With the development of computing facilities nowadays, computer simulations have become an important experimentation complement in the study of multiphase hydrodynamics. In particular, various computational approaches and numerical schemes have been adopted to study flow characteristics in downer systems (Ge et al., 2011). Cheng et al. (2001) reported a gas−turbulence solid−turbulence model to simulate the
hydrodynamics in the entrance region of a downer. They showed that predicted profiles of local solid fractions and particle velocity display good agreement with the experimental results. Bolkan et al. (2003) integrated hydrodynamic features of circulating fluidized beds into a computer simulation of a downer reactor. They reported an empirical correlation when estimating average cluster sizes. They also presented a correlation for estimating the particle–wall friction factor. The hydrodynamic behavior of the gas–solid suspension, within the downer both in the developing−flow as well as in the
downer fully developed flow regions, was successfully estimated.
Additionally, Li et al. (2004) reported another mathematical model to describe the hydrodynamics of the fully developed region in a downer reactor based on the energy−minimization and multi−scale (EMMS) principle. In this study, they
accomplished the following: a) a simulation of the influence of the friction coefficient, of the solid flow rate and of the superficial gas velocity on the flow pattern in the downer; b) a prediction of local solid concentration and gas/solid velocities; and c) a calculation of cluster size and local slip velocity.
In 2005, Ropelato et al. (2005) proposed a multiphase model to predict the hydrodynamics in downer reactors. The proposed model was based on an Eulerian−Eulerian Approach which included dissipation of turbulent kinetic energy.
These authors claimed that CFD techniques provided powerful tools to simulate downer reactor inlet fluid dynamics as well as chemical process optimization.
In addition, based on the Eulerian−Eulerian Two−Fluid Continuum Approach, Liu et al.
(2011) improved a unified second−order moment two−phase turbulence model to
simulate the dense gas–particle flows in downers. These incorporated a coefficient of restitution into the Particle–Particle Collision Equation. Results were in agreement with experimental observations.
Kim et al. (2011) employed a k1–e1–k2–k12 two–fluid model based on the kinetic theory of
granular flow (KTGF) to predict the flow behavior of gas and solids in downers. These researchers found that particles of small size as 70 µm in diameter apparently interact with the gas turbulence. The turbulence energy interaction between gas and solids was described by different k12 transport equations, while the particle dissipation by the large
scale gas turbulent motion was taken into account through a drift velocity. Johnson– Jackson boundary condition was adopted to describe the influence of the wall on the hydrodynamics. The simulation results based on CFD model were compared with the experimental. Good agreement was obtained.
Prajongkan et al. (2012) calculated mass transfer coefficients and Sherwood numbers in a circulating fluidized bed downer using the concept of additive chemical reaction and mass transfer resistances. These researchers found that the mass transfer coefficients and Sherwood numbers had minimum values with both the increasing reaction rate constants and the increasing system height. System turbulences and dispersion coefficients were also studied in a Circulating Fluidized Bed (CFB) downer by Chalermsinsuwan et al.
(2012) using a Eulerian Computational Fluid Dynamics Model based on the concept of the kinetic theory of granular flow and statistics. Using this approach, these researchers found that in the CFB riser, a dense−core annulus flow structure was observed, while in
the CFB downer, a dilute core−annulus flow structure was obtained. The particle cluster
in the CFB riser had more heterogeneity movements than that in the CFB downer, which could be explained by the system flow direction. About the particle cluster dynamics, the particle cluster diameters and concentrations in the CFB riser were higher than in the CFB downer. The particle cluster dynamics were increased with decreasing system height, due to the accumulation of solid particles.
Furthermore, Computational Fluid Dynamics combined with the Discrete Element Method (CFD−DEM) have also been applied for studies of different multiphase flows
(Lim et al., 2006). Zhang et al. (2008) used CFD−DEM to study particle cluster behavior
in a riser/downer reactor. They reported that there are two types of clusters in a riser and downer: one is in the near wall region where the velocities of particles are low; the other is in the center region where the velocities of particles are high. With this method, various flow structures were predicted in the developing regions in the downer. CFD−DEM was also used by Zhao et al. (2010) to numerically simulate the flow
behaviors in a downer with a newly designed distributor. They reported that uniformity of particle distribution was directly influenced by air supply conditions. The calculated particle and air velocity distributions as well as axial and radial distributions of solid holdups agreed quite well with the measurements using Electrical Capacitance Tomography (ECT). With the same approach, Zhao et al. (2010) studied the hydrodynamics in downers. They found distinct clustering phenomena in the downers. They reported that particles injected from the inlet, first streamed according to the inlet design, and then dispersed in the downer to a homogeneous state. After that, the clusters started to form, where the number of clusters and their sizes varied throughout the downer.
Shu et al. (2014) mentioned that there are a significant number of CFD studies in CFB downer reactor units. However, it is well−known that some parameters have an important
wall boundary condition, and c) parameters describing particle−particle collisions (restitution coefficient). These authors pointed out that it has long been recognized that proper determination of interphase drag forces is crucial for a successful simulation of a heterogeneous gas−solid flow. In particular, the restitution coefficient using the kinetic theory has a significant effect on the predicted hydrodynamics. Furthermore, these authors also mentioned that extensive numerical simulations have consistently shown that the hydrodynamics of gas−solid flow are sensitive to wall boundary conditions. In this respect, CFD in downers is a very active area of interest that continues attracting many researchers.