Numerical study of magnetohydrodynamic mixed convection and entropy generation of Al_2O_3-water nanofluid in a channel with two facing cavities with discrete heating

摘要

In this work, transient numerical simulations are carried out to investigate the effect of alumina nanoparticles with pure water as a base fluid on mixed convection with magnetohydrodynamic flow in a vertical channel with two facing identical open cubic cavities with discrete heating. The nanofluids are modeled using a single phase approach and the fluid properties are considered constant with temperature. The left and right vertical walls of the cavities are isothermal, all other bounding walls of the cavity and the channel are adiabatic, and a uniform magnetic field is applied in the horizontal direction. The governing Navier-Stokes equations in vorticity and stream function form coupled with the energy equation are solved using the control volume method on a nonuniform orthogonal Cartesian grid. A parametric study has been carried out for three different Hartmann numbers of (Ha = 0; 5; 10) Richardson numbers of (Ri = -1; 5), nanoparticle volume fractions of (phi = 0.0; 0.1; 0.2) and Reynolds number ranging from 300 to 700. The effects of the nanoparticle volume fraction and magnetic field on hydrodynamic and thermal characteristics and entropy generation for assisting/opposing buoyancy have been assessed. In general, it has been found that in the range of parameters considered in this study, the entropy generation is dominated by irreversibilities due to heat transfer for all values of phi. The results show that the vortex dynamics, heat transfer characteristics and the magnitude of irreversibilities in the entropy generation are strongly affected by the strength of the magnetic field applied and the nanoparticle volume fraction. Moreover, these results suggest that the modulation effect of the applied magnetic field can play an important role in practical applications for entropy generation minimization.