In this work the influences of solid viscosity and the way to scale-down traditional drag models on the predicted hydrodynamics of Geldart A particles in a lab-scale gas-solid bubbling fluidized bed are investigated. To evaluate the effects of drag models, the modified Gibilaro et al. drag model (constant correction factor) and the EMMS drag model (non-constant correction factor) are tested. And the influences of solid viscosity are assessed by considering the empirical model proposed by Gidaspow et al. (1997, Turbulence, Viscosity and Numerical Simulation of FCC Particles in CFB. Fluidization and Fluid-particle Systems, AIChE Annual Meeting, Los Angeles, 58–62) and the models based on kinetic theory of granular flow (KTGF) with or without frictional stress. The resulting hydrodynamics by incorporating the different combinations of the drag model and solid viscosity model into two-fluid model (TFM) simulations are compared with the experimental data of Zhu et al. (2008, Detailed Measurements of Flow Structure inside a Dense Gas-Solids Fluidized Bed.” Powder Technological 180:339–349). The simulation results show that the predicted hydrodynamics closely depends on the setting of solid viscosity. When solid viscosity is calculated from the empirical model of Gidaspow et al., both drag models can reasonably predict the radial solid concentration profiles and particle velocity profiles. When the KTGF viscosity model without frictional stress is adopted, the EMMS drag model significantly over-estimates the bed expansion, whereas the modified Gibilaro et al. drag model can still give acceptable radial solid concentration profiles but over-estimate particle upwards and downwards velocity. When KTGF viscosity model with frictional stress is chosen, both drag models predict the occurrence of slugging. At this time, the particle velocity profiles predicted by EMMS drag model are still in well agreement with the experimental data, but the bed expansion is under-estimated.

This study presents an approach for predicting temperatures and ignition time during the cook-off process of solid propellants, with a focus on identifying potential hot spots and conduction losses. A pool fire temperature model is introduced to enhance the understanding of the thermal stability of solid propellants, specifically ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene (HTPB). Numerical simulations were performed to analyze the thermal behavior of the AP/HTPB propellant under cook-off conditions using a three-dimensional, transient model with heating rates between 0.67 and 0.8 K/s. The ignition temperatures for the AP/HTPB propellant varied with distance and time, recorded at 641.40 K, 598.10 K, and 554.43 K. To streamline the computations, a simplified model with hot wall conditions for the outer steel wall of the propellant was adopted, accurately replicating the cook-off processes under specific experimental conditions and aligning closely with experimental findings. The similarity between the model predictions and experimental data provides a reliable means of assessing the explosion risk of AP/HTPB in cook-off scenarios, with relative errors of 0.845% and 2.03% for ignition temperature and ignition delay time, respectively. Additionally, the data generated from this model can aid in developing safety protocols for plant design, establishing safety zones, and planning emergency evacuation routes to mitigate potential hazards.

The discharge behavior of granular assembly inside a thin flat-bottomed silo is investigated by performing three-dimensional (3D) Discrete Element Method (DEM) simulations. The influence of boundary conditions in the thickness direction is evaluated by applying both wall conditions with varying friction coefficients and periodic boundary (PB) conditions. It is found that the introduction of frictional front and rear walls leads to the stick–slip motion of discharged particles and upward propagation of velocity waves, manifested by the formation of bubble-like sub-flow zones. In contrast, the adoption of periodic boundary condition in the thickness direction results in the resonant movement of particles. Statistical analyses indicate that the different modes of particle motion can be ascribed to the different topological features of particle contact networks. While the direction distribution of particle contacts in PB case is approximately isotropic, in the wall case, it exhibits notable anisotropy and a significant amount of particle contacts orient toward the frictional front and rear walls. The DEM simulation results demonstrate that under the conditions considered here, the horizontal profile of the particle vertical velocity in the wall case follows the Gaussian type, whereas that in the PB case presents a clear flattened feature. Analysis of the transient horizontal distributions of particle vertical velocity suggests that the Gaussian-type velocity profile can be attributed to the continuous emergence and updrift of the bubble-like sub-flowing zones.