(125g) Coupled Lbm-DEM Simulations of Gas Fluidised Beds

 COUPLED LBM-DEM SIMULATIONS OF
GAS FLUIDISED BEDS

J. R. Third^a* and C. R. Mueller^a**
^a ETH Zuerich
Leonhardstrasse 27

Zuerich, CH-8092 

* jthird@ethz.ch, **muelchri@ethz.ch

Gas-fluidised bed
reactors feature in numerous industrial processes, with the fluidised catalytic
cracking of hydrocarbons being perhaps the most important example. Other
notable applications of fluidised beds include fertiliser manufacture, the
production of high purity silicon and the gasification of coal. Despite the
importance of fluidisation to industry, many aspects of the behaviour of
fluidised particles remain poorly understood. Consequently, there is
significant industrial and academic interest in the computational modelling of
gas-fluidised beds.

Since its
introduction by Tsuji et al. (1),
the coupled computational fluid dynamics-discrete element
method (CFD-DEM)
technique has become one of the most widely used techniques for modelling
gas-fluidised beds. In these simulations each particle is modelled as a
distinct entity, whereas the fluid flow is modelled in a volume-averaged manner
using a large cell size, typically 3 times the particle diameter. Due to the
low spatial resolution of the fluid model, it is not possible to compute the
fluid-particle interactions directly in these simulations with the result that
these models require a closure relationship, often called the drag law. In
recent years closure relationships have been developed for CFD-DEM simulations
using the lattice Boltzmann method (LBM), e.g.
Beetstra et al.
(2). However, the drag law remains the greatest source
of error in CFD-DEM methods since even the most sophisticated expressions
describe only the average force experienced by a particle and make no
allowance for particle rotation or relative motion between the particles. The aim of this work is to examine the ability of the CFD-DEM technique
accurately to model gas-fluidised beds. The predictions
of a CFD-DEM model are compared with those of a coupled lattice Boltzmann
method-discrete element method (LBM-DEM) technique, which does not require a
closure relationship for the fluid-particle interactions.

Significant differences can be observed between the results of the two
modelling approached. For example, figure 1 shows the height of the centre of
mass of the particles above the distributor as a function of time for CFD-DEM
and LBM-DEM models. These data indicate that the LBM-DEM model predicts larger
fluctuations in the average height of the particles than the CFD-DEM model.
Furthermore, on a time-averaged basis the location of the centre of mass of the
particles is higher for the LBM-DEM model, suggesting that this model predicts
a larger bed expansion. This is consistent with a detailed analysis of the
fluid-particle interaction forces, which indicates that the force acting on the
particles in the LBM-DEM simulation is approximately 125 % of the force that
would be predicted by the equation proposed by Beetstra et al. (2).

Figure 1: Height of the centre of mass of the particles above the
distributor as a function of time for CFD-DEM and LBM-DEM models.

REFERENCES 

1.     Y.
Tsuji, T. Kawaguchi and T. Tanaka, Discrete particle simulation of
2-dimensional fluidized beds. Powder
Technol., 77(1): 79- 87, 1993.

2. R. Beetstra, M. A. van der Hoef and J. A. M. Kuipers, Drag force from lattice Boltzmann simulations of intermediate Reynolds number of past mono and bidisperse arrays of spheres. A.I.Ch.E. Journal 53(2): 489-501, 2007.