coalescence are proposed. The collision probability between two fluid particles and the collision probability between turbulent vortices and fluid particles are derived based on the space angle. According to the collision probability, the corresponding theoretical model is constructed for each kind of coalescence mechanism by using the idea of joint probability. The critical coalescence velocity predicted by the coupled model consisting of the near equation and the liquid film discharge equation was used to seal the coalescence model. Different from the contribution of turbulent vortices of equal size to aggregation, the contribution of turbulent vortices of different sizes to particle aggregation of fluid of given size is considered in the model of aggregation mechanism established in this paper. At the same time, the influence of the turbulent vortex life time and the mean collision free path between the turbulent vortex and the fluid particles on the coalescence is also taken into account in the model. In this paper, we also propose a second order longitudinal structure function suitable for omnipotent spectrum. In addition, the critical coalescence velocity predicted by the model and the evolution of droplet size distribution in the agitator are in good agreement with the experimental results. In addition, the direct numerical simulation of the coalescence process of two equal size droplets was carried out by using open source hydrodynamic software. Based on the numerical simulation results, the assumptions used in the existing models of drainage mechanism are analyzed. It can be found from the calculation results that the Reynolds lubrication approximation hypothesis has its limitations, and neglecting the influence of axial pressure gradient on the discharge of liquid may lead to the prediction of unreasonable results by the mechanism model.) the hypothesis of axisymmetric liquid film is reasonable in the early stage of discharge. However, the assumption that the boundary condition of constant force used at the boundary of liquid film is reasonable at the later stage of discharge is reasonable, but the boundary condition with constant velocity is quite different from the result of numerical calculation. These studies have guidance and reference significance for the establishment of a more reasonable model of drainage mechanism in the future.
【学位授予单位】：湘潭大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ021

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