Entropy generation analysis of different nanofluid flows in the space between two concentric horizontal pipes in the presence of magnetic field: Single-phase and two-phase approaches

In this paper, entropy generation analysis of different nanofluid flows in the space between two concentric horizontal pipes in the presence of magnetic field by using of single-phase and two-phase approaches was carried out. Single-phase model and two-phase model (mixture) are utilized to model the flow and heat transfer for Newtonian nanofluids in the space between two concentric horizontal tubes subjected to the magnetic field. The Reynolds and Hartman numbers ranges are 500 <= Re <= 1500 and 0 <= Ha <= 20, respectively. In this study, heat transfer of various nanofluids (Al2O3, TiO2, ZnO and SiO2) and their entropy generation have been investigated. The effect of diameter of particles (water-Al2O3 nanofluid) on heat transfer and entropy generation has also been studied. Average Nusselt number in terms of Hartman number and Reynolds number for different nanofluids for single-phase and two-phase models in various volume fractions, entropy generation due to friction, magnet and heat transfer in terms of radial direction for different Hartman numbers, Reynolds number and different nanofluids with different diameter of particles were obtained. We found that in all states, the Nusselt number is higher in twophase model than in single-phase model. The maximum pressure difference for single- and two-phase models occurs at maximum volume fractions and Hartman number. Also, as the diameter of the nanoparticle increases, the result will be an increase in the temperature of the walls, leading to an increase in entropy generation. Also, as the Hartman number increases, the amount of entropy generation increases. (C) 2018 Elsevier Ltd. All rights reserved.