5. Heat Pipes Flashcards
What are the triple and critical points?
The triple point is where the temperature and pressure of a substance in all three phases coexist (below the triple point we have solids forming)
The point on a phase diagram at which gas and liquid have the same density and so are indistinguishable (above the critical point we do not have the ability to change phase)
Why is it important to stay far from the triple point when designing a heat pipe?
At the critical point the differences between liquid and vapour disappear, and hence the latent heat of vapourization goes to zero.
A heat pipe degenerates into a pipe with a fluid in it, and all enhancements due to phase change disappears.
Why would a thermosyphon outperform a heat pipe when vertical?
With the heat source at the bottom, the liquid can fall under gravity down the walls of the pipe unhindered. The wick will essentially increase the resistance to flow.
Draw a graph of thermal capacity limits against temperature for a thermosyphon.
An upside down ‘u’ shaped graph
Why is convection better than conduction at transferring thermal energy?
Convection can ‘carry’ heat away, and if the fluid velocity is high this can occur quickly
Conduction relies on diffusion, which is dependent on material properties and tends to be slow
Describe the mechanism for heat pipes/thermosyphons
- Engineer a fluid flow path
- Heat a fluid in the hot zone
- Transport fluid to cold zone
- Cool fluid at cold zone
- Return cold fluid to hot zone
How do thermosyphons transport hot/cold fluid around
Hot vapour rises
Cold liquid condenses and falls with gravity
How do heat pipes transport hot/cold fluid around
Hot vapour travels to cold end (achieved by a pressure difference due to marginally different temperatures)
Cold liquid transported by capillary action
What desirable properties determine if a working fluid is suitable or not
Readily available
Non-toxic
Stable
Inexpensive
Compatible with wick material
High surface tension (for capillary action)
High latent heat of vapourisation (to shift lots of heat)
Critical and triple points temperature far from operating temperature
What is a wicks purpose and what are they usually made of?
Draw up the liquid and distribute it evenly
Made from pourous ceramic, sintered metals or knitted wire meshes
What are the 4 limitations of heat pipes
Sonic limit
Entrainment limit
Capillary (or Circulation) limit
Boiling limit
What is the sonic limit?
If vapour density is low, then its velocity has to be high to transport the same mass. Velocity can be difficult to maintain and causes problem at the speed of sound
What is the entrainment limit?
If vapour density is increased (by increasing temp and pressure), the vapour can shear off liquid from the wick and carry it in the wrong direction (Entrainment limit)
What is the capillary (or circulation) limit?
If vapour density is further increased, it becomes difficult to maintain the liquid flow in the capillaries (Capillary or Circulation limit)
What is the boiling limit?
When temperature approaches the critical point, boiling begins to occur in the wick, which reduced available flow paths (boiling limit)
