Numerical Simulation Of Gas-solid Flow And Liquid Distribution In Fluidized Beds With Wet Particles

Gas-solid fluidized bed reactors have been widely used in modern energy,chemical engineering,materials engineering,food industry and et al due to its great ability for heat-conduction as well as the capability to handle a large amount of particles successively.The property of objects processed by fluidized beds changes accordingly with the development of technology.A well-known case is the liquid-present fluidized bed which contain wet particles.The presence of liquid leads to strong viscosity among particles,which causes different fluidization behaviors in comparison to the non-cohesive system,thus influencing the chemical conversion efficiency and the running stability of fluidized beds.Therefore,the research on characteristics of gas-solid flow and liquid distribution may benefit the design and the optimization of these reactors.This thesis delivers a new wet particle contact model based on the hysteresis contact model and carries out separate studies on the gas-solid flow characteristics of uniform liquid distribution fluidized beds with and without immersed tubes and dynamic liquid distribution wurster bed upon CFD-DEM model.The key points of this thesis are summarized as follows:A new model for wet particle contact based on the hysteresis contact model is established.This model is able to measure energy dissipation from different stages during wet particle collision.It can not only predict the appearance of particle agglomerate but also consider the influence caused by liquid viscosity and liquid surface tension on this collision.In a word,this new model can avoid some limitations of the current model.This thesis has applied the new wet particle contact model to study the gas-solid flow characteristics of uniform liquid distribution fluidized beds,including flow pattern,bed pressure,volume fraction of solid,particle velocity and particle mixing.The author finds,according to the study result,wet particles begin to agglomerate under the influence of the presence of liquid,and bubbles become gas channels with an irregular border.Fluctuating pressure increases in the beginning and then decreases when cohesiveness increases.Dry particles follows a stable core-annular flow structure while particles with a high level of cohesiveness flow in an indictable and irregular way.The mixing rate decreases when the particles increase in the cohesiveness level,and this mixing would stop with a high cohesiveness level which leads to the defluidization.18 immersed tubes are arranged inside the liquid-present fluidized bed to analyze the fluidization characteristics which cover flow pattern,bed pressure,gas and solid velocity,particle mixing state and et al.as well as the influence of the tubes on the wet particle fluidization.It turns out immersed tubes with a high cohesiveness level can decelerate the particles within the bed and act as a holder to promote the progress of defluidization.In addition,the immersed tubes cause the particles to a sharp turn and disintegrate the core-annular flow into several small annuluses.The wet particles heap up on the immersed tubes.The fluidization quality of the immersed tubes on the top,middle part of the bed as well as the parts near the wall is the worst.On basis of this new wet particle contact model,this thesis takes the process of spray,heat transfer and evaporation into consideration to look into the characteristics of gas-solid flow and liquid distribution in a 2D wurster bed with different levels of spray intensity and hot air temperature.According to the results,the author finds that when particles pass through the spray region of central cylinder,they will come together to form into some agglomerate whose size is closely related to the spray intensity while the place where agglomerate disappears is related to the spray intensity and hot air temperature.The author even manages to find out the pattern that mass fraction of liquid on the particles decreases from top to bottom within the annular region,and it's even more obvious when with a high spray intensity.Spray intensity and hot air temperature are totally on the opposite sides when it turns to the influence on the liquid mass fraction within the bed.Either an increase of the spray intensity or a decrease of the hot air temperature will break the dynamic equilibrium of the liquid mass fraction within the bed.