Experimental And Numerical Investigation On Hvdrodvnamics And Mixing Performance Of Fractal Injectors

There are many fractal objects in nature,part of which is similar to the whole in a certain pattern.For instance,the hierarchical network of branches,for trees of different sizes,the smallest branches size is always similar.Fractal is the structure obtained after optimization of mass and energy transfer process in nature.By introducing this kind of high-efficient structure into chemical engineering from nature,the transfer and reaction process in the conveying equipment or reactor can be enhanced.Fractal theory has certain advantages for the quantitative description of nonlinear processes and complex systems,and the similarity between part and whole has a guiding significance for the design of amplification in chemical engineering process.It has become one of the development goals of chemical engineering to get rid of the cumbersome gradual amplification empirical method,and fractal theory provides a possible method.Fractal injector is a kind of fast liquid mixing equipment with fractal structure,and it also an example of the application of fractal concept in chemical industry.The liquid fast reaction system widely exists in chemical industrial processes such as polymer chemical industry,fine chemical industry,pharmaceutical biotechnology,and the performance of mixing directly affects the production process.Fractal injectors can achieve the overall mixing uniformity through a hierarchical fractal structure,and the problem of low mixing rate caused by laminar mixing and the high energy consumption caused by turbulent mixing can be avoid.In this paper,the mixing and flow field of a fractal injector with three-dimensional distribution of nozzles are studied under liquid-liquid homogeneous turbulence mixing operating condition.The design principle of the fractal unit of the injector is mainly discussed,the hydrodynamics and mixing characteristics of the fractal injector are investigated,and the influence of the fractal structure parameter and configuration on the mixing performance is revealed,which can provide theoretical basis and support for the industrial design and application of fractal injector.PIV experiment method is used to validate the jet flow field obtained by CFD simulation.The velocity field,turbulent kinetic energy distribution,vorticity distribution and other information of fractal injector with three-dimensional distribution nozzle are studied by CFD simulation method.Mixing characteristics of fractal injector were quantitatively evaluated through the unsteady CFD simulation and LIF experiment results.The main work and conclusions are summarized as follows:(1)The flow in the fractal element is simplified as submerged parallel multiple circular jets.Numerical simulations and experimental study on the near jet flow field and mixing process of submerged parallel multiple circular jets with spatial arrangement were carried out.The effects of jet number,jet spacing and inject Reynolds number on the flow field structure were especially analyzed.The results revealed that,compared with the parallel plane twin jets,there is no discoverable velocity stagnation point,and the flow field can be divided into three part:convergence region,merging region and combined region.By comparing the twin jets,triple jets and quad jets systems under the same jet Reynolds number,it is found that this mixing length between the jets is the shortest in the triple jets system,which indicates that the mixing rate and velocity decay rate of the triple jets system are faster than the other two.Analyzing the influence of jet spacing,it can be found that the merge point and combine point of submerged parallel multiple circular jets mainly depend on the spacing between jets.Compared with the traditional parallel plane twin jets system,the near-field is more sensitive to the change of jets spacing,and there is a linear relationship between jet spacing and location of characteristic points in the flow field.(2)Based on the research of the submerged parallel multiple circular jets flow field,the size of the fractal unit can be determined and a fractal injector with two generations was obtained.Numerical simulation study of its hydraulic and vortex structure is carried out,and the effect of fractal dimension on the flow and mixing characteristics of fractal injectors was analyzed.The results showed that,compared with the 2D fractal injector,the fractal injector with 3D distribution of nozzles can realize the uniform distribution and mixing in the whole vessel more quickly under the same total flow rate.However,more dense distribution of nozzles can't guarantee better mixing performance.Under the same total flow rate,the larger the number of nozzles,the smaller the inject Reynolds number,and the weaker the turbulence near the nozzles,which resulted the slower mixing rate.Reducing the fractal dimension properly can improve the mixing rate,micro mixing performance,and raise the volume utilization rate of the vessel.(3)On the premise of jet impingement pairs,large kinetic energy dissipation and obvious non-uniform flow field were observed in injector with strict fractal structure(overlapped fractal injector),the configuration is adjusted to staggered fractal injector,which can replace the impinging jet flow with crossflow.In the case of keeping same total flow rate,the flow and mixing characteristics of the injector with the statistically fractal structure were discussed.The results showed that after adjusting the configuration to the staggered,the uniformity of the flow field is improved,the distribution of the intense turbulence region becomes more dispersed,and the streamwise and spanwise vorticity take a symmetrical distribution.While improving the uniformity of flow field,the mixing rate in radial direction is sacrificed in staggered fractal injector.However the diffusion and mixing rate in radial direction mainly depends on the nozzles distribution,adjusting fractal dimension can improve the mixing performance in radial direction.On the other hand,compared with overlapped fractal injector,the diffusion and mixing rate in streamwise direction are increased.