Analysis Of Flow And Heat Transfer Characteristics Of Nanoparticles In Sound Field Micro Fluidized Bed

Fluidized bed is widely used in many fields because of its good two-phase back mixing ability and heat and mass transfer characteristics.With the new needs of industrial development,the size of fluidized bed reactor and the particle size of materials in the bed are decreasing continuously.Compared with traditional large-scale fluidized beds,micro-fluidized beds have outstanding advantages in particle fluidization state,heat transfer characteristics and operability.Nanoparticles have gradually become the research focus due to their unique properties.However,during the fluidization process of nanoparticles,there will be some problems,such as agglomeration phenomenon,lowering fluidization quality and low heat transfer efficiency between gas and solid.However,the advantage of external sound field lies in that it is not limited by physical parameters such as the type of nanoparticles and temperature,and it is assisted by radiation to further improve the fluidization state and effectively strengthen the heat transfer.Therefore,it is of great practical significance to study the flow and heat transfer characteristics of nanoparticles in micro fluidized bed under the condition of external sound field.In this paper,the flow and heat transfer of SiO₂ nanoparticles in micro fluidized bed are studied.Firstly,the flow field and sound field are coupled to simulate the fluidization process of nanoparticles in different sound pressure levels.By analyzing the simulation results,the parameters such as volume fraction distribution of nanoparticles in the bed,bubble state distribution,particle velocity and root mean square of bed pressure fluctuation are obtained.Simulation shows that local gas holdup,bubble frequency and average bubble rising speed all decrease with the increase of sound pressure level at lower gas velocity.At higher gas velocity,the local gas holdup and bubble frequency are directly proportional to the sound pressure level,while the average rising speed is inversely proportional to the sound pressure level.With the sound pressure level gradually increasing from 100-140d B,the fluidization quality of particles in the bed is gradually improved,and the uneven gas-solid mixing and throttling phenomenon are obviously reduced.However,when the sound pressure level reaches 140d B,the fluidization quality decreases.This is because the excessive sound pressure level forces the contact frequency of particles to increase,which leads to the difficulty of mutual adhesion and dispersion,and the effect of weakening agglomeration is not obvious.Study on the influence of sound field on gas-solid phase heat transfer in micro fluidized bed.Using the particle temperature model and heat transfer model,the temperature distribution diagram and Re number change diagram of nanoparticles are obtained.Calculate the heat transfer coefficient according to the bed temperature and particle pseudo temperature.It is found that the heat transfer coefficient does not change obviously before the sound pressure level is 80d B.After 80d B,the heat transfer coefficient increases with the increase of sound pressure level and reaches the maximum at 130d B.At the same time,it is found that the fundamental reason for the sound field to improve the heat transfer coefficient is that the energy of sound field strengthens the turbulence intensity between particle phase and gas phase.It is found that when the sound frequency is 120Hz and the sound pressure level is130d B,the fluidization and heat transfer of nanoparticles in the micro fluidized bed can be improved most obviously.