Study Of Core-Annular Flow Pattern And Particle Ejection Mechanism Within Bubbling Fluidized Bed

Fluidized bed has the advantages of high combustion efficiency,wide fuel adaptability and high reaction strength.It is widely used in petroleum,chemical,metallurgy,materials,environmental protection and other fields,and plays an important role in building a new industrial system of energy saving,emission reduction and sustainable development.Not only that,in modern industry and agriculture,there is also a prevalence of solid particles transportation,mixing and other practical needs.How to make solid particles have fluid properties,that is,fluidization,has a very important significance for the effective use of energy.In gassolid two-phase flow systems,dense-phase gas-solid bubbling fluidization is one of the most extensive forms of fluidization in the industry.Therefore,the study of the interaction between particles and bubbles and the hydrodynamic behavior of particles and bubbles based on the bubbling fluidized bed has important implications for exploring the mechanism of bubbling fluidization,and revealing the internal relationship between the physical properties of particles,fluidization speed and bubbling fluidization.In this paper,a combination of high-speed photographic techniques and Computational Fluid Dynamics(CFD)-Discrete Element Method(DEM)numerical simulation of four-way fluid-solid coupling was used to systematically study the bubbling fluidized bed by varying the fluidization spped and particle numbers.The main research work and results achieved in this paper are as follows:(1)The development history of fluidized bed technology is systematically reviewed,the relevant basic theory of particle fluidization is outlined,and the current research hotspots and research status in the field of bubbling fluidization at home and abroad are introduced in recent years.(2)The relevant theory of the CFD-DEM four-way coupled numerical simulation method is introduced,the advantages and disadvantages of the Euler Euler model and the Euler Lagrange model are compared,and the differences and connections between the drag coefficients of the five common drag models in currently simulating dense gas-solid two-phase flows are compared in detail.(3)The Small-Scale Challenge Problem(SSCP-I),published by the National Energy Laboratory of the United States in 2013,investigates the effects of different drag models on gas-solid two-phase flow processes.The gas-solid two-phase fludization characteristics such as instantaneous particle flow process,particle velocity vector distribution,bed height variation,gas phase mean pressure drop,and particle time-averaged velocity are compared,respectively,and a validation analysis of the drag model is performed in conjunction with the experimental data to provide a reference for numerical simulation of dense gas-solid two-phase flow.(4)Experimental measurements and numerical simulations of gas-solid twophase transient flow processes in fluidized beds at different particle concentrations were carried out to study the hydrodynamic behavior of bubbles.The results show that the simulations can accurately predict the bubbles morphology compared to the experimental results.When the number of particles is 30,000,only a single CoreAnnular flow pattern appears.When the number of particles is increased to 36,500,the single bubble in the spouted bed transitions into two and a double Core-Annular flow pattern emerges.As the number of particles is increased to 43,000,a complex multi Core-Annular flow pattern appears.These flow patterns are also observed in the experiments using high speed imaging camera.This paper analyzes and explains the causes of these flow phenomena from the dynamic characteristics of particle phase and fluid phase.These results have great significance in providing guidance for optimization of dense phase bubbling spouted beds.(5)Geldart class D particles with particle sizes of 2.5 mm at different fluidization speed were studied using a high-speed photographic technique combined with CFD-DEM four-way coupling,and comparative analysis of fluidization parameters such as bed height,equivalent diameter of bubbles,and ejection behavior of the particles in the fluidized bed was carried out.The results show that numerical simulations can predict more accurately the changes in bubble morphology and the behavior of particle ejection.The mechanism of particle ejection is varied under different inlet flow conditions.At the low inlet flow rate of 300 L/min,the particles ejected into the freeboard region are only from the bursting bubbles.As the inlet flow rate increases to 400 L/min,the ejected particles are not only from the bursting bubble but also from the bubble wakes.Particle ejection is mainly caused by the bubble expansion rate.These findings could serve as the reference for the optimal design of fluidized beds to achieve higher gas-solids mixing efficiency and combustion efficiency.