Numerical study of hydrodynamics in gas-solid reactors operating within bubbling fluidisation regime

Numerical simulation of flow behavior of particles in a gas-solid stirred fluidized bed

Disengagement of dispersed cerium oxalate from nitric-oxalic acid medium in a batch settler: Measurements and CFD simulations

Multiphase modeling of settling and suspension in anaerobic digester

Effects of fluidization velocity on solid stack volume in a bubbling fluidized-bed with nozzle-type distributor

Parametric study of particles homogenization in cold-flow riser reactors

Based on the two-fluid model, a three-zone drag model was developed, and the kinetic theory of granular flows and the Schneiderbauer solids wall boundary model were modified to establish a new three-dimensional (3D) unsteady mathematical model for high-pressure dense-phase pneumatic conveying in horizontal pipe. With this mathematical model, the influence of the three frictional stress models, namely Dartevelle frictional stress model, Srivastava and Sundaresan frictional stress model, and the modified Berzi frictional stress model, on the simulation result was explored. The simulation results showed that the three frictional stress models accurately predicted the pressure drop and its variations with supplementary gas in the horizontal pipe, with relative errors ranging from −4.91% to +7.60%. Moreover, the predicted solids volume fraction distribution in the cross-section of the horizontal pipe using these frictional stress models exhibited good agreement with the electrical capacitance tomography (ECT) images. Notably, the influence of the three frictional stress models on the simulation results was predominantly observed in the transition region and deposited region. In the deposited region, stronger frictional stress resulting in lower solids volume fraction and a higher pressure drop in the horizontal pipe were observed. Among the three frictional stress models, the simulation results with the modified Berzi frictional stress model aligned better with the experimental data. Therefore, the modified Berzi frictional stress model is deemed more suitable for simulating high-pressure dense-phase pneumatic conveying in horizontal pipe.

Solid volume fraction is a defining characteristic of multiphase gas–solid flows. In the case of gravity-driven particle curtains, it changes significantly along the flow direction. This has implications for the performance of falling-particle solar receivers, which use particle curtains to capture and store heat from concentrated sunlight. This study reports on the characterization of solid volume fraction as a function of height in gravity-driven particle curtains using optical techniques and simulations performed with a discrete element method (DEM) model. The experimental domain is a hopper-fed vertical channel, and the modeling domain is designed to match the experimental setup. Solid volume fraction was obtained from experimentally measured transmittance using two techniques: (1) a thermopile detector paired with a visible light source and (2) a high-speed camera. The DEM simulations were calibrated by identifying the coefficient of static friction between particles that resulted in equivalent mass flow rates to the experiments. All techniques, including an analytical model derived from first principles and empirical relationships, showed very similar trends of solid volume fraction decreasing along the flow direction, where it drops off rapidly initially before leveling off in a power law relationship. Both experimental and computational tests were performed with 1 mm and 2 mm particles, and solid volume fraction is found to decrease with increasing particle size at any location in the channel. Furthermore, the solid volume fractions calculated from transmittance measurements were found to be sensitive to the optical properties of the particles through Monte Carlo ray tracing simulations. To account for this effect, a new scale factor dependent on particle reflectivity is derived, which leads to significantly improved agreement between experimental and simulated results. A simple, closed-form equation is proposed to transform measured transmittance to a solid volume fraction while accounting for particle reflectivities. Even though solid volume fractions above 0.1 were not measured in this study, the technique developed is suitable to measure even larger solid volume fractions, especially for more reflective particles.

CFD modeling of heat transfer and hydrodynamics in a draft tube conical spouted bed reactor under pyrolysis conditions: Impact of wall boundary condition

Modification of kinetic theory of granular flow for frictional spheres, part II: Model validation

Numerical simulation of gas–solid flow with two fluid model in a spouted-fluid bed

The granular pressure and granular temperature underpin various models of granular flows while they are playing an increasing role in modeling of other phenomena in granular systems such as heat transfer, segregation, erosion, attrition, and aggregation. The development and validation of these theories demand experimental determination of these two quantities. Diffusing wave spectroscopy (DWS) is now an accepted technique for measurement of granular temperature in dense granular systems. Using granular temperature data obtained from DWS with the kinetic theory of granular flow, we have derived the granular pressure data for a liquid‐fluidized bed. The determined variation of the mean bed granular pressure with mean bed solid volume fraction compares favorably with previously published experimental data and theoretical models of others. Where discrepancies do occur, they may be attributed to differences in particle inertia, suggesting further work on granular pressure models is required. Finally, we report the variation of the granular pressure with height above the distributor for several mean solids volume fractions. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Slush nitrogen is a mixture of solid nitrogen particles and liquid nitrogen, and its flow characteristics in a horizontal pipe are investigated experimentally and theoretically in this study. Pressure drop of slush nitrogen is higher than that of subcooled liquid nitrogen due to the viscous and the mechanical frictions, and the dependence of pressure drop on the mean velocity varies under different flow conditions. The solid volume fraction distributions and the velocity throughout the pipe cross‐section of slush nitrogen flow are investigated with the Eulerian–Eulerian multiphase approach incorporated with the kinetic theory of granular flow. The flow patterns of slush nitrogen are determined as the pseudo‐homogeneous flow, the heterogeneous flow and the bedload flow from the experimental and numerical results. The relationship between the friction factor and the Reynolds number for slush nitrogen with various solid volume fractions is obtained by using the slush Reynolds number. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1762–1773, 2013

A computational fluid dynamics model based on the Eulerian–Eulerian multiphase framework was implemented to simulate the coal gasification processes in a circulating fluidized bed gasifier. The standard k–ε turbulence model is used for gas phase, and the solid phases are modeled with kinetic theory of granular flow. A lumped chemical reaction model was compiled based on the chemical percolation devolatilization model and the METC Gasifier Advanced Simulation gasification model. The user‐defined functions are developed to calculate rates of chemical reactions and to recirculate solids from the riser exit to the solid recycle inlet. Sensitivity analysis was performed on char–steam gasification reaction to choose an appropriate kinetic rate in this study. The calculated gas composition out of the riser was first compared with experimental data; then, the flow fields such as solid volume fraction, solid velocity, and species profiles were analyzed, indicating that the model gave reasonable predictions. Furthermore, the effects of the operating temperature, the steam/coal ratio, and the air/coal ratio on the exiting gas composition were numerically studied, and the predicted trends vs these parameters are reasonable and consistent with the findings in the literature, which shows the applicability of the Eulerian multiphase model to simulate coal gasification processes in circulating fluidized bed gasifiers. © 2015 Curtin University of Technology and John Wiley & Sons, Ltd.

Computational fluid dynamics modeling of slurry flow in horizontal pipes: Effect of specularity coefficient on hydraulic gradient

In dense gas–solid fluidized beds the heat transfer to immersed objects is strongly coupled to the hydrodynamic behavior. The objective of this study is to experimentally and numerically assess the heat transfer coefficient around a horizontal tube in a Geldart B bubbling fluidized bed and derive a numerical-correlative approach for predicting the angular dependent heat transfer coefficient. The considered system consists of corundum as the solid bed material and air as the fluidization gas, entering the cylindrical geometry through a Tuyere nozzle distributor. Experimental data are obtained from a pilot scale test-rig with different tubular heat transfer probes and evaluated in a comprehensive uncertainty analysis. The resulting magnitude and angle dependent variations of the heat transfer coefficient at different superficial gas velocities are compared to three dimensional numerical simulations. The applied CFD model of the fluidized bed treats both gas and powder as Eulerian phases. The size distribution of the particles is described by two granular phases with corresponding mean diameters and a sphericity factor to account for their non-spherical shape. The fluid–solid interactions in this Multi Fluid Model are considered by incorporating the Kinetic Theory of Granular Flow and a sphericity-adapted drag model. The hydrodynamics at the tube surface, e.g. solid volume fraction, gas and particle velocities, are used for a novel numerical-correlative calculation of the angle dependent heat transfer coefficient between the bed material and the immersed tube. Special focus is set on depicting the particle contact time at the tube surface appropriately. The numerical results show the correct tendency of an increasing heat transfer coefficient with rising gas velocity and are partially in good agreement with the experimental observations.