Effect of the cooling system on the fluid movement

As we seen before, the total thermal resistance of the cooler will vary with the flow rate, this is mainly due to convection through R_cv . Therefore, when performance data is defined, it is necessary to specify the thermal resistance with respect to the flow rate. In fact this only applies to waterblocks as for air cooled heatsinks, which are generally sold with a fan, the flow rate is fixed. The effect of the cooling system on the fluid movement is only related to waterblocks.

Why be concerned with this ?

Imagine two different waterblocks which have exactly the same thermal performance (same resistance R) at any flow rate ! How to decide between them ?

Because of fluid friction on the waterblock's surface, of the internal circuit layout (curve, elbows...) and because of the variations in cross section (input, output ...), a considerable part of the power that the pump provides to the fluid will be lost. The consequence is a flow decreased by the presence of the waterblock, and thus less effective convection heat transfer !

These losses, which are reflected in the flow rate, are measured with what is called the head loss. In practice, we will thus look for a waterblock which causes less pressure loss, and this will enable us to decide between two waterblocks having same thermal resistance.

Thermal resistance and pressure losses are the two inseparable and indispensable means of performance measurement of any waterblock.

As the pressure losses and their influence on the fluid circuit will also depend on all parts of the circuit, I reference you to the next article which will deal with all this.

Finally, the thermal radiation

Another type of heat transfer with conduction and the convection, the thermal radiation doesn't need any material, solid or fluid, to exist. Even in a vacuum heat can be exchanged by radiation.

How does it work ?

Thermal radiation is closely associated to electromagnetic radiation, which can be described as material energy (heat, ...) converted into light energy "type" and conversely. Light here does not mean necessary visible, thermal radiation also occurs in the infra-red spectrum.
This type of heat transfer will depend on the temperature and type of materials of the various elements considered. In the case which concerns us, the radiation of a given waterblock or heatsink will thus depend on the remainder of the environment which it "sees" (case, psu, motherboard ...).

In fact, relative to the other types of heat transfer, radiation will be significant only when there exist significant differences in temperatures between these elements. Sun (5900 °C) and fire (2800 °C) heat by thermal radiation, in these two examples the temperature are significantly different compared to the ambient temperature.
The good news here is that concerning CPU cooling, the heat transfer by radiation is negligible, thus let's ignore it ...

Voila ! that's all folks, while waiting for the next part about pressure losses and influence of the fluid circuit. I wish to thank Deejeecee, Christophe, Rosco, Karamilo and Salmatt for their returns on the preliminary versions of this article, and Bill Adams for his "polish" on the english translation.