Fludised Bed – coupled Simulation of OpenFoam and Liggghts
Coupled CFD and DEM Simulation
A particle bed is fluidised by an airstream coming from the bottom. It shows the results of direct interaction of a fluid phase and a particle system. For the simulation Liggghts 3.8.0 and OpenFoam 5.x were used. This example will also be major part of a future tutorial where we will explain in detail, how to set up a simulation using the liggght / openFoam coupling.
In this video you can see three columns. The left column shows the fluid motion in a cut view vertical trough the cylindrical pipe that was used in the simulation. It is easy to see, that the fluid motion within the particle bed is faster than above the particles. This is due to the void fraction of the particles. The particles narrow the space which can be filled with gas, in this case air. Since not all particles are moving with the air, the cross section that can be used by the fluid is smaller than the cross section of the pipe. This leads to higher velocities, which are represented by the red colour.
The middle column shows the void fraction also as a cut view vertical trough the pipe. A void fraction of 1 mean there will only be air which is equal to a 100% void. The lower end of the scale shows a void fraction of 30 % which comes close to the most dens packing for the particle grain size distribution I used in this simulation. The moving gas bubbles that become visible are formed by the lifting particles upwards. These bubbles are not always so easy to see in the particle motion.
The right column shows the particle motion. The gas bubbles can be seen here as well. Some particles are moved upwards with the motion of the fluid. Sometimes these particles fly high enough to hit the lid on the top.
This video you can see here is in slow motion. But if you are focused on the particles in the right column, you can see that particles with a larger size tend to settle to the bottom of the cylinder. This effect is called segregation and is always happening when a particle system is in motion and particles of different sizes or density are mixed together. Larger and particles with a lower density will move to the to, while smaller particles will flow to the bottom. Size here the air is flowing from the bottom upwards and smaller particles are easier to been blown away, the air movement is lifting the smaller particles upwards. The larger sized particles stay at the bottom. The smaller particles are fluidised easier which leads also to segregation.
If we have a look to the void fraction at void fraction column, we also can see that air channels are forming in the lower third of the particle bed, wile the bubble movement is happening above this.
I prepared a second rendering of the particles where it is very clear how the larger particles settle to the ground. Therefore, I coloured the particles by size. I’m running the animation much faster so the settling process is more visible.