Fluidized beds are essential reactors and dryers in many industries, prized for their ability to achieve high rates of heat and mass transfer and provide uniform temperature distributions. This is achieved by passing a fluid (usually a gas) upwards through a bed of solid particulates at a high enough velocity to cause the solid/fluid mixture to behave like a fluid. However, the hydrodynamics within these systems are incredibly complex, involving a dynamic interplay between the gas phase and the solid particles. Predicting and optimizing this multiphase flow is a significant engineering challenge. To truly capture the intricate particle dynamics, a simple Fluidized bed CFD simulation is insufficient. We need to account for particle-particle and particle-wall collisions, which dominate the behavior of the dense bed. This is where a coupled Fluidized bed DEM CFD approach becomes essential. This project presents a high-fidelity simulation using ANSYS Fluent to model the gas flow and ANSYS Rocky to model the granular particle dynamics via the Discrete Element Method (DEM). This powerful coupling allows for a complete simulation of all forces, providing insights to improve dryer and reactor performance. The methodology is benchmarked against established research, such as the work by Azmir, et al. [1] and He, et al. [2].

• Reference [1]: Azmir, Jannatul, Qinfu Hou, and Aibing Yu. “CFD-DEM simulation of drying of food grains with particle shrinkage.” Powder Technology343 (2019): 792-802.
• Reference [2]: He, Yi, Andrew E. Bayly, and Ali Hassanpour. “Coupling CFD-DEM with dynamic meshing: A new approach for fluid-structure interaction in particle-fluid flows.” Powder Technology325 (2018): 620-631.

Figure 1: Example of bubbling phenomena in a fluidized bed, showing particle velocity distribution from the reference paper [2].

Simulation Process: CFD-DEM Modeling – Coupling ANSYS Rocky and Fluent

The simulation process began with the creation of the cylindrical domain geometry and a fine, structured mesh using ICEM software. The core of this project lies in the advanced physics setup that leverages the seamless integration of ANSYS Fluent and ANSYS Rocky. This Fluidized bed DEM simulation is built on a sophisticated Eulerian-Lagrangian framework.

In this framework, ANSYS Fluent solves the continuum equations for the fluid phase (the Eulerian part), while the ANSYS Rocky solver tracks every single particle individually (the Lagrangian part). While Fluent has its own Discrete Phase Model (DPM), for dense particle systems like a fluidized bed, a dedicated DEM solver is required to accurately model the crucial particle-to-particle interactions. The coupling with ANSYS Rocky facilitates a true 4-way coupling: it models how the fluid affects the particles, how the particles displace and affect the fluid, and, most importantly, the direct collisions and frictional forces between the particles themselves and between particles and the walls. This robust ANSYS Rocky CFD workflow is what makes a realistic simulation of the complex bed hydrodynamics possible.

Post-processing: CFD-DEM Analysis of Particle Dynamics and Bed Expansion

The simulation results provide a powerful insight into the complex mechanics of the fluidized bed. The contour of particle rotational velocity in Figure 2 reveals the intense microscopic interactions driving the macroscopic behavior. The analysis shows particles spinning at speeds ranging from nearly stationary up to a remarkable 546.7 rad/s. This rotation is not random; it is a direct result of the frictional collisions between particles and with the walls, a phenomenon captured perfectly by the Fluidized bed DEM Fluent model. The regions of high rotational velocity (green and yellow) correspond to areas of intense agitation and mixing, primarily within the rising gas bubbles that churn the bed. This tumbling motion is critical as it constantly renews the particle surface exposed to the hot gas, dramatically enhancing heat and mass transfer.

Figure 2: Particle rotational velocity from the ANSYS Rocky CFD simulation, highlighting the intense particle interactions.

The temperature distribution, shown in Figure 3, tells the story of the system’s overall efficiency. The particle temperatures are contained within a very narrow range, from 300.08K to 301.57K. This incredible thermal uniformity is the primary goal of a fluidized bed and is a direct consequence of the vigorous mixing shown in Figure 2. The continuous, chaotic motion ensures that no single particle remains in a hot or cold spot for long, leading to a near-isothermal bed. The bubble-like structures, visible in both contours, are the engines of this process. These bubbles of gas rise through the bed, dragging particles upwards in their wake and causing the energetic collisions and rotations that lead to perfect thermal mixing. This successful Fluidized bed Rocky simulation beautifully visualizes how the bed expands and mixes, confirming the powerful capability of the coupled CFD-DEM approach to solve complex industrial problems.

Figure 3: Particle temperature distribution from the Fluidized Bed CFD analysis, demonstrating excellent thermal mixing.

The animation extracted from the transient (unsteady) CFD simulation is shown below: