Experimental and Numerical Investigations into the Charge Distribution and Particle Velocity Characteristics in the Freeboard of a Bubbling Fluidized Bed
Due to the intensive and frequent collisions and interactions between particles with each other and with the wall, tribo-charging is inevitable in the fluidization process. The electrostatic charge on the particles can influence the particle flow characteristics in the fluidized bed, resulting in problems such as agglomeration, wall fouling and even electric discharge. It is therefore necessary to investigate the electrostatic characteristics and the interplay between electrostatics and hydrodynamics in the fluidized bed. Several methods have been proposed to characterize the electrostatics in the fluidized bed such as Faraday cups and electrostatic probes. The parameters of local charge levels, charge spatial distributions and particle charge density are determined in these methods. In addition, particle velocity characteristics are also determined using several methods, including optical probes, particle image velocimetry and radioactive particle tracking. However, none of the techniques is capable of measuring both the charge distribution and the particle velocity characteristics. More work is required to achieve an in-depth understanding of the relationship between the electrostatics and the particle flow dynamics in the fluidized bed. In this study, experimental and numerical investigations into the charge distribution and particle flow characteristics are conducted. Wire-mesh electrostatic sensor arrays are installed in the freeboard of a bubbling fluidized bed. The charged particles in the sensing volume of the sensor arrays generate induced charge on the electrodes. The induced charge on the electrodes is measured and used to reconstruct the cross-sectional charge distribution in the freeboard. Meanwhile, the axial and lateral velocities of particles are estimated by cross-correlating the signals from the corresponding electrodes. To study the electrostatic charging effect, numerical simulations of the bubbling fluidized bed are conducted using coupled computational fluid dynamics (CFD) and discrete element method (DEM). Charge on particles is computed from an established triboelectric model. The particle behavior in the freeboard region is numerically tracked using the DEM method. With the available experimental and numerical results on charge distribution and particle velocity characteristics, the electrostatic properties in the freeboard region and their relationship with the particle hydrodynamics under different superficial air velocities are thoroughly investigated.