A little Theory
What is "heat flux" ?
The notion of heat flux, or more precisely “heat flux density” is not so easy to appreciate. Let's see why.
For simplicity, heat flux and heat flux density are often equated to the same quantity. However :
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A heat flux can be defined as the power (Watt) passing through a finite surface. Its unit is therefore the Watt ;
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A heat flux density can be defined as the power (Watt) passing through a unit of this area. Its unit is therefore Watt/m². It is this definition which is used and called by abuse of language "heat flux".
A medium, whether solid, liquid or gaseous, is crossed by a thermal flux from one side to the other when it is subjected to a thermal transfer on one of its boundaries. The flux is not necessarily constant, but is generally damped as one moves away from the heat exchange boundary.
For this reason, the heat flux that interests us is generally that at the very boundary of heat transfer. There then exists a relationship of continuity between the flux transported to this boundary (by conduction, convection or electromagnetic radiation) and the flux transmitted into the environment. The “true” flux propagated at the boundary is then given by Fourier’s law. We can summarize this explanation with the summary below:
Types of measurables transfers
It is important to emphasize that heat flux is the result of a combination of:
- well-defined generating conditions (temperature, emissivity, etc.), whether constant or variable over time;
- a propagation medium subjected to these generating conditions, a medium that possesses specific geometric (thickness) and thermal (conductivity, density, heat capacity) characteristics.
Therefore, given generating conditions, it is entirely possible to obtain different heat fluxes depending on the propagation medium. This is why some materials are used as thermal insulators (the heat flux through them is low, as in fiberglass) and others as heat sinks (such as copper in computer cooling devices). This may seem obvious, but when it comes to precision heat flux measurement in critical, high-value applications (nuclear power plant heat exchangers, solar concentrators, turbomachinery, etc.), the major consequence is this: there is no single, unambiguous heat flux measurement, meaning no value that can be considered exactly equal to and corresponding to the heat my process undergoes. And yes! Because the heat flux sensor inserted into the process being characterized does not have the same geometric and thermal characteristics as the process itself; otherwise, we would have to manufacture a sensor out of concrete or even wood...
With this in mind, it is necessary to approach heat flux measurement with caution: for some users, it is a relative measurement, allowing them to characterize energy transfer by modifying certain operating parameters (parametric analysis, optimization). For others, it is a reference measurement, knowing the characteristics of the sensor, allowing them to derive a transfer function to their system to obtain the "true flux" that passes through it.