Experimental and Numerical Investigation of the Motion of an Object Immersed in a Fluidized Bed of Fine Particles

Characterizing the motion of a very large particle, called object, in a fluidized bed of fine particles is essential to understand the hydrodynamic phenomena occurring in gas-solid fluidized beds involving biomass, solid waste, or agglomerates. To this end, the position of a single spherical polymeric object immersed in a cylindrical fluidized bed of fine and coarse sand was tracked for a few hours by the radioactive particle tracking (RPT) technique. For each bed material, the experiments were carried out at two excess gas velocities, i.e. U_e = 0.25 m/s and U_e = 0.50 m/s. The analysis of the RPT data showed that the object rise takes place principally in the drift of the bubbles. Moreover, the mean object rise velocity to the mean bubble velocity was constant (~ 0.22), regardless of the fluidization medium and the tested excess gas velocity. It was also noticed that the residence time of the object in the emulsion phase depends on the fluidization medium and this parameter influences the object rise frequency. To understand the physical phenomena involved at meso-scale and to guide/validate the mathematical modeling, the experimental cases were simulated by an Euler-nfluid approach based on the kinetic theory of granular flow representing both gas and sand phases. To track accurately the motion of the object, a discrete forcing method based on a porous medium approach was proposed which could strictly ensure the conservation laws at the close vicinity of the interface. The interface is approximated as a plane in each cut-cell. The domain contains the object structure, which is considered a real part of the calculation domain. The use of an immersed boundary method to perform fluid-structure interaction is powerful as it allows large or slight displacement without any dependency on the mesh.