Investigation of permeability and porosity effects on the slip velocity and convection heat transfer rate of Fe3O4/water nanofluid flow in a microchannel while its lower half filled by a porous medium

In this study, we numerically investigated the forced heat-transfer and laminar flow in a two-dimensional microchannel whose lower half was filled with a porous medium. The nanoparticles used were Fe3O4 and a water-based fluid. The nanoparticles were considered in the form of a completely stable suspension in a water-based fluid. The nanofluid flow in this microchannel was modeled employing the Darcy-Forchheimer equation. We also hypothesized that there was a thermal equilibrium between the solid phase and nanofluid for energy transfer. And the walls of the microchannels were assumed at a constant temperature higher than the inlet fluid temperature. Also, the slip boundary condition was assumed along the walls. The effects of Darcy number, porosity and slip coefficients, and Reynolds number on the velocity and temperature profiles, and local Nusselt number were studied in both porous and non-porous regions in this research. In this study, the Darcy number was assumed to be Da = 0.1 and 0.01, Reynolds number Re = 25, 50, and 100, slip coefficient B = 0.1, 0.01, and 0.001, the porosity of the porous medium = 0.5 and 0.9, and the volume percentage of the nanoparticles phi = 0%, 2%, and 4%. With the Darcy number decreasing, the local Nusselt number increased in the non-porous region, and decreased in the porous region. And this phenomenon was observed for the first time. The increase in the Reynolds number increased the heat transfer in both regions. For instance, the local Nusselt number increased 4 times with the Reynolds number changing from 25 to 100 under the same conditions. The decreased Darcy number in the porous medium increased the amount of slip velocity near the walls in the non-porous region, and on the other hand, the decreased Darcy number in the porous medium reduced the slip velocity in the porous region. Also, the jump observed in the slip velocity, was due to the presence of the fluid velocity in the microchannel width. (C) 2017 Elsevier Ltd. All rights reserved.