Hydrodynamics studies on macro- and micro-flow structure with effects of particle properties in a circulating fluidized bed

A systematic and comprehensive hydrodynamics study was conducted in a narrow rectangular circulating fluidized bed (2-D CFB) to investigate the effects of particle properties on the characterization of macroscopic and microscopic flow behaviors. The CFB system consisted of a 7.6 m riser with a 19 mm x 114 mm cross section and a downcomer with a large storage section at the top. FCC, glassbeads and sand particles were employed to study the effect of particle density, size and sphericity. Differential pressure measurement, optical fiber probe and visualization technique were applied. Experimental results were examined focusing on the solids distribution for the different types of particles. The particle aggregation was also investigated involving the mechanisms of cluster formation and characteristics of particle aggregates.;The characteristics of microscopic flow were studied with the same types of particles as for the macroscopic flow study in the CFB riser under various operating conditions. A new digital imaging system consisting of a high-speed video camera, light source and image process programs was designed and developed to enable the high density fast flow to be visualized directly, particularly focusing on the particle aggregations. The particle aggregates were characterized by analyzing the images frame by frame and were classified into different form types. A new model was proposed to predict the form of cluster in the fully developed region. The instantaneous solids concentration data obtained by the optical fiber probe were also investigated to identify particle aggregates and characterize the cluster properties. The effects of particle properties on the characteristics of clusters, including the cluster forms, number fraction, time fraction, cluster frequency and mean existence time, were studied. A new method was proposed and realized the measurements of cluster velocity and size with the solids concentration signals detected by the optical probes. The results indicated the smaller and lighter particles have higher tendency to aggregate than the larger and heavier particles. The particle properties were also considered as factors influencing the cluster velocity and size.;This research combined the intrusive and non-intrusive measurements to study the macro- and micro-flow structure in the CFB riser. The obtained results agreed well with others derived from different measurement techniques. Furthermore, to a great extent, the studies of the two scopes are supportive for each other.;Keywords: Rectangular Circulating Fluidized Bed, Macroflow, Microflow, Particle properties, Visualization, Optical fiber probe, Solids concentration, Particle velocity, Particle aggregation, Cluster velocity, Cluster size;The macroscopic flow structure in the CFB riser was studied over a wide range of operating conditions with five types of particles. Extensive experiments were carried out using an optical fiber probe system, which can measure the solids concentration and particle velocity simultaneously. The results showed the lateral and axial profiles of solids concentration and corresponding particle velocity were influenced by the particle properties as well as operating conditions. The influences of particle density, size and sphericity on the solids concentration and particle velocity were studied in detail. It was found that the heavier particles had denser solids distribution and lower particle velocity profiles laterally and axially. The larger particle size led to a higher solids concentration and slower particle acceleration as well as a lower particle velocity along the entire riser. The particle sphericity influenced the solids distribution by increasing the solids concentration and decreasing the particle velocity with the increase of particle sphericity. The effects of riser geometry, distributor design and operating condition on the solids distribution were also studied. To achieve a good understanding of the mechanism for the particle properties to affect the flow, the slip velocity and the force balance for particles moving in gas was investigated. Correlations between the local particle velocity and solids concentration were proposed to work for each type of particles and to indicate the influence of particle properties on the solids distribution.