JW Lecture 4 - Drop & Bubbles, Centrifugal Motion

Question 1

Internal circulation effects

Answer 1

The effect of internal circulation is small when a liquid drop falls in a gas and is large when a gas bubble rises in a liquid. If the fluid within the drop is very viscous, the amount of energy which has to be transferred in order to induce circulation is large, and the circulation effects are therefore small.

Question 2

How can centrifugal fields be generated?

Answer 2

1. In a centrifuge: fluid is introduced in a rotating bowl

2. Introducing fluid at a high velocity into a cylindrical/conical flask

Question 3

What does centrifugal force depend on?

Answer 3

The mass of the object, the speed of rotation, and the distance from the centre.

Question 4

What is the centrifugal acceleration the product of?

Answer 4

The radial position, and the square of the angular velocity.

Question 5

Would separation under gravity be quicker or in centrifuge?

Answer 5

In a centrifuge due to the enhanced gravity force. But density difference between particle and fluid shouldn’t be small for separation.

Question 6

How do hydrocyclones work?

Answer 6

The tangential inlet causes the fluid to rotate and thus, these devices do not have moving parts. The overflow contains a suspension that is more dilute than the feed and has a finer particle size distribution. The underflow is a suspension more concentrated than the feed and has a coarser particle size dist.

Question 7

What velocities need to be considered within a hydrocyclone?

Answer 7

1. Tangential velocity: gives rise to particles subject to the centrifugal force and is critical to the operation.
2. Radial velocity: there is a net flow of liquid towards the centre and a net flow of solids away from the centre. It is much smaller than the tangential velocity.
3. Axial velocity: takes the material to the two outlets. The suspension near to the wall of the hydrocyclone forms axially to the underflow and the material near to the centre of the hydrocyclone flows axially to the overflow.

Question 8

LZvV

Answer 8

Because there is axial flow upwards into the overflow and downwards into the underflow, there is a locus where there is no net velocity in the axial direction, known as the locus of zero vertical velocity (LZvV). Particles that orbit at a radial distance greater than the LZvV will tend to report to the underflow. Particles orbiting at a radical distance greater than the LZvV will tend to report to the underflow, whilst those orbiting at a distance less will be carried out into the overflow. Particles that orbit at the LZvV will have no preference for the overflow or underflow, equal chance of entering either exit. This is defined as being the cut size.