Recent progress towards hydrodynamic modelling of dense gas-particle flows

In this paper a state-of-the-art review will be presented on hydrodynamic modeling of dense gas-particle flows as encountered in the fluid\ud bed family of gas-solid contactors. After a brief introduction the different classes of fundamental hydrodynamic models will be discussed together with their physical basis and mutual advantages and disadvantages. Thereafter some typical results will be presented on first principles modeling of dense\ud gas-fluidized beds. Finally the conclusions will be presented and areas which need substantial further attention will be indicated

The influence of particle properties on pressure signals in dense gas-fluidised beds: a computer simulation study

A hard-sphere discrete particle model of a gas-fluidised bed was used in order to study the influence of particle properties on pressure signals in dense gasfluidised beds. In the model the gas-phase hydrodynamics is described by the spatially averaged Navier-Stokes equations for two-phase flow. For each solid particle the Newtonian equations of motion are solved taking into account the inter-particle and particle-wall collisions. Pressure fluctuations inside the bed are strongly affected by the coefficients of restitution and friction: the more energy is dissipated in collisions the stronger the fluctuations are. The root mean square of the pressure fluctuations showed an almost linear dependency on the amount of energy dissipated in collisions during the simulation

Discrete particle simulation of bubble and slug formation in a two-dimensional gas-fluidised bed: a hard-sphere approach.

A discrete particle model of a gas-fluidised bed has been developed and in this the two-dimensional motion of the individual, spherical particles was directly calculated from the forces acting on them, accounting for the interaction between the particles and the interstitial gas phase. Our collision model is based on conservation laws for linear and angular momentum and requires, apart from geometrical factors, two empirical parameters: a restitution coefficient and a friction coefficient. A sequence of collisions is processed using techniques which find their application in hard-sphere simulations which are commonly encountered in the field of molecular dynamics. The hydrodynamic model of the gas phase is based on the volume-averaged Navier-Stokes equations. Simulations of bubble and slug formation in a small two-dimensional bed (height 0.50 m, width 0.15 m) with 2400 particles (dp = 4 mm, material: aluminium, p = 2700 kg m¿3) showed a strong dependency of the flow behaviour with respect to the restitution and friction coefficient. A preliminary experimental validation of our model was performed using a small scale "two-dimensional" gas-fluidised bed (height 0.30 m, width 0.15 m, depth 0.015 m) with 850 ¿m ballotini glass particles (p = 2930 kg m¿3) as the bed material. Results compared fairly well with the results of a simulation which was performed with 40,000 particles using realistic values for the restitution and friction coefficients which were obtained from simple independent experiment

Granular dynamics simulation of cluster formation in dense riser flow

The occurrence of clusters is one of the most characteristic features of dense gas-solid flow in the riser section of a Circulating Fluidized Bed (CFB). The existence of such clusters has a profound influence on the performance of a CFB unit as a chemical reactor. It is therefore of great importance to gain more insight into this phenomenon. Although the existence of clusters (regions of locally higher solid fraction) in dense riser flow is well accepted in the chemical engineering community, a clear definition is still lacking (Chen, 1995). Clusters were observed experimentally by (among many others) Horio and Kuroki (1994) using a laser sheet technique. They found that the clusters had a characteristic parabolic shape. Tsuo and Gidaspow (1990) used a Two-Fluid approach with constant solids viscosity in order to simulate the riser section of a CFB. They reported the formation of cluster-like structures as well as the typical core-annulus flow structure where the average solids concentration is considerably higher near the wall. Other Two-Fluid approaches incorporating the kinetic theory of granular flow (Sinclair and Jackson (1990), Nieuwland et al. (1996)) did not focus particularly on cluster formation but more on the radial segregation of solids. This type of model possesses a peculiar dependency on the magnitude of the coefficient of restitution where a small deviation from unity causes the flow structure to change completely while the agreement with experimental findings deteriorates. This was (among others) pointed out by Hrenya and Sinclair (1997) who also reported that when particle phase turbulence was included this dependency became far less pronounced. Tanaka et al. (1996) performed simulations of gas-solid flow in a vertical duct using the Lagrangian approach for the solid particles. They employed the Direct Simulation Monte Carlo (DSMC) method to describe the particle dynamics. In this DSMC method the simulated particles actually represent a certain number of ‘real‘ particles. A Monte Carlo procedure is invoked to determine the collision partners and the geometry of the collision. Hence this method does not account for actual particle-particle and particle-wall interaction in a direct way. Moreover the modified Nanbu method used in their work does not guarantee exact conservation of energy (Frezzotti, 1997) which can be an important drawback especially since the collision parameters turned out to be of key importance in their simulations. In this paper we present an extension of our Eulerian Lagrangian simulation technique (Hoomans et al. (1996)) for dense gas-solid two-phase flow in a riser which features a direct incorporation of the particle-particle and particlewall interaction