How to maximize heat loss with the three methods of heat transfer
If we want to maximize the rate of heat transfer instead of reduce
it, how might we do so? While specific needs may vary, this should
give a summary of the three methods of heat transfer that we have
talked about and their relationship to heat loss.
First, how can we take advantage of conduction to increase the rate
of heat transfer? Choosing a material that has a high conductivity
like aluminum is a great start. This will decrease the time that it
takes for heat to travel across the material. Additionally, we could
change the shape of what material we are conducting heat through. We
could decrease the thickness that the heat has to travel across, and
also increase our conductive material's surface area to increase the
rate of heat transfer from the material to the environment. These
factors are largely what guides the designs of heat sinks. They need
to distribute a significant amount of heat from their heat source to
their environment, so they are often designed as a metal, somewhat
thin, and fan-like so that it can easily distribute heat to its
environment.
Now how can we use heat transfer via convection to increase the rate
of heat transfer? If you are familiar with how you will feel colder
on a windy day even though the air temperature is the same, you
probably know how this works. Moving cooler fluids through an area
where otherwise they would be stagnant increases the temperature
gradient between the surface and the fluid to maximize the rate of
heat transfer. The natural buoyancy that comes from warmer fluids
will help with convective heat transfer as cooler fluids come in to
replace it, we call this natural convection, but sometimes, this
isn't enough for our cooling needs. Instead, we will use forced
convection, where we increase the amount of fluids that are flowing
across our surface to increase convective heat transfer. By taking
advantage of moving fluids, we can increase the rate of heat
transfer away from our surface to the environment.
Lastly, how can we use heat transfer via radiation to increase heat
loss? Well, based on the Stefan-Boltzmann Law, we can increase the
rate of heat transfer by increasing the surface area, increasing the
temperature, or increasing the emissivity. Increasing the
temperature is out of the question for maximizing heat loss, but we
can increase the surface area and the emissivity. When increasing
the surface area, we do need to be careful to make sure that emitted
electromagnetic radiation isn't just re-absorbed by the parallel
surface area fins. Also, we can increase the rate of heat transfer
by covering the surface with a material that has a high emissivity
coefficient.
To put these three mechanisms of heat transfer together, how can we
maximize heat loss? By using a material with a high conductivity,
increasing the surface area between the surface and it's
environment, using the forced movement of fluids to increase
convective heat transfer, and coating our surface with a
high-emissivity material, we can maximize the rate of heat transfer
from our surface to its environment. We won't particularly worry
about the high surface area reducing the rate of heat transfer via
radiation, because they are often insignificant compared to the
benefits we get from the increased rate of heat transfer via
conduction. Again, what strategies we should employ to increase the
rate of heat transfer for an object will vary depending on the
conditions of said object, but this summary should give a good
overview of what design considerations to take.
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