Comparative methods in convective heat transfer enhancement by nanofluids entropy generation

Heat transfer enhancement technologies are widely adopted in several industrial applications; thus, this topic covers an important role also in the research field. Solutions are applied from refrigeration to aerospace industry, solar energy collectors, nuclear systems and so on in order to develop thermal devices, featured by high efficiency, reduced size, low costs, and significant energy save. In this way, passive or active methods can be adopted. Methods involving special surfaces like extended, coated, and rough surfaces, vortex generators, inserts, or additives in the working fluids are grouped in the “passive” techniques. Jets, systems involving surface or fluid vibrations, electrostatic fields, and so on, are listed in the “active method group,” which requires external supplies of energy; it should be underlined that nanofluids can be employed directly as a passive technique and in active techniques. A comparative analysis with different methods is given by means of different techniques related to the first and second law of thermodynamics. The first law analysis is accomplished by the Nusselt number, the friction factor, the Euler number, and the performance evaluation criterion (PEC), and this last coefficient can be defined in two ways, as will be shown. The second law analysis is performed by the entropy generation method (EGM). In the present work, the second law analysis by the EGM is employed in order to obtain comparisons to carry out some optimal configurations for forced convection in ducts and tubes with different thermal boundary conditions.