First principle of thermodynamics
Considering its most simplified version, it establishes the relationship
between :
- the thermal power P
absorbed by the fluid in contact with the solid
- the difference between the temperature at the output To and at the input Ti
of the cooler (hsf or wb)
- the volumetric flow rate D
(in m 3 / s)
- the specific heat of fluid c
- the density of the fluid r

The thermal power P
is proportional to all the remainder, and that gives :
P
and Ti
are given and do not depend on the cooler considered
(P
depends on the CPU and Ti
from what occurs before the input). We are looking
for the coldest solid surface, therefore the fluid temperature should
also be the coldest possible along its course.
Thus it is necessary to look for To
closest to Ti.
Writing this first principle of thermodynamic like that :

then a better flow rate D,
a better density r and a better
specific heat c is desired (actually the product of both).
Concerning flow rate, this joins what we saw thanks to the interpretation
with the Fourier's Law, i.e. the more it is, the more velocity also
is. With regard to the two remainders, density and specific heat,
it depends entirely on the choice of the fluid, since they are physical
properties.
The last thing in connection with this principle, let us compute
To
- Ti and
you will notice that this difference is rather small, generally
smaller than 1 °C for water with the usual flow rate of
a water cooling system.
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