湍流条件下气液界面传质机理研究

发布时间：2018-07-10 11:24

本文选题：界面传质 + 抵达频率密度　；参考：《湘潭大学》2015年硕士论文

【摘要】：目前关于气液两相界面传质的理论研究尚不成熟和完善。在实际工业生产过程中,许多单元设备的传质效率还较低,难以满足生产需求,亟待优化升级。因此研究界面传质机理并构建相应的理论模型对于指导单元设备的设计和优化、强化传质效率具有重要意义。本文基于现有模型存在的不足,将采用现象学方法和湍流结构理论,构建基于宽能谱分布、随机界面作用的流体微元或漩涡传质理论模型。本文分两个部分。第一部分为气-液表面传质机理模型的构建。传统的传质系数模型主要是基于渗透理论或表面更新理论的改进模型。这些模型考虑传质过程是由非稳态分子扩散控制。而基于经典的大涡和小涡传质模型预先假定传质由某波段能谱的流体结构控制。本文则在全能谱的范围内考虑不同尺度流体微元结构对传质的贡献,因此上述假设不再需要。本文重点研究了气泡界面传质过程,采用特征线法以及浓度梯度薄层近似,推导出了一个新的液相侧传质系数模型。模型的推导结合了非稳态浓度对流扩散方程以及气泡与流体微元的相互作用。该模型考虑了气泡与旋涡之间的有效相对运动才会促使漩涡与气泡表面接触,并基于随机基础作用的概念从理论上推导了不同尺度漩涡抵达表面的频率密度分布。该抵达频率密度分布可以用来理论解释全能谱各区波段的流体微元结构对传质系数贡献的差异性。此外,本文还考虑了气泡尺寸、气泡变形与振动、以及旋涡数密度分布对传质影响。模型预测的结果与文献报道的实验数据吻合很好。第二部分首先采用数值方法验证了特征线以及浓度梯度薄层近似的合理性(浓度梯度层厚度整体上低于漩涡尺寸的3.18%),并证明了即使在单涡的传质过程也应看成是非稳态过程。数值计算表明:相同界面接触时间,处于耗散区的漩涡能带走更多的溶质,传质效率更高。通过推导的LBM模型数值求解了非稳态的浓度方程,与本文解析模型相比较,发现界面液相侧的切向分子扩散对传质影响较弱。当忽略切向分子扩散,传质系数的误差低于1%。表明忽略该扩散具有一定合理性。
[Abstract]:At present, the theoretical study on gas-liquid two-phase interface mass transfer is not mature and perfect. In the actual industrial production process, the mass transfer efficiency of many unit equipment is still low, it is difficult to meet the production demand, so it is urgent to optimize and upgrade. Therefore, it is of great significance to study the mechanism of mass transfer at the interface and to construct the corresponding theoretical model for guiding the design and optimization of unit equipment and enhancing the efficiency of mass transfer. In this paper, based on the shortcomings of the existing models, the theoretical model of fluid micro-element or vortex mass transfer based on the wide spectrum distribution and random interface action will be constructed by using the phenomenological method and the turbulent structure theory. This paper is divided into two parts. The first part is the construction of gas-liquid surface mass transfer mechanism model. The traditional model of mass transfer coefficient is based on permeability theory or surface renewal theory. These models consider that mass transfer processes are controlled by unsteady molecular diffusion. Based on the classical mass transfer models of large and small vortices, it is assumed that mass transfer is controlled by the fluid structure of a certain band energy spectrum. In this paper, the contribution of fluid micro-element structures of different scales to mass transfer is considered in the range of the omnipotent spectrum, so the above hypothesis is no longer necessary. In this paper, the mass transfer process at the bubble interface is mainly studied. A new model of liquid side mass transfer coefficient is derived by using the characteristic line method and the concentration gradient thin layer approximation. The model combines the convection-diffusion equation of unsteady concentration and the interaction between bubble and fluid element. The effective relative motion between bubble and vortex is considered in this model to promote the contact between vortex and bubble surface, and based on the concept of random fundamental action, the frequency density distribution of vortex arriving at different scales is deduced theoretically. The density distribution of the arrival frequency can be used to explain the difference of the contribution of the fluid microelement structure to the mass transfer coefficient in each region of the omnipotent spectrum. In addition, the effects of bubble size, bubble deformation and vibration, and vortex number density distribution on mass transfer are also considered. The results of the model prediction are in good agreement with the experimental data reported in the literature. In the second part, numerical method is used to verify the rationality of the characteristic line and the density gradient thin layer approximation (the thickness of the concentration gradient layer is lower than 3.18% of the size of the vortex as a whole), and it is proved that even the mass transfer process of the single vortex should be regarded as an unsteady process. The numerical results show that the vortex in the dissipative region can take more solutes away and the mass transfer efficiency is higher at the same interface contact time. The unsteady concentration equation is solved numerically by the derived LBM model. Compared with the analytical model in this paper, it is found that the effect of tangential molecular diffusion on mass transfer at the liquid side of the interface is weak. When tangential molecular diffusion is ignored, the error of mass transfer coefficient is less than 1. It shows that it is reasonable to ignore the diffusion.
【学位授予单位】：湘潭大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ021.4

【共引文献】