MHD nanofluid free convection and entropy generation in porous enclosures with different conductivity ratios

MHD natural convection of Cu-water nanofluid in a square porous enclosure is investigated using a parallel LBM code, considering temperature dependence of viscosity and viscous dissipation. Effects of nanofluid concentration (phi = 0-0.12), Rayleigh (Ra = 10(3)-10(6)), Hartmann (Ha = 0-20) and porous-fluid thermal conductivity ratio (K* = 1-70) on heat transfer and entropy generation are investigated. It is shown that K* is a very important parameter, and porous media with low K* numbers can confine convection effects, but by increasing K* both conduction and convection effects can substantially improve. Also, magnetic field always has negative impact on Nu, however this impact can be controlled by phi and K*. A magnetic instability has also observed in Ra = 10(4), and Nu exhibits a sinusoidal variation with Ha. It is proved that, depending on K*, Ra and Ha values, use of nanofluid with porous media to enhance heat transfer can be either beneficial or detrimental. Also, for given K*, Ra and Ha numbers an optimal phi exists to improve heat transfer. Finally, entropy generation study performed and results state that in low and high Ra values the thermal and frictional entropy generation are respectively dominant, while for moderate Ra they have the same order of magnitude. (C) 2017 Elsevier B.V. All rights reserved.