Taking advantage of or preventing convective heat transfer
Now that we have talked about some ways convection works in our
natural environment, what are some ways that we take advantage of
convection in designs?
One way that we try to prevent convection is by air sealing our
buildings. Because of the temperature difference between outside air
and inside air, thermal convection can occur, and a windy day
outside can amplify the thermal convection. By air sealing
buildings, we can minimize the thermal convection into or out of
buildings, and thereby cut cooling or heating costs, prevent
moisture problems, and get more control over the indoor environment.
There is a point where there is too much air sealing, when the
ventilation in a building is inadequate for health purposes. In
general however, it is beneficial for people to air seal their
buildings to reduce energy costs.
Another way that we try to prevent convection is in our building
insulation. Back on the conventional insulation page, I brought up
the question of if air is such a great insulator, why don't we just
fill our insulating spaces with air? One obvious reason is that it
would be very difficult to seal the air in the insulating area, but
this could be overcome with large bags of air or some other way to
contain the air. However, this solution doesn't prevent thermal
convection between the two insulation walls. The air on one side of
the insulation will be relatively warmer than the air on the other
side of the insulation, which creates a temperature difference that
can cause thermal convection inside the insulation. If we have no
solids in the insulation, there will be a convective current in the
insulation that will decrease the effectiveness of air as an
insulator. Most of the effective conventional insulators that I
mentioned, like mineral wool, areogel, and polystyrene have a small
pore size that prevents the movement of fluids in the insulator.
This forces thermal energy to transfer via convection rather than by
convection, which makes rate of heat transfer across insulation much
closer to conductive heat transfer without the influence of
convective heat transfer.
Photomicrograph of 4
m
polystyrene particles.
Promkotra, Sarunya. (2004). MICROMECHANICAL TESTING OF
TWO-DIMENSIONAL AGGREGATED SUSPENSIONS OF COLLOIDAL PARTICLES.
10.13140/RG.2.2.24589.95202.
Now that we have talked about preventing convection, we will discuss
how some engineering designs harness thermal convection to increase
heat transfer rates. One that we have already mentioned is the heat
sink that is used in cooling computer systems. As already stated,
the heat sink will be designed with a high surface area to increase
the the rate of conductive heat transfer to the surrounds, which is
usually air. However, the rate of conductive heat transfer decreases
with a decreasing temperature gradient, so keeping the temperature
gradient between the heat sink and the air as high as possible is
ideal. The main way that the temperature gradient is maintained is
with convection. Hot air from the heat sink will naturally dissipate
with natural convection processes, but this is often not fast enough
for many heat transfer needs. In these cases forced convection will
be used to increase the fluid movement through the heat sink and
maintain the temperature gradient. By using forced convection to
increase the rate of heat transfer from the heat sink to its
environment, a heat sink system can effectively control the
temperature of computer systems and other machines.
Heat sink and fan used to increase the cooling rate of the
computer.
By fir0002flagstaffotos [at] gmail.comCanon 20D + Canon 70-200mm
f/2.8 L - Own work, GFDL 1.2,
https://commons.wikimedia.org/w/index.php?curid=1403949
There are many ways that heat transfer via convection is taken
advantage of in our lives. Once we know how convective heat transfer
works, it is easier to see the design considerations that have been
utilized to take advantage of or prevent convection. The examples I
have discussed are just a few that exist in our daily lives, but
many more are out there, so keep an eye out and more convective heat
transfer systems will appear.
