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.