Multiphase Flows

Explained

Multiphase flows are of great interest for industry as they are encountered in many different fields. Examples include bubbly flows and granular flows. Despite this fact many unanswered questions about the fundamental aspects of these flows still exist. At SMR we are trying to solve part of these questions.

Bubbly Flows

Direct numerical simulation of bubbly flows is mostly done using our Front Tracking (FT) model; the model tracks the interface between one or more bubbles and the surrounding liquid explicitly using a triangular mesh. This allows an accurate calculation of the surface tension force and the bubbles are allowed to deform. Research with this model is shifting from single rising bubbles towards swarm effects and reacting flows. The scale of the simulations lay in the order of 10-2 m. Besides the FT model, we also have a Volume of Fluid (VoF) model which operates on the same scale.

The Discrete Bubble Model (DBM) is a parallel code which is typically used to simulate lab-scale bubble columns. Bubbles are now tracked by discrete elements in the flow. Hydrodynamic closures from the direct numerical simulations are used to model the drag, lift and virtual mass forces. Bubble-bubble interaction studies are performed using this type of model; coalescence, breakup and clustering are important parameters for a full description of an industrial scale bubble column. The DBM is the gas-liquid version of the Discrete Particle Model (DPM, see below).

A Multi Fluid Model (MFM) is a CFD code which employs a phase fraction parameter to account for the different phases. It is capable of simulating large scale flow structures at industrial scales.

Granualar Flows

Granular (particle) systems are found in a variety of production processes, such as fluidized catalytic cracking, polymers, drugs or fertilizers. For particulate flows, we also use the multiscale modelling approach. At the smallest scales, we use lattice-Boltzmann simulations to provide hydrodynamic closures for gas-particle systems. The knowledge of these closures for different void fractions and particle packings is important for further modelling.

The next scale is the Discrete Particle Model (DPM), capable of tracking up to a few million discrete particles in a single simulation. Gas-solid fluidized beds, spout fluidized beds, vibrated beds and chute flows are examples of simulations performed with the DPM in our group. For bubbling fluidized beds, information on bubble formation and gas flow from the DPM are used in the larger-scale Two Fluid Model (TFM). The TFM uses a phase fraction parameter to account for the particulate phase and the gas phase. The TFM is mainly used to account for bubble clustering behavior in fluidized beds. The largest scale fluidized beds can be performed in the Discrete Bubble Model (similar to the DBM in bubbly flows). In this model, a bubble is modelled as a discrete entity while the particle phase is modelled as a continuum. The key feature of this model is that it is capable of simulating true industrial scale reactors.