Frictional losses in a bubbling fluidized bed with horizontal flow of solids

Abstract

This study investigates the frictional losses in a horizontal flow of solids fluidized under bubbling conditions, relevant to applications like combustion, gasification, drying, and waste incineration. Experiments were conducted in a fluid-dynamically down-scaled setup simulating a bed of Geldart type B solids, fluidized under typical industrial conditions for thermochemical conversion processes, corresponding to sand (particle density: 2650 kg/m³; diameter: 950 µm) fluidized with air/flue gas at 900 °C. The rig features a bubbling bed with a closed horizontal loop for controlled solids circulation, equipped with in-bed pressure probes to measure horizontal pressure drop. Horizontal solids velocity was evaluated by performing magnetic solids tracing experiments. Key parameters varied included solids velocity (0–0.10 m/s), channel width (0.58–1.0 m), and settled bed height (0.67–0.83 m).

The results clearly indicate a horizontal pressure gradient (15–485 Pa/m), which is proportional to the solids' mean velocity (0–0.101 m/s in upscaled terms). An inverse relationship between the pressure gradient and channel width was also identified. Rheological analysis indicates shear-thinning behavior, with wall shear stress ranging from 10 to 140 Pa for shear rates of 2 × 10⁻³–0.45 s⁻¹ (on an up-scaled basis). Existing models for non-Newtonian flow were found to underestimate the impact of geometric parameters. An alternative correlation is proposed, and friction coefficients are calculated. Analysis of friction coefficients against the Reynolds number confirms laminar flow. Additionally, a strong positive correlation between the generalized Reynolds and Péclet numbers highlights the impact of viscous forces in solids mixing. Lastly, the friction factor analysis, based on granular flow rheology, indicates that friction dynamics occur within the dense flow regime.