(448s) Incorporating the Effect of Fluidized-Bed Temperature in CFD Simulation through Particle-Particle Interaction Coefficient

Authors: 
Uddin, M. H., University of Nevada, Reno
Coronella, C., University of Nevada, Reno
Abstract

Experimental studies on the hydrodynamics of dense gas-solid fluidized beds at elevated temperatures are very limited. This lack of studies is due to difficulties associated with measuring techniques under these conditions. However, numerous industrial applications of fluidized beds are at high temperatures, e.g., combustion and gasification, and it is desired to better understand the effects of temperature on hydrodynamics. The first-principles based computational fluid dynamics (CFD) is an effective tool to explore the complex hydrodynamics behavior in gas-solid fluidized bed at elevated temperature. It is well accepted that the changes of fluid physical properties, such as density and viscosity are not sufficient to describe the hydrodynamic behavior of fluidized beds at high temperature. Particle-particle interactions play a vital role at high temperature, which in turn has a substantial impact on the hydrodynamics of a fluidized bed. In this study, a series of CFD simulations was performed using glass beads particles (Geldart-B) in a two dimensional bubbling fluidized bed at different temperatures . Effects of particle-particle interactions were studied by changing the restitution coefficient, and its effect on the hydrodynamic properties (i.e., bubble size, bubble shape, bubble rise velocity, pressure drop fluctuations, and bed expansion) were investigated. The coefficient of restitution has an effect on bed hydrodynamics which is most pronounced when near the point of inelasticity. Our CFD results show that decreasing the coefficient of restitution results in larger bubbles. A similar result is experimentally reported for high-temperature fluidized beds (Sanaei et al. 2007. "Effect of Temperature on Hydrodynamics of Fluidized Beds"; Velarde et al. 2016. "Development Of An Endoscopic-Laser Piv/Dia Technique For High-Temperature Gas-Solid Fluidized Beds." Chemical Engineering Science.)

We explain the effect of particle-particle interactions on bubble size through the two-phase theory of fluidization. When particles experience inelastic collisions, i.e. no loss of energy, there is no change in total particle momentum. However, elastic collisions result in a loss of kinetic energy and reduced particle velocity. , As the fluid passes by particles with reduced velocity, the bed experiences increased relative fluid velocity. In two-phase theory, any fluid flow exceeding the minimum fluidization velocity passes through the bed as bubbles. Thus, decreased coefficient of restitution can result in increased bubble size.

The interpenetrating two-fluid model (TFM) using Eulerian-Eulerian flow field method employed in the open source MFIX software is used for all simulations studied here. The complete set of mass and momentum balance equations are solved through numerical simulation to obtain gas and solid flow fields at high temperatures. The simulation data analyzed and results were then compared with recent experimental results. Velarde et al. (among others) showed that high temperature beds exhibit relatively large bubbles, with an increased possibility of slugging behavior. We believe the effect of increased bed temperature can be accounted for in TFM simulations by decreasing the coefficient of restitution.