Cluster Velocity and Slip Velocity in the CFB Riser: Experiment and Simulation

Cluster is a common phenomenon in the circulating fluidized bed, which influences obviously the transfer efficiency of mass and heat. Experimental studying on the cluster velocity was carried out in a thin 2D fluidized bed and Lagrange approach was employed to computing the velocity of the clusters which captured by a high speed camera. The statistical analysis was carried out based on the moving direct of the clusters. The experimental results show that the cluster velocity can be accurately computed by the Lagrange method. The velocity of the upward clusters is slightly greater than that of the downward cluster. The cluster velocity increases with increasing the superficial gas velocity and most of cluster velocity ranges from -3 to 3 m/s. The number of the downward clusters is greater than that of the upward clusters in the peripheral region, while the upward clusters were slighter more than the downward cluster in the central region of the riser. The number of the downward cluster is about 60-70% of the total clusters. The x-direct component of cluster velocity is low, which is the larger in the central region and the smaller in the peripheral region of the riser. Comparison of cluster slip velocity (between cluster velocity and gas velocity) and particle slip velocity (between particle velocity and gas velocity) has been implemented with two-fluid model (TFM). The simulation results show that the cluster slip velocity is nearly equal to the particle slip velocity in the bottom region, whereas it is obviously larger than the particle slip velocity in the top region. The downward cluster slip velocity is greater than that of the upward cluster because of its inverse direct to the gas flow, which results in the shorter during time of the downward cluster.