湍流场中流体颗粒破裂机理研究及其模型构建

发布时间：2018-05-04 04:19

本文选题：破裂频率 + 湍流涡　；参考：《湘潭大学》2017年硕士论文

【摘要】：湍流分散体系(如气-液、液-液等)中流体流动时伴随发生的流体颗粒(以下简称流粒,气泡或液滴)的破裂现象通常决定了流粒在流场中的分散状态、粒径分布和相界面积,因而对整个体系的传质、传热和反应性能有着重要影响。深入理解湍流中流粒的破裂机理并构建出相应的理论模型可为多相反应器的设计、优化和工业放大提供分散相粒径分布和相界面积分布方面的重要理论依据。本文通过对以往破裂频率模型进行分析,提出了前人模型中采用的“流粒尺寸总是落在惯性子区”和“小于或等于流粒尺寸的湍流涡才能引起流粒破裂”的假设具有明显不合理性(二者结合,相当于认为只有惯性子区的湍流涡才对流粒破裂有贡献)。此外,以往破裂频率模型大都是借鉴气体分子反应速率的建模方式,以流粒与湍流涡的碰撞频率与破裂概率的乘积来获得流粒的破裂频率。但由于受湍流脉动随机性的影响,湍流涡的寿命时间也具有一定的随机性,这使得湍流涡与流粒之间的碰撞较气体分子情形要更为复杂。前人模型由于是直接照搬气体分子碰撞模式,因而无法体现湍流涡寿命时间对流粒与湍流涡碰撞的影响。不同于前人模型,本文将对流粒破裂有贡献的湍流涡的分布区域扩展至了整个湍流能谱范围(即同时包括含能涡子区、惯性子区以及耗散子区);进而在全能谱分布函数的基础上和考虑湍流涡寿命时间的前提下,提出从构建湍流涡与流粒的碰撞概率模型以及引入临界破裂速度的思路来推导流粒破裂频率模型。通过引入湍流涡与流粒的相对速度、碰撞角度、碰撞自由程等反映碰撞物理过程的参数,并考虑流粒与湍流涡的初始相对距离、湍流涡寿命时间内涡旋所能运动的相对距离对碰撞的影响,本文获得了可导致流粒破裂的碰撞频率及其相应的流粒破裂频率模型、子尺寸分布模型。本文提出的破裂频率模型和子尺寸分布模型的预测结果表明:流粒破裂频率随着湍流动能耗散速率的增加而增大,但随着流粒尺寸的增大,流粒破裂频率呈现出先增大后降低的趋势。越高的湍流动能耗散速率对应着湍流涡传递给流粒的能量越多,因此流粒发生破裂的频率越大;此外,在湍流动能耗散速率较低时,液滴子尺寸分布以等尺寸破裂概最高,但当湍流动能耗散速率增大时,非等尺寸破裂概率逐渐增大,液滴子尺寸分布曲线变得平坦;表面张力越小,流粒越易发生破裂,相应的流粒破裂频率越大、非等尺寸破裂概率增大;模型预测结果与实验数据表现出一致的演变趋势,且与搅拌槽中液滴累积尺寸分布吻合良好。
[Abstract]:In turbulent dispersion systems (such as gas-liquid, liquid-liquid, etc.), the breakup of fluid particles (hereinafter referred to as flow particles, bubbles or droplets) associated with fluid flow usually determines the dispersion state of flow particles in the flow field. The particle size distribution and phase boundary area play an important role in the mass transfer, heat transfer and reaction performance of the whole system. A thorough understanding of the fracture mechanism of particles in turbulent flow and the construction of corresponding theoretical models can provide an important theoretical basis for the design of multiphase reactors, optimization and industrial amplification of dispersed phase particle size distribution and phase interface integral distribution. Based on the analysis of the previous rupture frequency model, The assumption that the particle size always falls in the inertial sub-region and the turbulent vortex smaller than or equal to the particle size can cause the flow particle rupture is obviously unreasonable. It is considered that only turbulent vortices in the inertial subregion contribute to the rupture of convective particles. In addition, most of the previous fracture frequency models were based on the gas molecular reaction rate model, and the fracturing frequency was obtained by using the product of collision frequency and rupture probability of flow particle and turbulent vortex. However, due to the randomness of turbulence pulsation, the lifetime of turbulent vortices is also stochastic, which makes the collision between turbulent vortices and particles more complicated than the case of gas molecules. Because the previous model is a direct model of gas molecule collision, it can not reflect the impact of turbulent vortex lifetime time convection particle and turbulent vortex collision. Different from previous models, this paper extends the distribution region of turbulent vortices which contribute by convection particle rupture to the whole turbulent energy spectrum (that is, including the energetic vortex subregion). The inertial subregion and the dissipative subregion are further considered on the basis of the omnipotent spectrum distribution function and the turbulent vortex lifetime time. In this paper, the probability model of collision between turbulent vortex and particle and the method of introducing critical rupture velocity are proposed to deduce the model of particle rupture frequency. By introducing the relative velocity of turbulent vortex and particle, collision angle, collision free path and other parameters to reflect the physical process of collision, the initial relative distance between particle and turbulent vortex is considered. The effect of relative distance of vortex motion on the collision during the turbulent vortex lifetime is studied. In this paper, the collision frequency and the corresponding particle rupture frequency model, the sub-size distribution model, are obtained. The prediction results of the fracture frequency model and the sub-size distribution model proposed in this paper show that the particle rupture frequency increases with the increase of turbulent kinetic energy dissipation rate, but with the increase of particle size. The flow particle rupture frequency increased first and then decreased. The higher the turbulent kinetic energy dissipation rate is, the more energy the turbulent vortices transmit to the flow particles, so the higher the frequency of the flow particles rupture is, in addition, when the turbulent kinetic energy dissipation rate is lower, the droplet size distribution is the highest in the same size fracture. However, when the turbulent kinetic energy dissipation rate increases, the non-equidimensional fracture probability increases gradually, and the droplet size distribution curve becomes flat, the smaller the surface tension, the more prone the flow particle is to rupture, and the higher the corresponding flow particle rupture frequency is. The prediction results of the model are consistent with the experimental data and agree well with the cumulative size distribution of the droplet in the stirred tank.
【学位授予单位】：湘潭大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TQ021.1

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