CHEE2945 – Lecture 11
Bulk powder:
- Consider some volume of particles in a container. The free space between
particles, voids, is usually filled by a fluid.
- The volume fraction, ϕ , is the total volume of the particles divided by the
volume of the bed (in the above case, a container).
- Voidage, ε , is the volume of voids divided by the volume of the bed.
- The volume fraction plus voidage equals 1 (ϕ + ε=1).
- Voidage is easy to determine, as gaseous voids can be filled with liquid
gradually, and the volume added when to fill all the voids can be easily
determined from added mass.
- Particles flow easily when all the particles are a similar size. If smaller
particles are inserted into the voids, the voidage decreases and the particles
will not flow as easily.
Granular materials:
- A granular material (GM) is a conglomerate of discrete, solid, macroscopic
particles.
- They are characterised by a loss of energy by interaction between them, e.g.,
by friction during collision.
- They are not influenced by thermal motion fluctuations.
- They have a diameter greater than 1 micron.
- For example: rice, coffee, salt, corn flakes, coal, ball bearings, powders.
- These are common in pharmaceuticals, agriculture, and energy production.
Granular material properties:
- GM’s are not constituted of a single phase of matter but can have flow
properties of Newtonian fluids.
- They dissipate energy very quickly.
- They may exhibit properties of solids, liquids, or gases.
- For example, sand in a bucket can bear weight, and hence, acts as a solid. If
the bucket was tipped over, their flow would be liquid like. If air was pushed up
through the sand, it would create a fluidised bed, causing the GM to exhibit
gaseous properties.
- The properties exhibited by the GM depends on the energy of the individual
particles.
Flow differences between GM’s and fluids:
Bulk powder:
- Consider some volume of particles in a container. The free space between
particles, voids, is usually filled by a fluid.
- The volume fraction, ϕ , is the total volume of the particles divided by the
volume of the bed (in the above case, a container).
- Voidage, ε , is the volume of voids divided by the volume of the bed.
- The volume fraction plus voidage equals 1 (ϕ + ε=1).
- Voidage is easy to determine, as gaseous voids can be filled with liquid
gradually, and the volume added when to fill all the voids can be easily
determined from added mass.
- Particles flow easily when all the particles are a similar size. If smaller
particles are inserted into the voids, the voidage decreases and the particles
will not flow as easily.
Granular materials:
- A granular material (GM) is a conglomerate of discrete, solid, macroscopic
particles.
- They are characterised by a loss of energy by interaction between them, e.g.,
by friction during collision.
- They are not influenced by thermal motion fluctuations.
- They have a diameter greater than 1 micron.
- For example: rice, coffee, salt, corn flakes, coal, ball bearings, powders.
- These are common in pharmaceuticals, agriculture, and energy production.
Granular material properties:
- GM’s are not constituted of a single phase of matter but can have flow
properties of Newtonian fluids.
- They dissipate energy very quickly.
- They may exhibit properties of solids, liquids, or gases.
- For example, sand in a bucket can bear weight, and hence, acts as a solid. If
the bucket was tipped over, their flow would be liquid like. If air was pushed up
through the sand, it would create a fluidised bed, causing the GM to exhibit
gaseous properties.
- The properties exhibited by the GM depends on the energy of the individual
particles.
Flow differences between GM’s and fluids:
