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Flow nature of a fluid
The flow regime can be either laminar or turbulent,
depending, for a given fluid, on the cooling system geometry in
contact with the fluid and on the fluid velocity.
For a given cooling system, the flow will become turbulent beyond
a certain velocity of the fluid and it will be more especially as
velocity increases.
The flow is laminar when it is possible to identify streamlines
in the fluid which will slip one on each others ( caution ! not
without friction) ; for instance, by injecting dye in the flow,
we would see it moving gently while becoming slightly deformed .
A flow is turbulent when it is no more laminar, that is to say we
cannot identify these streamlines any more ; the dye is diffused,
mixed, chaotic, nothing easily identifiable except the presence
of vortices.
This latter is one of the turbulence characteristics, which shows
a whole set of vortices having very different size. This characteristic
practically results in one of the properties of turbulence which
concerns us in heat exchange : the mixing, we will
return again to this topic ...
The following picture shows the differences between laminar and
turbulent flow, for the latter the dye is much more mixed and homogenized.

Photo from the book " An album of fluid
motion, Van Dyke, The Parabolic Press "
Now that we have in mind how a fluid is characterized, it is still
necessary to dig a little more to manage to understand how the convection
coefficient h behaves. In what follows, we will see what
happens to the fluid close to the solid's surface, as it is here
where everything of interest will occur. This zone bears the sweet
name of the boundary layer.
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