ALL OF ADVANCED TOPICS TO MEMORISE Flashcards
Why are phase changes commonly employed?
To increase energy transfer rates because during a phase change the order and bonding within a substance alters. This absorbs or releases energy.
Is the change in energy during a phase change associated with a change in temp?
NO.
What is the name given to the energy change?
Latent heat.
Does forming bonds release or use up energy?
Releases energy! breaking bonds uses up energy.
Why are phase changes attractive for cooling applications?
Because phase changes such as vaporisation absorb energy.
Can you show how a cooling tower works?
Draw diagram
What are the advantages of an evaporative cooler?
Simple, low cost, no toxic or difficult to work with chemicals
What are the disadvantages of an evaporative cooler?
Efficiency is environment dependent, requires a constant supply of water, humidifies air
How can the disadvantages of evaporative cooling be removed?
If we simply use phase changes to move heat from one place to another.
How does a heat pipe look and work?
Draw diagram
What are heat pipes limited by?
The speed at which vapour can flow from the hot to the cold end and the speed at which the liquid can flow from the cold to the hot end. (This can be rapid due to the low pressure inside the pipe and the pressure difference at each end)
Can you draw the resistor network for a heat pipe?
Draw diagram
Why can’t heat pipes operate at very high/very low temps?
At too high a temp all the working fluid will vaporise and condensation won’t occur. Also can’t operate at too low a temp because then the working fluid will not undergo phase change.
