Numerical Simulation On Concentration Field Of An Oscillatory Flow Reactor With Orificial Baffles

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.