Radiation and heat sinks Flashcards
What condition has to be true for matter to emit radiation
It must have an absolute temperature greater than 0K
What is the energy transformation related to thermal radiation
Thermal to electromagnetic
What is the definition of a black body in terms of its ability to absorb
A black body will absorb any radiation that falls on it, regardless of its wavelength
What is the frequency of the radiation emitted by a black body dependant on
Its temperature
Why is radiation only important for high temperatures
The T^4 term in the equation means that the as the temperature increases there will be a dramatic increase in the energy emitted
What are the 3 possible things that can happen to incident radiation when it meets a surface
It can be absorbed, reflected or transmitted
When considering thermal radiation why can transmitted radiation usually be ignored
Thermal radiation is normally infrared and very few materials are trans
In general what are heat sinks designed for
To increase heat transfer by increasing the surface area exposed to cooling fluid
What is the performance of a heat sink dependant on
Air velocity, Material used, Fin design and surface treatment
What are materials used for heat sinks
Aluminium alloys
Diamond
Composite’s such as copper tungsten
Why is aluminium used instead of copper in heat exchanges despite copper having a higher thermal conductivity
It is lighter, cheaper and easier to extrude
Why is the efficiency of a fin not a good overall representation of the ability for a fin to transfer heat
A high efficiency is simply a measure of how much heat it does transmit compared to how much is physically possible, a very short fin can have a very high efficiency but will not exchange a great deal of heat
What is meant by spreading resistance in therms of heat sinks
What the heat source is not uniformly distributed across the base of a heat sink resulting in some fins being at lower temperatures, this increases the overall thermal resistance
What can be done to combat the problems associated with spreading resistance
Use a material with a higher thermal conductivity
Use a thicker base
Use a heat pipe as the base
What are shape factors in terms of thermal radiation
Shape factors consider the overlap or orientation between to shapes, ie not all the radiation will go from one surface to another
