(254e) CFD-DEM simulation of the onset of fluidization for large particles in a bed of fine sand

Due to the unique advantages of fluidized beds for solid mixing and heat transfer, they are widely used for the thermal processing of solid fuel particles. In practice, such systems are composed of non-conventional fuel particles such as coal or biomass that are completely different in shape, size, and density from the bed material such as sand. In such cases, correct understanding of the main fluidization parameters such as the minimum fluidization velocity (U_mf) of the bed components plays a key role in the successful design and control of the systems. However, the onset of fluidization of irregular particles is not clear as it may vary depending on the physical properties of the particles, the initial mixing conditions of the conventional and irregular particles, and/or the fraction of the non-conventional particles in the bed.

In this study, we aim at predicting the U_mf of large and light biomass particles (objects) mixed with fine sand particles (fines), with the help of coupling the computational fluid dynamics (CFD) and the discrete element method (DEM). To this end, fine particles were primarily simulated using the coarse-grained CFD-DEM, in which a group of fine particles was lumped into a larger numerical parcel. In the conventional CFD-DEM, the CFD cell size is usually 3-4 times larger than the bed particles. This ratio cannot be held when there is a substantial size difference between the bed components as it significantly increases the computational error. To address this issue, we adopted the diffusion-based averaging method and mapped the solid concentration and solid velocity of the object into the CFD cells. The capability of the adopted numerical approach for predicting the motion of the object in a bed of fine particles was verified by comparing the upward and downward velocities of the object with the corresponding experimental data. Further simulations are in progress to predict the U_mf of the objects varying in size and density in a bed of sand particles.