Numerical investigation of flow characteristics, heat transfer and entropy generation of nanofluid flow inside an annular pipe partially or completely filled with porous media using two-phase mixture model

Simultaneous application of nanoparticles and porous media to enhance heat transfer inside an annulus is investigated numerically. Two-phase mixture model along with Darcy-Brinkman-Forchheimer relation has been implemented for nanofluid flow simulation in porous media. Different configurations consisting of various porous layer thicknesses, porous layer positions (at inner/outer wall of the annulus), and its permeabilities are analyzed as a function of nanoparticle concentrations and Reynolds from the view point of the first and the second laws of thermodynamics. A new PN (performance number) - defined as the ratio of enhanced heat transfer to pressure loss is introduced to better judge the first law's performance of configurations. Results showed that the configuration's parameters, nanoparticles concentration and Reynolds number have considerable effects on both the performance and entropy generation numbers. For configurations with high permeabilities (Da = 0.1, 0.01), PN has an increasing trend with porous layer thickness; while for configurations with low permeabilities (Da = 0.0001), PN has a decreasing trend with porous layer thickness; and for configurations with a moderate permeability (Da = 0.001), an optimum thickness corresponds to PN. The study of the second law also reveals that an optimum porous media thickness exists for each nanofluid flowing in a porous medium at a specific Reynolds. (C) 2015 Elsevier Ltd. All rights reserved.