Entropy generation analysis of laminar flow of a nanofluid in a circular tube immersed in an isothermal external fluid

•Entropy generation of nanofluids under isothermal external fluid is studied.•Water–Al2O3 and ethylene glycol–Al2O3 are two nanofluids used.•Nanoparticles addition decreases the entropy generation by heat transfer.•Nanoparticles addition increases the entropy generation by fluid friction.•Nanoparticles addition is recommended for less viscous fluids at low Reynolds number.

This paper is an analytical study of entropy generation in the laminar flow of nanofluids in a circular tube. The tube is immersed in an isothermal external fluid – which is the most general thermal boundary condition but has not been studied in much detail in literature. Two nanofluids, namely – water–Al2O3 and ethylene glycol–Al2O3 have been chosen for this study. The effects of the external Biot number, non-dimensional temperature difference and volume fraction on the entropy generation characteristics of the flow have been shown through graphs and the physical reasoning behind the observed trends has been discussed threadbare. It is shown that the addition of nanoparticles is beneficial only at smaller Reynolds number and for less viscous base fluids. Most importantly, it is proved that the entropy generated in the case of a tube immersed in an isothermal external fluid is bounded by those for uniform heat flux and uniform wall temperature boundary conditions.