CHEE2945 – Lecture 12
Rheology:
- Rheology is the study of the flow of materials.
- At contact points between particles in a dry powder, the particles will exert a
force on each other. The force due to gravity can be split into a normal and
tangential vector. The normal force vector is known as the normal stress, and
the tangential one is shear stress.
- Consider a jelly-like block between two plates. If a normal force is applied, it
will compress and form a pill-like shape. If a shear force is applied, it will
deform and form something of a parallelogram shape.
GM’s in hoppers:
- In a hopper, particles can form a solid arch, with sufficient strength to hold up
the particles. This is undesirable, and so the hopper should be designed in
such a way to avoid this.
- There are two main types of flow of granular materials in hoppers:
o Core flow.
o Mass flow.
- Core flow occurs when the powder flows in a channel created by the powder
itself. Particles in the center line move faster than particles at the wall.
- In core flow, powder in the lower regions of the hopper is stagnant until the
hopper is almost empty.
- In mass flow, all the particles are always in motion. It has a uniform flow and
can be closely approximated as steady state. The bulk density of the
discharge is roughly constant. It is also independent of the height of the
hopper.
- The flow of GM’s in hoppers depends on the hopper angle. A shallow angle
gives core flow, while a sharper angle gives mass flow. The hopper angle
required to ensure mass flow is dependent on the particle-particle friction, and
particle wall friction. A particle which gives mass flow with one powder may
give core flow with another.
- Core flow gives low wear on walls, particles have different residence time, and
segregation can occur, as particles get stuck in corners.
- In mass flow, all particles have similar residence times, and there is no
particle segregation. Mass flow stresses are generally lower, so there is less
compaction of the powder. However, friction between particles and wall can
cause degradation or contamination.
Stress analysis of GM’s:
Rheology:
- Rheology is the study of the flow of materials.
- At contact points between particles in a dry powder, the particles will exert a
force on each other. The force due to gravity can be split into a normal and
tangential vector. The normal force vector is known as the normal stress, and
the tangential one is shear stress.
- Consider a jelly-like block between two plates. If a normal force is applied, it
will compress and form a pill-like shape. If a shear force is applied, it will
deform and form something of a parallelogram shape.
GM’s in hoppers:
- In a hopper, particles can form a solid arch, with sufficient strength to hold up
the particles. This is undesirable, and so the hopper should be designed in
such a way to avoid this.
- There are two main types of flow of granular materials in hoppers:
o Core flow.
o Mass flow.
- Core flow occurs when the powder flows in a channel created by the powder
itself. Particles in the center line move faster than particles at the wall.
- In core flow, powder in the lower regions of the hopper is stagnant until the
hopper is almost empty.
- In mass flow, all the particles are always in motion. It has a uniform flow and
can be closely approximated as steady state. The bulk density of the
discharge is roughly constant. It is also independent of the height of the
hopper.
- The flow of GM’s in hoppers depends on the hopper angle. A shallow angle
gives core flow, while a sharper angle gives mass flow. The hopper angle
required to ensure mass flow is dependent on the particle-particle friction, and
particle wall friction. A particle which gives mass flow with one powder may
give core flow with another.
- Core flow gives low wear on walls, particles have different residence time, and
segregation can occur, as particles get stuck in corners.
- In mass flow, all particles have similar residence times, and there is no
particle segregation. Mass flow stresses are generally lower, so there is less
compaction of the powder. However, friction between particles and wall can
cause degradation or contamination.
Stress analysis of GM’s:
