The heat fluxes transferred between systems, or between parts of the same system, are governed by a set of equations which take into account the thermal potential difference (TPD), the importance of the transfer areas concerned and the physicochemical properties of the examined flows or solids.
These equations are known as heat transfer laws. They make it possible to evaluate the energy fluxes and the flux densities involved in a given heat exchange. They also make it possible to determine the temperature profiles in the systems studied.
In this chapter, we analyze the different physical processes through which heat transfer occurs. This analysis is conducted with a view to elaborating the equations and laws expressing the heat fluxes induced by the different transfer mechanisms.
2.2. Mechanism and law of conduction
We already know that heat can spread according to three physical mechanisms or modes of transfer: conduction, convection and radiation.
In conduction, energy exchange occurs through contact-continuity between the mass particles; the particles directly in contact with heat get excited and transmit this excitation through their contact with adjacent particles.
Consider, for example, the case of a metal bar heated at one end by an electrical resistance (see Figure 2.1).
Mass particles that are in direct contact with the electrical resistance (end 1) receive a significant amount of heat.