Study On Gas-Solid Flow Structure And Heat And Mass Transfer Characteristics Of Wet Particle In A Vibrated Fluidized Bed

Coal is China’s main energy source,making great contributions to China’s economic development.Under the goals of carbon peaking and carbon neutrality,the efficient and clean utilization of coal is a key measure to promote the green development of the coal industry.China is rich in lignite resources,lignite usually contains a large amount of water,so the utilization of lignite is greatly restricted,lignite drying has become an important process in the improvement of lignite quality.Vibrated fluidized bed(VFB)drying technology is widely used owing to its high gas-solid contact efficiency and the ability to overcome the disadvantages of channeling,dead zone,local overheating,and wet particle agglomeration.At present,the results show that a VFB of lignite particles belongs to the wet particle system.The presence of liquid filling granular gaps makes the formation of liquid bridges with a certain volume between particles,which will have a non-negligible impact on the flow characteristics and interparticle collision characteristics of particles in a VFB.Furthermore,this will affect the fluidization quality and heat and mass transfer characteristics.So the thesis studied the effects of different operating parameters on the gas-solid flow characteristics and heat transfer characteristics of wet particles in a VFB.The influences of operating parameters on the drying characteristics of lignite in a VFB were studied.The main research work of this thesis is as follows:Firstly,based on the comprehensive analysis combining the CFD-DEM coupled simulation,high-speed dynamic camera,and of quartz impact force sensor measurement,the gas-solid flow characteristics of wet particles in a VFB were studied.Through the combination of the high-speed dynamic camera and numerical simulation,the influence of vibration energy on the fluidization behavior of wet particles was analyzed.The change of bed height and the solid volume fraction in a VFB of wet particles were analyzed.Moreover,the collision characteristics in a VFB of wet particles were revealed,and the axial velocity fluctuation,particle collision frequency,and particle collision intensity were analyzed.The results show that the introduction of vibration energy significantly reduces the minimum fluidized gas velocity and improve the fluidization quality of the wet particles.The fluidization state is stable under the conditions of U=1.4 Umf,f=25 Hz,A=2 mm,and the solid volume fraction distribution is relatively uniform.When the vibration parameters and gas velocity exceed critical values,the fluctuation of the wet particles becomes too violent and the fluidization quality deteriorates accordingly.The axial average velocity fluctuations of particles increase with the increase of amplitude or frequency,and the particle velocity changes with the same sinusoidal fluctuation law as the vibration frequency.The collision frequency first decreases and then increases with the increase of amplitude,and decreases and then increases with the increase of frequency.According to the statistical analysis of the quartz impact force sensor signal,the average collision force of wet particles gradually attenuates along the bed height and increases with the increase of vibration frequency and amplitude.Secondly,based on the CFD-DEM coupling model,the numerical simulation studies of the heat transfer characteristics in a VFB of wet particles under hot conditions were carried out.The heat transfer characteristics of wet particles in a fluidized bed with or without the introduction of vibration were comparably studied,the effects of vibration frequency,amplitude,gas velocity,and airflow temperature on the heat transfer characteristics were studied.The results show that some particles agglomerate in the upper part of the bed in a conventional fluidized bed(CFB).The introduction of vibration energy destroys particle agglomeration,which effectively improves the efficiency of gas-solid contact and solid-solid contact of wet particles in a VFB.The average temperatures of particles in a VFB increase faster than that in a CFB,and the average particle temperature increase by 0.72 % within 5 s.The average temperature of particles decreases with the increase in bed height.The average temperature of the particles increases with the increase of amplitude,and first increases and then decreases with the increase of frequency.The average temperature difference between particle layers increases with the increase in bed height,from 29.98 K at the height of 10 mm to 2.44 K at the height of 70 mm.With the increase of amplitude,the Nusselt number distribution of particles is more uniform,and the heat transfer capacity of the particle bed is enhanced.The Nusselt number distribution of particles shows periodic fluctuation with a frequency identical to the vibration frequency.The particle Nusselt number increases with increasing gas velocity,and the increase of gas velocity improves the heat input and gas-solid contact efficiency,so the heat transfer efficiency of particle layer increases.Finally,the drying characteristics of lignite in a VFB were studied,and the effects of operating parameter including gas velocity,amplitude,frequency,and airflow temperature on the drying characteristics of lignite were studied.The drying curves and drying rate curves at different conditions were analyzed by combining the fluidization and collision characteristics of wet particle in a VFB.The results show that the introduction of vibration energy significantly increases the lignite drying rate compared with the same conditions in a VFB.The average temperature of lignite particle in a VFB increases faster than that in a VFB.The drying rate of lignite increases with the increase of gas velocity and temperature.The increase of gas velocity makes the evaporative water can be carried out quickly from the gas-solid interface,which accelerates the diffusion of lignite moisture from the inside to the surface.The increase of gas temperature increases the temperature gradient of the gas-solid interface and the convective heat transfer coefficient,so the drying rate of lignite increases.The drying rate of lignite increases with the increase of vibration frequency and amplitude.According to the study of previous numerical simulation,the increase of vibration intensity strengthens the particle collision behavior,and enhances the gas-solid interface turbulent mixing,thus improves the convective heat and mass transfer efficiency and drying rate.The dissertation consists of 74 figures,7 tables and 115 references.