There are examples of multiphase flows everywhere. Naturally occurring multiphase flows might include air bubbles rising in a glass of sparkling water, sand particles carried by wind, rain drops in air. In industry, illustrations might be the injection of air bubbles in a bubble column, separation of particles in a cyclone separator or the spray drying of milk in a spray dryer.
In order to study flow process using computer simulation, we first need to describe it using equations. These ‘transport’ equations are obtained by applying the conservation laws of mass, momentum and energy to each fluid phase in the flow domain.
From these transport equations we ascertain volume fraction, velocity, and temperature for each phase. Since the phases are generally moving at different velocities and have different temperatures, there are exchanges of momentum and energy between the phases. Correct modelling of these inter-phase exchanges is one crucial factor in a successful simulation.
Taking inter-phase momentum exchanges as an example, the following forces can be identified: drag, turbulence drag, lift and virtual mass. These are exerted between the phases due to their relative motions.
Fortunately, the required equations, models and their solution methods are readily available in
STAR-CD from CD-adapco.
The best way to illustrate the power of this computational technique in flow analyses is by way of examples, described below.
Mixing vessels operating in multiphase flow regimes are commonly found in the chemical and process industries. Examples include; catalyst particles that are introduced into vessels to promote specific reactions or gas bubbles that are injected in order to provide chemical species for reactions such as oxygen from air bubbles.
Fig.1. demonstrates the computed flow pattern and void distribution in a mixing vessel with a downward pumping, pitched blade impeller. We can clearly see the recirculating flow generated by the impeller in the lower region, and the bulk circulation over the whole tank. The void fraction plot shows that some bubbles are trapped by the recirculating flow resulting in increased gas volume fraction towards the centre of the recirculation.
Images like this provide valuable information to an engineer, promoting better understanding of the flow dynamics, the spatial distribution of the phases and what these mean in terms of reactions, heat and mass transfers.
Liquid-liquid extraction column
Liquid-liquid extraction is often used in the petrochemical industry to promote mass transfer between two fluids. To provide maximum contact between the two fluids a counter-current flow arrangement is used as in the example shown in Fig. 3.
The heavier fluid is introduced through a central inlet at the top of the column and a distributor screen is used to distribute the fluid. The lighter fluid enters the column through the central inlet at the bottom. Perforated trays are placed horizontally in the column to provide further contact between the two fluids in similar fashion to a distillation column. The two fluids can leave the column via the bottom or the top outer annuli. In Fig. 3, we can clearly see the expected collection of the heavier fluid on the trays, the rolling-off at the tips of the trays and the cascade
down the column.
Simon Lo is Chemical and Process Industry Manager, CD-adapco, London, UK. www.cd-adapco.com