Investigating fluidisation of granular solids

‘Fluidisation occurs when the upwards force of a gas on an individual grain is equal to the downwards force of gravity acting upon each grain’

Mark Rober ex-NASA scientist

‘When the gas velocity is high enough that the drag force on the particles equals the weight of the particle, the substance becomes fluidized ’

Source 1 in bibliography

Fluidisation is a scientific phenomenon that makes granular solids act almost like a thick liquid. This occurs when a gas, at the right rate, is passed through a granular solid, such as sand. The upwards force of the gas pushing on each grain is equal to the downwards force of gravity acting upon each grain. This means that the grains can move around, and flow almost like a fluid. As you can see from the diagram and image below, there are large gaps between grains. Interestingly, the effect is more pronounced with irregularly shaped grains.

Source 1 and 6 in bibliography

This effect is largely to do with reducing internal friction Internal friction is the friction when the grains move pass one another. This can affect the viscosity of a fluid. Fluids with more internal friction have more viscosity. As an example, honey has more internal friction than water, and as a result is more viscose. Viscosity is defined by this equation: force x distance / area x velocity.

As the gas velocity increases, the type of fluidisation changes. In our experiment, we reproduced bubbling fluidisation. In this type, the top surface of the bed becomes less defined, and the gas visibly bubbles up through the granular substance. This is shown in the image below. As you can see, there are many different types of fluidisation This image was taken from the paper ‘Introduction to fluidization’. There is more information about this source in the bibliography.

When Olympic divers are practising a new skill or technique, they need a safe learning environment. To provide this, compressed air is released from an outlet and Rises through the water. Now this example is not with a granular solid, but the effect is the same. The water molecules are forced apart, they move around more freely.

The reason that diving into water can hurt is surface tension. Water molecules on the surface of the pool cannot bond with as many other water molecules as those deeper can, so they form stronger bonds with those next to them. This is why water is sometimes described as having a ‘skin’. But the compressed air moving upwards breaks these stronger bonds, therefore breaking the surface tension of the water and making it safer to dive into.

Source 3 and 4 in bibliography

To coat an object entirely in powdered paint, you need to use a fluidised bed. As we know, air moving up through a granular solid lowers internal friction and allows the grains to move freely, like particles in a liquid. This means that an object can be completely submersed in the powder, and covered evenly.

There is often a problem when discharging grain from a silo. Problems such as bridging and ratholing can mean that the grain does not flow smoothly from the outlet. With the application of a fluidised bed in the funnel cone of the silo, these problems can be solved. The grain is discharged cleanly, and the fluidised bed does not affect the grain in any way that could influence its value. Refer to source 4 in bibliography for an animated video of how this works

Experiment: Why do some object float in the fluidised bed and others sink?

Hypothesis: objects with a high buoyancy force will always float whereas those with a low buoyancy force will always sink.

For a visual description of the experiment, please refer to the video I submitted Conclusion: This hypothesis is wrong, but before we answer why, we must address what buoyancy force actually is.

Buoyancy force is defined by this equation:

Buoyancy force = fluid density x object volume x gravity

So I used this formula to calculate the buoyancy force for the objects I used in the experiment. They came out like this:

Billiard ball: 11903.659 Newtons (buoyancy force in sand)

Ping pong ball: 5264.506 Newtons (buoyancy force in sand)

As we can see from the answers above, the billiard ball has a higher buoyancy force. If we go by the original hypothesis, the billiard ball should float better than the ping pong ball. But this was not the case. Why? Well, in reality it is a tug of war between the buoyancy force of an object and the mass it has. While the billiard ball had a higher buoyancy force than the ping pong ball, it also had a far higher mass, meaning that it sunk while the ping pong ball floated.

Source 6 in bibliography

As this project is based upon an artefact, and not a piece of writing, I will detail how I build the fluidised bed with the help of Mr Carter

Materials used

• 15mm pvc pipe

• 4x 15mm quick-fit flomasta pvc elbow joints

• 7x 15mm quick-fit flomasta pvc tree joints

• 40 litre plastic box

• 15mm quick-fit ball valve

• 15mm nipple clip to connect to air system

To begin with, I cut 4 lengths of the 15mm pvc pipe to almost the length of the box (I had to allow for the joints). In these lengths, I drilled two 3mm holes spaced at right angles, and continued this down the length of the pipes, each hole spaced 2.5mm apart. Then, I cut 6 very short lengths to connect the joints together. After that, I drilled two 16mm holes in the side of the box so that the air from the compressor could enter. Here is a quick sketch showing the layout simply.

Although the finished product worked perfectly well, there are a couple of things that I think would really improve the item. Firstly, instead of 4 crosspieces of pipe, I think there should have been five. This would mean that the air is released more evenly through the sand. However, this was not possible because of the size of the joints I was using. Finally, I think that I could have done with more sand in the box to make the effect better I did order a bag while I was at school, but it did not arrive in time for me to test it.

1. Introduction to fluidization by Ray Cocco, S.B . Reddy Karri and Ted Knowlton

At Particulate solid research, INC (PSRI) . Published November 2014 https://www.aiche.org/sites/default/files/cep/20141121.pdf

2. What is internal friction? – definition, angle and coefficient by Ali Motamedi at study.com https://study.com/academy/lesson/what-is-internal-friction-definition-anglecoefficient.html

3. Pulsair systems.inc https://www.pulsair.com/diving-pool-bubbler-sparger/pool-bubbler-faq/

4. Surface tension of water, why is it so high? By Susanna Lauren. Posted on Biolin scientific website https://www.biolinscientific.com/blog/surface-tension-of-water-why-is-it-sohigh

5. Silo discharge – animation by tridents siperm GmbH uploaded on the 9th of June 2016 https://www.youtube.com/watch?v=Twp0OY0nn5w

6. Liquid sand hot tub – fluidized bed uploaded to YouTube by Mark Rober 4 years ago https://www.youtube.com/watch?v=My4RA5I0FKs