Study On The Characteristics Of Particle Clusters In A High Density Circulating Fluidized Downer Reactor

To investigate the gas-solids hydrodynamics,particle cluster characteristics in particular,a two-dimensional circulating fluidized bed system is built with a downer of 5m in this study.Under a wide range of operating conditions with superficial gas velocity(U_g)from 1-7 m/s and solids circulation rate(G_s)between 100-700 kg/m~2s,distributions of averaged solids holdup and particle velocity and the characteristics of article cluster are studied in details.Effects of the operating conditions on the gas-solids flow are also studied systematically.Results show that axial distributions of the averaged solids holdup and particle velocity are non-uniform because of the inlet effects to some extent.Axially,averaged solids holdup distribution shows a dense part near the inlet section and a dilute part at the outlet of the downer.In the fully developed section near the downer bottom,the averaged solids holdup is about 0.7?10.9%and the averaged particle velocity ranges from 1.6 m/s to 6.7 m/s.Laterally,the averaged solids holdup is relatively smaller in the center than that near the wall,while the corresponding averaged particle velocity is higher in the center than that at the wall region.In comparison with low density operating conditions,solids holdup distribution is more non-uniform under high density operating conditions.The averaged solids holdup increases with the decrease of U_g and/or the increase of G_s.The averaged particle velocity increases with the increase of U_g.However,effects of G_s on the distribution of the averaged particle velocity are relatively complicated.When U_g is small(U_g?3 m/s),the averaged particle velocity increases with increasing G_s.With higher U_g(U_g>3 m/s),the averaged particle velocity changes slightly with the increase of the G_s.Based on the above research,hydrodynamics of particle clusters and the effects of operating conditions are further studied.Important time-averaged parameters of the particle cluster include cluster solids holdup,cluster time fraction and cluster frequency.It is found that cluster solids holdup distribution shows a dense part near the inlet section and a dilute part at the outlet of the downer which is similar as the distribution of the averaged solids holdup.In the developed section,the cluster solids holdup,cluster time fraction and cluster frequency is about 0.7?15.5%,30.3?40.4%and 390?780 Hz respectively.The lateral distribution of the cluster solids holdup and cluster time fraction shows a dilute region in the center and dense region near the wall,while the cluster frequency is much higher in the center than that near the wall.Effects of the operating conditions on the distribution of both cluster solids holdup and cluster time fraction is similar.These two parameters increase with the decrease of U_g and/or the increase of G_s.For cluster frequency,it increases with the increase of the U_g.It shows that the cluster frequency increase with the increase of G_s,when G_s is small(G_s?300kg/m~2s).When G_s is up to 500 kg/m~2s,cluster frequency decreases with the increase of G_sbecause of the intensified solids-solids interactions in-between the cluster.The major dynamic parameters of the cluster are cluster velocity,the chord length calculated based on the duration time and cluster velocity.Cluster velocity ranges from3.6?7.5 m/s which is relatively larger than the averaged particle velocity.Effects of the operating conditions on the cluster velocity is similar to that on the averaged solids holdup.The chord length of the cluster is from 1.6?3.3 mm and increases with decrease of U_g and/or the increase of G_s.For low density operating conditions,cluster chord length changes dramatically from 1.6?3.3 mm.It focuses on 2.5?2.9 mm with high density operating conditions.For cluster duration time,it is from 0.30 ms to 0.98 ms and decreases with the increase of U_g.For the effect of G_s on cluster duration time,it decreases with the increase of the G_s when U_g equals to 1 m/s.With high U_g up to 3 m/s,the cluster duration time increases when solids circulation rate increases from 500 kg/m~2s to 700 kg/m~2s.