Influence Mechanism Of Characteristic Flow Structures On Heat Transfer Characteristics In Gas-Solid Fluidized Bed

The vigorous bubble turbulence and particle circulation in the gas-solid fluidized bed make it have excellent mixing performance,heat transfer and mass transfer performance,and have wide applications in chemical engineering and related fields.Bubbles,particle vortex,particle agglomerate,and spray cloud zone,as the typical flow structure in a gas-phase polyolefin fluidized bed,determine the heat transfer and mass transfer characteristics of the fluidized bed reactor.In particular,when condensate is introduced into the fluidized bed reactor,the heat transfer characteristics of the reactor also change due to changes in hydrodynamic characteristics such as droplet-particle and droplet-bubble interactions.Therefore,it is of great significance to study the influence of the characteristic flow structure of fluidized bed reactor on heat transfer characteristics.In this thesis,the gas phase polyolefin fluidized bed reactor is taken as the research object.Firstly,the CFD simulation of single bubble and continuous jet bubble behavior in two-dimensional fluidized bed 1s earried out to study the particle vortex and electrostatics on bubble characteristics,particle behavior and heat transfer.Through the wet particle fluidization experiment,continuous spray heating fluidization experiment and CFD simulation,the diffusion path and evaporation behavior of the liquid in the fluidized bed and its effects on bubble characteristics,particle flow pattern and heat transfer were studied.The main research work and results include:1.By coupling the two-fluid model with the heat transfer model the generation and dispersion process of the particle vortex during the single bubble rise is quantitatively characterized for the first time.It is found that as the bubble rises,the particle vortex gradually diffuses to the entire fluidized bed,and drives the particles to mix,causing the bubble to split and inhibit the gas flow,so that the gas flow rate between the bubble and the emulsion phase is reduced by 6.5%compared with the calculated value of the Davidson model.Conducive to the heat transfer between the emulsion phase and the bubble phase;due to the particle vortex enrichment effect on the particles,hot spots are easily generated in the particle vortex,and the region with a large local heat transfer coefficient is always located below the bed.Furthermore,the electrostatic model is coupled with the two-fluid model and the heat transfer model.It is found that the charged state of the particles causes the force at the boundary of the bubble to change,the diameter of the bubble decreases,the top of the bubble becomes sharper,the velocity of the bubble increases,and the gas flow rate between the emulsified phase and the bubble phase Compared with the calculated value of the Davidson model,the calculated value is reduced by 13.5%.The particle charging also makes the dispersion of the particle vortex stronger,and the enrichment of the particle is enhanced.It is more likely to generate hot spots in the particle vortex and near the wall surface of the fluidized bed.2.In the central jet bubbling fluidized bed,the influence of continuous jet bubbles and electrostatics on the gas flow rate,particle trajectory and heat transfer behavior was obtained by CFD simulation.According to the bubble distribution,the jet bubbling fluidized bed is divided into a bubble interaction zone,a transition zone and a wall zone in a radial direction.According to the granular temperature distribution,the jet bubbling fluidized bed is divided into a gas injection zone and a bubble growth zone along the longitudinal direction.Due to the strong interaction between bubble-bubble,bubble-particle vortex,particle vortex-particle vortex,the gas interchange coefficient is increased by 58.5%compared to the calculated value of the Davidson model.When the particles are charged,the particle distribution in the jet bubbling fluidized bed is uneven,the bubble movement is more disordered,the bubble shape is more irregular,the bubble diameter and the number of bubbles is significantly reduced;the particle vortex entrains and enriches more particles.This causes the particle volume fraction near the wall to rise,causing hot spots at the wall.On this basis,the particle trajectory is studied by the fully coupled CFD-DEM simulation method.It is found that when the particles are not charged,the particles released in the gas injection zone are entrained by the bubble and ejected into the free space,and then fall along the wall.The particles released in the transition zone will be vortexed by the particles and circulate inside the bed.When the particles are charged,the particles released in the gas injection zone and the transition zone may be precipitated and adhered to the wall3,In the wet particle fluidization experiment,by analyzing the kurtosis and skewness of the pressure pulsation signal,the critical transition conditions forjudging the wet particle flow pattern from turbulence to laminar flow are proposed when the fluidization state is abnormal.And the kurtosis is more sensitive to the transition of the fluidized state.By analyzing the incoherent spectrum and coherent spectrum of the pressure pulsation signal,it is found that when the bed is defluidized,small-sized bubbles or local particle pulsations may still exist in the fluidized bed.When the decay index of the incoherent spectrum is from-5/3 to-1,the flow pattern is laminar,the particle pulsation is weak,and there is basically no energy dissipation in the fluidized bed When the decay index is-5/3?-3,the flow pattern is turbulent,and there will exist structures such as uniform particle aggregates.The force analysis of the wet particles shows that the relative value of the liquid bridge force and the drag force determine the bubble diameter and the minimum fluidization velocity of the wet particle fluidized bed.4.In the two-dimensional fluidized bed with heating system,innovatively use reversible temperature-sensitive color-changing particle tracer technology,using its discoloration characteristics at different temperatures,combined with PIV technology and DIA technology to achieve the experimental characterization of particle diffusion behavior.It is aimed to reveal the influence of spray flow rate,nozzle position,heating temperature on bubble characteristics,particle pulsation characteristics,spray zone characteristics and heat transfer.It is found that a small size particle agglomerate is formed near the nozzle,which promotes the diffusion of liquid.The wall spray has the strongest promoting effect on bubble generation,while the top spray greatly weakens the bubble generation,and generates large-sized agglomerates and settles above the gas distribution plate,not conducive to heat transfer.Compared with the side wall spray,the spray area is increased by 40%at the top spray.The average particle velocity field analysis results show that liquid evaporation will produce more particle vortex but the particle vortex distribution 1s more dispersed,which weakens the bulk particle circulation.Using the established multi-zone heat transfer model,it is found that when the liquid is sprayed at the center of the bubble,it will reach the thermal equilibrium state in about 100 s.Increasing the bed height has little effect on the inter-layer particle exchange.When the liquid is sprayed on the wall surface,the inter-segment exchange is small and the temperature difference in the bed is significantly increased.5.The CFD model of the fluidized bed was established considering the effects of liquid coating on the particles,liquid evaporation and droplet-particle collision on the particle motion,bubble characteristics and heat transfer characteristics of the fluidized bed were studied.The results show that for the monodisperse particle fluidized bed,when the liquid is horizontally sprayed,the jet action increases the average particle height and bubble frequency of the fluidized bed;when the liquid is sprayed at 30°,the jet shearing action causes the average particle height of the fluidized bed.Decreased significantly.There is a dense droplet accumulation zone,a liquid discharge core zone and a liquid evaporation zone in the vicinity of the nozzle.A small number of free droplets will be dispersed into the main fluidized bed under the action of the jet.When the liquid is sprayed at 30°,the shape of the core area is obviously changed,and the heat transfer coefficient at the nozzle position is significantly increased,but the heat transfer coefficient of the area above the nozzle height is lower than that of the dry fluidized bed.For the fluidized bed of bi-dispersed particles,the liquid jet reduces the degree of separation of the fine particles,the degree of particle turbulence increases significantly,and a large range of particle vortexes are formed in the vicinity of the nozzle,and the fine particles accumulate at the wall surface.As the proportion of fine particles increases,the number of particles encapsulated by the liquid and the average film thickness increase,the heat transfer between the particles and the gas is weakened,indicating that the spray is detrimental to the heat transfer of the fine particles.