Gas-solid Drag Model Research Articles

Effect of a cluster on gas–solid drag from lattice Boltzmann simulations

Fast fluidization of fine particles leads to formation of particle clusters, which significantly affects the drag force between the phases. Existing gas–solid drag models, both empirical and theoretical, do not account for the effect of the clusters on the drag force, and as a result, the computational studies using them are unable to capture the inherent heterogeneity of fast fluidization beds. The limitation of the current drag models is generally attributed to poor understanding of the effect of the clusters. In this study, the effect of a single cluster on the drag force has been investigated by conducting lattice Boltzmann simulations of gas–particle flow under a wide range of the overall voidage and particle Reynolds numbers. It was observed that simulations with the particles in a cluster configuration gave considerably lower drag than those with particles in a random arrangement. Furthermore, for the cluster voidage between maximum to 0.7, a significant drag reduction was observed when the inter-particle distances within a cluster was decreased. The simulations with a constant cluster voidage of 0.7 showed that the drag force decreased on decreasing the overall voidage from the maximum voidage to approximately 0.96; however any further decrease in the overall voidage caused a steep increase in the drag force. The results of this study are important in quantifying the drag reduction due to the formation of clusters.

Evaluation of Drag Models for Predicting the Fluidization Behavior of Silver oxide Nanoparticle Agglomerates in a Fluidized Bed

The fluidization characteristics of nanoparticle agglomerates have been experimentally and numerically studied in a fluidized bed. The experimental studies were carried out in a bed containing silver oxide dry powder belonging to group B of Geldart’s classification with a primary particle size of 30 nm. Pressure drop measurements using an optical fiber technique allowed the effects of particle loading and inlet gas velocity on the fluidization characteristics of the particles to be determined. Interparticle adhesion forces between silver oxide nanoparticles give rise to the formation of agglomerates with a wide size distribution. Furthermore, a considerable number of bubbles are formed in the bed with agglomerate bubbling fluidization and low bed expansion. Numerical simulations were also performed to evaluate the sensitivity of gas–solid drag models. An Eulerian multiphase model was used with different drag models. A mean particle size of 175 μm was chosen for the numerical simulations, and the results o...

Space averaging on a gas–solid drag model for numerical simulations of a CFB riser

Proper modeling for fluid dynamics is central in understanding any industrial multiphase flow. For the study of gas–solid flow in a circulating fluidized bed (CFB) riser, the model based on the Eulerian description of the phases is widely used. Gas–solid flows in fluidized bed units are heterogeneous, and resolving them in numerical simulations requires a very fine mesh spacing and a short time step size. Such constraints on the mesh and time step size result in very time consuming calculations even for pilot scale fluidized bed studies. A small scale CFB riser 3 m in height and 0.40 m in width was examined in order to study effects of different scales such as mesh and time step size. When using coarse scales, the information about the mesoscale structures is lost illustrating the dependence of results on the discretization scales. The same set of equations is thus not valid for the simulation using coarse meshes. A study on space averaging was done to formulate the correction factor to the drag model with some defined notations. It was found that the correction factor for the drag model was dependent on the location in the flow domain.