Horizontal secondary gas injection in fluidized beds: Solid concentration and velocity in multiphase jets

Abstract

High speed gas injection through nozzles is used in fluidized bed technology for controlling particle size or adding a reactant. Experiments were carried out to investigate the flow field of glass beads (x1,2=92μm) in multiphase jets (with respect to the applied pressure). Particle image velocimetry (PIV) was used in a semicircular fluidized bed with optical access, to analyze the particle movement. It was found that higher gas pressure at the nozzle inlet initially leads first to higher particle velocities but decreases the particle degree of mixing. In subsonic conditions the trend of the measured data could be predicted by a simple force balance model. Additionally, the transition from subsonic to choked flow conditions inside the nozzle could be distinguished by analyzing the jet opening angle. The fluidization velocity showed no significant influence in these investigations. It was also proven by solid concentration (1−ε) measurements with capacitance probes: Increasing kinetic gas energy at the nozzle inlet leads to lower solid concentration at the jet axis. At very high gas pressures there were almost no particles in the jet. Finally, a relatively new measurement technique was used for flow analysis. A fast gantry X-ray CT was used to analyze the turbulent flow without disturbing it. Thanks to a calibration the solid distribution could be made visible. This showed an entrainment zone close to the nozzle exit. The results showed that the kinetic energy of solids can be increased by applying higher gas velocities. However, very high gas velocities lead to reduction of solid entrainment into the jet.