**1. Introduction**

Heat transfer is the field in Thermodynamics in charge of the study of the generation, conversion, exchange and use of energy in form of heat (thermal energy) between different systems. Heat can be transferred using several mechanisms such as conduction, convection, and radiation. The proper understanding of these modes of heat transfer leads to a successful design of any device in which thermal energy is involved. That is why it is so important to study and to analyze heat transfer in any application.

Thermal energy is present in any aspect of our lives. We can daily experiment heat transfer phenomena at home, when cooking or taking food out of the fridge [1], when using a vehicle to go to work [2] or turning the heating or air-conditioning system on in our office [3]. Sometimes this thermal management is not evident but it is essential for our comfort and lifestyle [4]. Besides, heat transfer is of vital importance in all the electric power plants. Whether some fuel is being burnt to get steam or if we want to maintain a PV panel temperature to a certain level, again a proper management of thermal energy is crucial to the operation of the system [5, 6].

And nowadays it is even more important to perfectly control thermal transfer when designing renewable energy systems where every unit of energy requires to be used; or when installing an efficiency measure in a process to prevent thermal losses [7, 8].
