Spatially-resolved characterization of the flow structure in a bubbling fluidized bed with solids crossflow

The aim of this study is to characterize the flow structure in a bubbling fluidized bed with a horizontal solids crossflow. Such configurations are integral to the processes of chemical looping combustion, indirect gasification, drying, and coating, where solids mixing and precise control of the solids residence time distribution are critical for maximizing reactor efficiency and/or product quality. This work combines Eulerian-Eulerian computational fluid dynamics (CFD) modeling with experiments in a fluid-dynamically down-scaled cold-flow model. The experiments provide a physical basis for assessing the reliability of the numerical predictions, while the CFD analysis delivers detailed spatial and temporal characterization of flow phenomena not directly accessible via measurement. The closed-loop unit incorporates a module that induces convection of the solids crossflow along a horizontal transport channel, which has an up-scaled cross-sectional width of 0.92 m and a loop length of 10.35 m. The unit is designed and operated according to Glicksman's simplified scaling laws to replicate industrial conditions for thermochemical processes that involve bubbling fluidization, specifically, a bed of sand-like particles (density, 2650 kg/m 3 ; mean diameter, 950 μm) operated at 800 °C. In this setup, two operational parameters are varied: the bed height (0.67–0.83 m at settled state); and the solids crossflow rate (yielding mean solids velocities in the range of 0–0.15 m/s). The CFD model is validated against measurements of the bed voidage, solids velocity, and solids dispersion coefficient. Thereafter, the simulated data are analyzed along the horizontal flow direction, so as to provide high-resolution insights into the local distributions of convective (solids velocity) and dispersive (turbulent kinetic energy) solids transport. The results indicate that at low crossflow rates (yielding Pe ≈ 0.03–0.64), the solids flow is dominated by the formation of counter-rotating vortical structures, resulting in substantial regions within which solids move in the direction opposite to the crossflow. At high crossflow rates (yielding Pe ≈ 1.29–4.52), the vortical flow structures are largely disrupted, resulting in flow patterns in which the particle trajectories are more elongated and less streamlined, with only a small fraction of the solids exhibiting reverse flow. Importantly, macroscopic dispersion increases due to the longer and less-streamlined paths that the particles follow. Analysis of dispersion at the micro-scale reveals that the turbulent component, driven by bubble and cluster motions, is two orders of magnitude greater than the laminar counterpart associated with random particle motion and collisions. Vertical dispersion peaks at mid-height in the dense bed, whereas horizontal dispersion is highest near the gas distributor and at the bed surface. These observations indicate that bubble and wake formation, as well as bubble bursting, are the primary mechanisms driving the horizontal mixing of solids in bubbling fluidized beds. • Eulerian–Eulerian CFD approach is used to study solids transport mechanisms in bubbling beds with crossflow. • Low crossflow produces stable, counter-rotating vortex structures in the bed. • High crossflow disrupts vortex coherence, raising macro-dispersion of solids. • Increased bed height produces larger vortex macrostructures and higher solids dispersion. • Bubble- and cluster-induced mixing is two orders of magnitude higher than random motion.