Study On Particle Mixing Characteristics In Turbulent Fludized Bed Based On Image Method

The gas-solid turbulent fluidized bed(TFB)is widely employed in chemical,energy and other industrial processes because of its excellent mass and heat transfer and gas-solid contact efficiency.In previous studies on turbulent fluidized beds,the static bed height is so high that solids holdup will be more than 0.3.The gas-solid phase catalytic reaction process is prone to secondary reactions.The particle mixing characteristics determine the distribution range of particle residence time which directly affects the catalytic ability.In order to investigate the particle mixing characteristics in TFB under low static bed height,this paper takes the fluidized bed with 2 m height and a cross-section of 400×20 mm as the research objects.The high-speed camera is used to obtain the original image,the main research contents are as follows:(1)An image velocimetry calibration experiment was established in a liquid-solid system,and the deviation between the results and the standard value is within5%.According to the quantitative relationship between the iamge grayscale and the solids holdup obtained by the calibration experiment,the spatial distribution of particle concentration under low static bed height was obtained.From the results,the particle concentration exhibits"exponential"and"C-shaped"distributions in the axial direction.(2)The characteristics of cluster were quantitatively analyzed based on the OTSU method.The results show that the characteristic parameters of cluster exhibit different distribution trends under different static bed heights.The cluster frequency increases with the increase of superficial gas velocity at a static bed height of 0.05 m,but it decreases from 29.44 Hz to 21.4 Hz at a static bed height of 0.20 m;The equivalent diameter of most clusters is between 6 and 8 mm,and it will increases slightly with the increase of the static bed height;The axial distribution of the cluster solids concerntration are highly consistent with the trend of the cross-sectional average solids holdup;The cluster positive velocity is affected by many factors exhibits the"exponential"and"S-shaped"axial distribution trends under different operating conditions.(3)The particle diffusion coefficient was calculated based on random walk theory,and the key variable time interval was obtained using sensitivity analysis.The results show that the axial diffusion coefficient ranges from 0.0003 to 0.005 m~2/s and exhibits"S-shaped"multi-segment distribution characteristics in the axial direction.Axial distribution of solids axial diffusion coefficient increased significantly with the increase of superficial gas velocity,the increase exceeded more than 300%at the top of TFB.Under the same operating conditions,the solids lateral diffusion coefficient is an order of magnitude lower than axial diffusion coefficient,it exhibits an"exponential"distribution trend with a range from 0.0003 to 0.005 m~2/s.The comparison shows that the axial particle diffusion coefficient has a strong correlation with the particle velocity,while the lateral particle diffusion coefficient has a stronger correlation with the particle concentration.The passage of cluster enhances local mixing,but no significant correlation is shown between solids diffusion coefficient and cluster characteristic parameters.