Flow Structure In Fluidized Bed Research Articles

Insights into gas-solid flow structures in fluidized beds up to 1600 °C via analysis of gas residence time distributions

This study investigates gas residence time distributions (RTDs) in a fluidized bed of Al₂O₃ particles at temperatures ranging from ambient to 1600 °C, focusing on the impact of temperature on gas back-mixing and flow patterns. We find that at low temperatures (up to 300 °C), gas flows in a near-plug flow pattern, represented by narrow RTD curves with early peaks. As temperature increases to 600 °C, the RTD curves broaden, featuring delayed peaks due to bubble breakup and enhanced gas drag. Between 800 °C and 1200 °C, significant deviations from plug flow emerge, driven by stronger interparticle forces and increased gas transfer to the emulsion phase. Interestingly, beyond 1200 °C, the RTD curves shift back toward plug flow, influenced by intensified interparticle forces and physicochemical changes in the bed materials.

Micromechanical analysis of flow behaviour of fine ellipsoids in gas fluidization

The macro-scale flow behaviour of granular materials in gas fluidization is governed by particle-particle and gas-particle interactions, which are affected significantly by particle size and shape. Understanding the micro-scale flow structure of fine non-spherical particles is essential for process design, optimisation and control. In this work, the combined approach of computational fluid dynamics for gas phase and discrete element method for particles is used to study flow and force structures of fine ellipsoids in gas fluidization. The results reveal that fine particles have vortex flow structure in fluidized beds, and the vortex flow becomes more significant particularly for oblate particles. The microscopic structure analysis demonstrates that when aspect ratio deviates from 1.0, the packed beds could experience compaction, and in expanded beds, the bed expansion ratio increases. The effect of particle size is investigated, showing that with the decrease of particle size, the mean coordination number decreases due to more significant role of van der Waals force. Ellipsoids exhibit higher orientation order which varies greatly with gas velocity and particle size. In fluidized beds, ellipsoids tend to flow in small projected area to the fluid flow direction to reduce the flow resistance. The bed expansion criteria established in the literature are confirmed still valid for ellipsoids, but the gas velocity range for expanded bed interval is much wider than spheres.

Multi-scale analysis of flow structures in fluidized beds with immersed tubes

Multi-scale analysis and non-linear analysis were combined to investigate the hydrodynamics of fluidized beds with and without horizontal tubes. Pressure fluctuations were measured and analyzed employing discrete wavelet analysis, recurrence plot analysis, and recurrence quantification analysis. A systematic procedure was followed to determine wavelet parameters. At low gas velocities, the energy of macro-structures reduces with the addition of the first tube and then increases with the addition of a second tube. However, there is no notable difference at high gas velocities. Determinism is high for the bed without tubes, which can be attributed to the periodic behavior of bubbles. Determinism decreases with the addition of tubes because the breakage of bubbles results in less periodic behavior. The three methods of analysis used in this study captured the effects of immersed tubes on the hydrodynamics of fluidized beds. Recurrence quantitative analysis was found to be a powerful and easy-to-use method that can capture the nonlinear characteristics of fluidized beds much more quickly than conventional methods of nonlinear analysis. This method can thus be effectively used for the online monitoring of hydrodynamic changes in fluidized beds.

Ultrafast X-ray computed tomography for the analysis of gas–solid fluidized beds

Gas–particle flow in fluidized beds is generally complex and difficult to observe. But exact information on voidage distribution and solid transport is urgently needed for assessment, monitoring, modelling, and optimization of fluidized bed operation. So far, there was a lack of suitable measurement and imaging techniques to disclose the complex flow structures in fluidized beds with high spatial and temporal resolution. The ultrafast X-ray computed tomography technique, which has been developed in recent years, is superior for such types of multiphase flows and its performance for the analysis of fluidized beds has been demonstrated in this study. Spatial resolution of two millimetres and temporal resolution of several thousand cross-sectional images per second allow at the same time imaging and analysis of voidage structures as well as single particle movement. In this study, the capability of imaging fluidized bed behaviour at different column diameters and gas flow rates has been analysed.

Impact of solid sizes on flow structure and particle motions in bubbling fluidization

Knowledge of solid motions and flow structures in fluidized beds is of significant importance to a number of industrial processes, such as combustion, gasification of solid fuels, drying of particulate materials, oxidation or reduction of ores, and catalytic and thermal cracking. Many parameters, such as pressure drop, bed geometry, solid size and density, can affect the solid flow structure in a fluidized bed. In this study, experiments were designed to investigate the impact of solid size. Through PEPT studies, we found that the solid flow structure and the bubble pattern in a fluidized bed with an inner diameter of 150 mm vary significantly with solid particle size. Three flow structures have been found. For glass beads with a large size (> 700 μm), a single large circulation cell is observed within the whole bed, and particles move upwards at one side of the bed to the splash zone, and then return to the bed bottom along the opposite side of the bed. When the particle size is in the range 250–450 μm, particles move upwards across the whole area of the bed at relatively uniform velocity in a layer 30 mm deep immediately above the air distributor. Above this layer, solids move inwards and travel upwards in the centre of the bed to the splash zone, and then return to the bottom of the bed in an outer annulus. When the particle size is in the range 80–200 μm, the fluidized bed can be divided into three sections. In the bottom section, solids travel upwards in the outer annulus, and move down in the bed centre. In the top section, solids travel upwards at the centre of the bed to the splash zone and then return to the intermediate height of the bed via the outer annulus. In the intermediate section of the bed (60–100 mm above the distributor), the annular upward solid flow from the bottom section encounters the annular downward flow from the top section. The two solid flows merge and change direction towards the bed centre where the particles are mixed and redistributed to the circulation cells in the upper and lower sections. The bubbling pattern also varies with the particle size. The bubble size and their rising velocity decrease with decreasing of the particle size.

INSTABILITIES IN FLUIDIZED BEDS

▪ Abstract This paper reviews recent advances in our understanding of the origin and hierarchy of organized flow structures in fluidized beds, distinction between bubbling and nonbubbling systems, and stages of bubble evolution. Experimental data and theory suggest that, at high particle concentrations, the particle-phase pressure arising from flow-induced velocity fluctuations decreases with increasing concentration of particles. This, in turn, implies that nonhydrodynamic stresses must be present to impart stability to a uniformly fluidized bed at very high particle concentrations. There is ample evidence to support an argument that, in commonly encountered gas-fluidized beds, yield stresses associated with enduring particle networks are present in the window of stable bed expansion, where the particles are essentially immobile until bubbling commences. However, some recent data on gas-fluidized beds of agglomerates of cohesive particles suggest that there exists a window of bed expansion where the bed does manifest a smooth appearance to the naked eye and the particles are mobile; at higher gas velocities the bed bubbles visibly. The mechanics of such beds remain to be fully explained.