Effect of the fluid movement on heat exchanges
After having seen in the preceding article how conduction could
move heat, we now will see how this "displacement" can be improved,
precisely by setting in motion a material which will absorb and
transport heat : the fluid, either water or air.
This is what we call thermal convection.
We will subsequently consider the case where fluid movement is
forced, i.e. it has been set in motion by an independent component
(a fan or a pump). That's forced convection, as opposed to natural
convection (which is much less effective) for which the fluid is
set in motion by Archimedes's Principle (the heated fluid is less
dense and rises like a balloon).
As for conduction, we will express heat exchange through the concept
of thermal resistance, i.e. in the form P
= R DT
, where DT
(pronounce delta T
) is a temperature difference which will be specified later.
Thermal resistance : additional details
Some physical laws (Laws of Conservation which express the fact
that something is preserved, the thermal power here) are very simple
when applied to one-dimensional physical situations (1D, they can
be also 2D or 3D). These laws reveal a parameter (R
here) which is independent of the value of certain variables
which describe a physical phenomenon ( DT
and P
here). In a real situation, thus 3D, one will seek to express
the same laws in the same simple form. It is an approximation in
fact, which will be as good as the phenomenon is more significant
in one direction (dimension) compared to the others. Despite this
fact, thermal resistance is nevertheless a reliable concept and
is largely used. When this concept cannot be used, things are immediately
getting infinitely more complicated and necessitate experimental
methods and numerical simulations.
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