Moisture Distribution in Fluidized Beds with Liquid Injection

The effect of the electrostatic charge of particles was investigated in fluidized beds with conductive and nonconductive walls. For this purpose, changes in the wall surface charge density and particle contact time near the wall were examined. Computational fluid dynamics/discrete element method (CFD‐DEM) was utilized to simulate the accumulation of particle and wall‐induced electrostatic charges in a quasi‐two‐dimensional fluidized bed. Particles and walls were initially considered neutral and gained charge with time. The simulations showed that aggregated particles are formed due to the presence of the opposite electrostatic charges. Agglomerates are produced by joining the aggregated particles together. Agglomerates have a layered‐like structure and are made of positive and negative particles. Accumulation of these agglomerates and aggregated particles on the wall forms wall sheeting, which has a layered‐like structure. In other words, particles do not accumulate on the wall individually. A significant accumulation of particles near the wall was seen in the presence of electrostatic charges compared with the neutral bed. Also, the contact time of particles on the wall in the conductive case was more prolonged than in the nonconductive case. The results indicate that the CFD‐DEM approach is a helpful technique for predicting particles behaviour in the presence of the electrostatic charge.

Computational fluid dynamics (CFD) combined with the discrete element method (DEM) are powerful tools for analyzing dense gas–solid flows. However, the computational cost of CFD–DEM will be unfeasibly great when simulating large-scale engineering applications with billions of particles. Accordingly, the coarse-grained (CG) CFD–DEM method is applied to solve this problem. This investigated method replaces several smaller particles with larger ones called parcels, aiming to reduce the number of particles and fully consider the collision of particles between composition parcels and the collision of particles within composition parcels. First, high-speed photography verifies the numerical simulation's reliability. Then, the CG CFD–DEM was used to analyze the transient spatial distribution, transient average velocity, pressure drop, bed height, and the mixing state of particles in a dense gas–solid fluidized bed. The CG CFD–DEM was also compared with the CFD–DEM results, which showed a good agreement with the calculation results and proved the accuracy and applicability of the method. Finally, the computation time of the CG CFD–DEM was evaluated, showing a significant decrease in computation time with an increasing coarse ratio (k). This investigation can provide theoretical reference for the numerical simulation of the CG CFD–DEM method in dense gas–solid flow.

Comprehensive investigation and comparison of TFM, DenseDPM and CFD-DEM for dense fluidized beds

The combined discrete element method/computational fluid dynamics (DEM/CFD) approach allows for the description of reacting granular assemblies passed by a gas flow. Especially for thermally thick particles, the spatial resolution of the solid object volumes, their surfaces, and of the interstitial flow domain controls the quality of results while at the same time driving computational costs. To evaluate the differences between resolved and unresolved DEM/CFD approaches, pyrolysis of a bulk of spherical wood particles enclosed by a cylindrical heating surface and passed by hot nitrogen is numerically examined. The unresolved simulation is based on the averaged volume method (AVM). For the resolved simulation, the so‐called blocked‐off (BO) method is applied. The results show that the total mass conversion rate of solid particles into gaseous volatiles is faster when employing the resolved BO approach. The better spatial resolution of local flow field and particle surface representation leads to a more detailed prediction of convective and radiative heat transfer to the particles, but is associated with the penalty of a six‐fold computing time.

Bubbles as mesoscale structures play an important role in the development of the inhomogeneous drag force model, which is key to the prediction of fluidized beds. A coupled computational fluid dynamics–discrete element method is employed in this work to investigate the distribution of interphase drag force around a clouded bubble in an incipiently fluidized bed. A method for the determination of the bubble boundary is proposed. The results reveal that there is a stronger positive effect of drag force around the clouded bubble because of the presence of bubble crown and wake, which is different from the cloudless bubble. The bubble-induced local inhomogeneous coefficient is also proposed to evaluate the ensemble inhomogeneous impact around the bubble.

Fluidization is a well-established technique for processing particulate materials, characterized by intensive interactions between solid particles and the fluid flow. Among the various design parameters of fluidized bed dryers, bed height plays a critical role, directly influencing flow resistance, heat and mass transfer, and consequently, the system’s energy efficiency and overall performance. A thorough understanding of the mechanisms governing fluidized bed dynamics is therefore essential for process optimization. In this study, a three-dimensional gas-solid fluidized bed was simulated using a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) framework. This approach enables precise tracking of particle motion and distribution, allowing the temporal evolution of bed height to be quantified. Simultaneously, flow characteristics such as gas velocity, pressure, and particle distribution were analyzed under various operating conditions. Before performing a sensitive simulation study, the model was validated against data from previously published studies, showing strong agreement with established empirical correlations and confirming the reliability of the numerical framework. Building on this validation, a parametric study was conducted to examine the effects of key operating variables - namely, inlet gas velocity and bed height - on the hydrodynamic behavior of the system. The obtained results demonstrate that these parameters have a significant impact on flow uniformity and overall bed stability.

Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD

A novel particle-resolved direct numerical simulations (PR-DNS) method for non-spherical particles is developed and validated in the open-source MFiX (Multi-phase Flow with Interphase eXchanges) code for simulating the suspension of non-spherical particles and fluidisation. The model is implemented by coupling superquadric Discrete Element Method-Computational Fluid Dynamics (DEM-CFD) with the immersed boundary method. The model was first validated by applying it to analyse fluid dynamic coefficients ( $C_{\!D} , C_{\!L} , C_{\!T}$ ) of superellipsoids and cylinders at different Reynolds numbers, and the PR-DNS results closely matched those of previous methods, demonstrating the reliability of the current PR-DNS approach. Then, the model was applied to the simulation of the fluidisation of spheres and cylinders. The PR-DNS results were compared with both particle-unresolved superquadric DEM-CFD simulation and experimental data. The pressure drop, height distribution and orientation distribution of particles were analysed. The results show that the PR-DNS method provides a reliable method for reproducing fluidisation experimental results of non-spherical particles. In addition, the comparison of the drag correction coefficients predicted by existing models with that obtained from PR-DNS results indicates the need for a new drag model for particle-unresolved simulation of non-spherical particles.

CFD-DEM simulation of fluorination reaction in fluidized beds with local grid and time refinement method

Acetylene hydrogenation reactions (AHR) are typically carried out in fixed bed reactors (FBRs) with high diameter‐to‐particle diameter (D/d). In this study, first, a real catalyst bed was built using the discrete element method, and the suitability of the bed was assessed using empirical equations. Second, a discrete element method‐computational fluid dynamics (DEM‐CFD) model was developed using CFD in conjunction with the reaction mechanism to simulate the AHR in the FBR and was compared with the experimental results. The results show that the DEM‐CFD can well predict the 3D interactions of velocity, temperature, and species for different D/d. The results can provide favorable theoretical guidance for the optimization of FBR and process intensification.

An augmented coarse-grained CFD-DEM approach for simulation of fluidized beds

Super-quadric CFD-DEM simulation of chip-like particles flow in a fluidized bed

This paper discusses the thermal and exergy efficiency analysis of the hydrothermal liquefaction (HTL) process, which converts sewage sludge into biocrude oil in a continuous plug–flow reactor using a linear Fresnel solar collector. The investigation focuses on the influence of key operational parameters, including slurry flow rate, temperature, pressure, residence time, and the external heat transfer coefficient, on the overall efficiency of biocrude oil production. A detailed thermodynamic evaluation was conducted using process simulation principles and a kinetic model to assess mass and energy balances within the HTL reaction, considering heat and mass momentum exchange in a multiphase system using UDF. The reactor’s receiver, a copper absorber tube, has a total length of 20 m and is designed in a coiled configuration from the base to enhance heat absorption efficiency. To optimize the thermal performance of biomass conversion in the HTL process, a Computational Fluid Dynamics–Discrete Element Method (CFD-DEM) coupling numerical method approach was employed to investigate improved thermal performance by obtaining a heat source solely through solar energy. This numerical modeling approach allows for an in-depth assessment of heat transfer mechanisms and fluid-particle interactions, ensuring efficient energy utilization and sustainable process development. The findings contribute to advancing solar-driven HTL technologies by maximizing thermal efficiency and minimizing external energy requirements.

CFD–DEM simulation of a pseudo-two-dimensional spouted bed comprising coarse particles

To investigate the energy dissipation mechanisms within the pump and improve the computational accuracy of the solid–liquid flow numerical simulations, in this study, an improved CFD-DEM (Computational Fluid Dynamics - Discrete Element Method) method has been presented. First, the improved method of CFD-DEM is introduced, which mainly considers the turbulent dissipation of particles in the near-wall region and velocity field reconstruction. Then, the simulation results before and after the method's enhancement are compared. Finally, the analysis of the energy characteristics of the liquid phase flow field in the solid–liquid flow is conducted. Research shows that the modified CFD-DEM method significantly improves the accuracy of the particle distribution predictions, with the numerical results for head and efficiency being much closer to experimental values. In the high-speed regions of the impeller flow field, primarily located behind the pressure side of the blades, the liquid phase flow velocity and pressure fluctuations are less affected by changes in solid phase concentration. In the fluid region of the centrifugal pump, the energy loss caused by entropy production is significantly concentrated in the volute and impeller regions. Specifically, the entropy production dissipation in the volute region accounts for the substantial portion of the total entropy production, approximately 67%–68%, while the entropy production dissipation in the impeller region accounts for about 19.7%–20.4%. As the solid phase concentration increases, the energy dissipation within the pump gradually rises, and the total vorticity at the impeller inlet also increases correspondingly, with the vorticity distribution being related to the number of blades. The findings provide a reference for further exploring solid–liquid flow within centrifugal pumps.