【作者】 袁灿盛；

【导师】 吴嘉；

【作者基本信息】 浙江大学 ， 化学工程， 2008， 硕士

【摘要】 振荡流反应器的数值模拟,还停留在对反应器速度场的模拟研究阶段。研究应用计算流体力学技术对振荡流反应器的浓度场进行数值模拟研究,为模拟化学反应打下一个坚实的基础。同时,浓度场的模拟对于注入分散特性的研究,对于振荡流反应器的流动模型的研究都具有重大的意义,它为完成这方面的工作提供了另一种方法。本文借助商业计算流体力学软件Gambit和CFX,对五腔室圆环挡板振荡流反应器进行几何建模,考虑了反应器的进、出料管与注入分散管等复杂的几何结构;采用了混合结构化与非结构化网格对反应器的结构进行了数值离散,在物理边界处加密网格,建立了包含568,699个网格的计算模型,计算的准确性与速度兼顾。鉴于实验过程中流动的复杂性,采用均相自由表面流模型建立流动的传递方程,采用网格变形模拟振荡边界条件,对湍流过程用κ-ε方程来进行模拟。数值计算阶段对时间导数项的离散采用二阶向后欧拉差分,对流项用High Resolution格式。以前针对浓度场的研究,大多是用浓度计测出一系列点处的浓度值。这种方法受浓度计灵敏度限制比较大,测量的方法会对原来的速度场会产生干扰性影响。因为无需浓度计测量,数值模拟方法不但不会对速度场产生干扰,而且还能给出示踪剂随的质量传递和速度场之间的关系,有助于从机理上分析示踪剂的运动规律。对振幅为2.7mm,振荡频率为1.67Hz,无净流条件下,示踪剂浓度的演化情况的模拟表明:振荡流反应器中主体流动与漩涡构成的二次流动对腔室中心区域的交替控制促成了强烈的径向混合,使注入管心区的示踪剂在一至二个振荡周期中迅速分散到整个管截面范围。二次流漩涡相对封闭的流场结构也影响了其内部流体与主流区流体间的质量传递,因此示踪剂在轴截面上的均匀分布需要10个以上的振荡周期。研究把模拟得出的浓度场用于分析注入分散过程振荡流反应器的混合效率。定义一个腔室中的浓度方差C作为判断混合效率的依据,结果表明,C值的变化分成两个阶段:第一阶段,C值随时间快速下降,注入的示踪剂从注入口传递到整个腔室;第二阶段,C值随时间缓慢变化,腔室内的示踪剂分布逐渐均匀,并与周围腔室进行传递,C值是腔室内混合和腔室间混合共同作用的结果。研究模拟了不同振荡条件下的浓度场,并分析振荡条件对振荡流反应器混合效率的影响,结果表明,基于长程对流混合和短程湍流扩散双模式传递模式的二项指数衰减曲线方程能够较好地描述示踪剂浓度分布方差值随时间的变化(C～t关系)。用参数表征的混合效率说明振荡雷诺数越大,混合效率越高,但所需要的振荡装置的负荷和能耗也越大,实际选取振荡装置时应综合考虑这些问题。研究模拟了不同净流条件下圆环挡板振荡流反应器的浓度场,结果表明,较小净流下速度场与间歇过程基本一致,只是漩涡中心位置有所不同;大净流则改变速度场,示踪剂在主流带动下向出口很快传递。更多还原

【Abstract】 Numerical simulation of oscillatory flow reactor stays on the period of simulating velocity field of reactor. In this research, the concentration field is stimulated, which is a solid foundation for simulating reaction using computational fluid dynamics. At the same time, simulation of concentration field has great significance in researching characteristic of injection dispersion process and flow model of oscillatory flow reactor. It supplies another method to achieve these works.In this paper, the geometry of an orificial baffled oscillatory flow reactor with five cells was constructed with complicated inlet, outlet and center pipes included by employing the commercial package for computational fluid dynamics of Gambit and CFX. A method of hybrid structure and unstructured mesh was carried out to discretize the whole column and distributing more meshed at the physical boundary, considering both the computational veracity and speed, and 568,699 grids were generated for further computation. Taking the complexity of flow in the experiment into account, the transport equations were set up according to homogenous free surface model, with mesh deformation to simulate oscillatory boundary conditions.κ-εequations were added if the flow pattern was turbulent. In the stage of numerical calculation, the derivative term of time employed a scheme of second order backward Euler, and the convective term a scheme of High Resolution.The concentration field is studied by testing the concentration of a series of points in former researches. This method is constrained by the sensitivity of instrument and it will interfere the original flow field. Without the necessary of using instrument, the method of numerical simulation not only keeps the same velocity field of reactor but also obtains the relationship between mass transfer of tracers and velocity field, which is useful to analyze the moving rules of tracers. Simulation on evolvement of tracer’s concentration under condition of amplitude 2.7mm, oscillatory frequency 1.67Hz and without net flow show: violent radial mixing was caused by control in turn of mainstream and secondary flow region which was constituted by vortices. The mixing forced tracers to transfer to radial face in one or two oscillatory periods. The relative close flow structure of vortices affected mass transfer between inner flow and mainstream flow, so uniform distribution of tracer in axis face needed more than ten oscillatory periods.Concentration field comes from simulation is used to analysis mixing efficiency of oscillatory flow reactor in injection dispersion process. Define concentration variance C of a cell as criterion of mixing efficiency and the results come that the change of C can be divided into two periods: the first period C changes very quickly with time pass and tracers transfer from injection holes to the whole cell; the second period C changes slowly and the tracers gradually become uniform, at the same time, the tracers in the third cell exchanged with near cells. C was a result cooperated by mixing in the cell and mixing between cells.Simulation of concentration field under different oscillatory conditions and analysis of oscillatory condition to mixing effect of OFR come that the second order exponential function, which was based on two pattern transfer consisted of long distance mixing and short turbulent dispersion, can be successfully used to fit the change of concentration variance vs. time. Parameter was used to symbolize mixing effect and it showed higher Oscillatory Reynolds Number, higher mixing efficiency. But in practice, choosing oscillatory equipment also needs to consider large energy dissipation comes from high oscillatory.Simulation of concentration field under different net flow condition comes that compared with batch process, velocity field under small net flow was almost the same and the small difference is position of the centers of vortices, however, large net flow would be result to the change of velocity field and tracers were transferred quickly to outlet with mainstream.更多还原