Direct numerical simulations of collision dynamics of wet particles

Fluidized beds involving liquid injection have a wide industrial application ranging from physical operation, like agglomeration and coating, to chemical processes including catalytic oxidization, fluid catalytic cracking, condensed-mode polyethylene (1). The injection of the liquid results in wet particles, which behave completely different from dry particles and hence lead to much more complicated hydrodynamics of fluidized beds (2). Nevertheless, a fundamental description of the dynamics of wet particles is predominantly missing, which however is crucial for prediction of fluidization behavior effecting on the product quality. Despite significant investigation, experimental studies of wet collisions under actual fluidization condition (e.g., low particle velocity, thin liquid layer) are virtually impossible to perform and control. Direct numerical simulations can complement experiments by providing quantitative predictions of the micro-mechanical collisional behaviour of one or more particles with well-defined and easy-controlled system parameters. Jain et al. (3) demonstrated that the experimentally observed phenomena of collision between a particle and a wet wall can be reproduced by a hybrid model combining the volume of fluid (VOF) method and the immersed boundary method (IBM). Such simulations will be extended in this work to investigate the effects of liquid layer thickness, impact velocity, particle size and surface tension on the wet restitution coefficient () under normal collisions as well as oblique collisions. The motion of a solid particle is described by the IBM (Figure 1), which enforces a no-slip condition at the particle surface. Whereas, the VOF (Figure 2) describes the motion of the gas-liquid interface by a piece-wise linear reconstruction of the interface. Please click Additional Files below to see the full abstract

Drag Force on Bubbles in Bubble Swarms

Direct Numerical Simulations with a Front Tracking model have been used to study the hydrodynamic behavior of bubble swarms for the air/water system. In this work, we focus on the effect of the presence of neighboring bubbles on the drag as a function of the void fraction, especially at higher void fractions. Mimicking bubble swarms with a relatively small number of bubbles in a periodic domain, the number of bubbles was varied while keeping the overall void fraction constant. We have observed a strong hindered rise effect, while bubbles tend to arrange themselves in horizontal clusters. The observed minimum in the drag coefficient as a function of the number of bubbles at a fixed void fraction was related to the maximum number of bubbles that can coordinate themselves in a horizontal plane. The normalized drag coefficient increases for higher void fractions (i.e. hindered rise). Again, significant effects of horizontal bubble clustering on the drag coefficient were found and explained

Assessment of a subgrid-scale model for convection-dominated mass transfer for initial transient rise of a bubble

The mass transfer between a rising bubble and the surrounding liquid is mainly determined by an extremely thin layer of dissolved gas near the bubble interface. Resolving this concentration boundary layer in numerical simulations is computationally expensive and limited to low Péclet numbers. Subgrid-scale models mitigate the resolution requirements by approximating the mass transfer near the interface. In this contribution, we validate an improved subgrid-scale model with a single-phase simulation approach, which solves only the liquid phase at a highly-resolved mesh. The mass transfer during the initial transient rise of moderately deformed bubbles in the range Re = 72–569 and Sc = 102–104 is carefully validated. The single-phase approach is able to mirror the two-phase flow field. The time-dependent local and global mass transfer of both approaches agree well. The difference in the global Sherwood number is below than 2.5%. The improved subgrid-scale model predicts the mass transfer accurately and shows marginal mesh dependency

Progress in Applied CFD. Selected papers from 10th International Conference on Computational Fluid Dynamics in the Oil & Gas, Metallurgical and Process Industries

A hybrid collision integration scheme is introduced, benefiting from the efficient handling of binary collisions in the hard sphere scheme and the robust time scaling of the soft sphere scheme. In typical dynamic dense granular flow, simulated with the soft sphere scheme, the amount of collisions involving more than two particles are limited, and necessarily so because of loss of energy decay otherwise. Because most collisions are binary, these collisions can be handled within one time step without the necessary numerical integration as needed in a soft sphere method. The remainder of the collisions can still be handled with the classical soft sphere scheme. In this work the hybrid collisions integration scheme is shortly described and tested with a bounding box problem. The hybrid scheme is capable of solving the same problem as a classic soft sphere scheme but is roughly one order of magnitude faster.publishedVersio

Novel efficient hybrid‐DEM collision integration scheme

A hybrid collision integration scheme is introduced, benefiting from the efficient handling of binary collisions in the hard sphere scheme and the robust time scaling of the soft sphere scheme. In typical dynamic dense granular flow, simulated with the soft sphere scheme, the amount of collisions involving more than two particles are limited, and necessarily so because of loss of energy decay otherwise. Because most collisions are binary, these collisions can be handled within one time step without the necessary numerical integration as needed in a soft sphere method. The remainder of the collisions can still be handled with the classical soft sphere scheme. In this work the hybrid collisions integration scheme is shortly described and tested with a bounding box problem. The hybrid scheme is capable of solving the same problem as a classic soft sphere scheme but is roughly one order of magnitude faster