Natural convection of multi-walled carbon nanotube-Fe.sub.3O.sub.4/water magnetic hybrid nanofluid flowing in porous medium considering the impacts of magnetic field-dependent viscosity

The study ahead deals with the natural convection of MWCN-Fe.sub.3O.sub.4/water magnetic hybrid nanofluid flowing in a porous medium. The flow domain is affected by an inclined magnetic field influencing the dynamic viscosity. The dependency of the flow and heat transfer characteristics on Rayleigh number, Ra = 10.sup.3-10.sup.6; Hartman number, Ha = 0-50; inclination angle of the magnetic field, [phi] = 0°-180°; magnetic number, [delta].sub.0 = 0-2.0; porosity, [epsilon] = 0.1-0.9; Darcy number, Da = 10.sup.-7-10.sup.-1;and volume fraction of the composite nanoparticles, [phi] = 0, 0.1 and 0.3% is studied numerically. At low Rayleigh number Ra = 10.sup.4, dispersing the nanocomposite particles increases the average Nusselt number Nu.sub.avg, while that decreases the Nu.sub.avg when Ra = 10.sup.5 and 10.sup.6. The dependency of viscosity on the magnetic field decreases the Nu.sub.avg at 0° < [phi] < 135°, which is due to an increase in overall viscosity of the nanofluid. After that ([phi] [greater than or equal to] 135°), the average Nusselt number is greatly enhanced by increasing [phi] from 135° up to 180°. There is no meaningful change in average Nusselt number of the hybrid nanofluid by increasing the inclination angle of magnetic field in the absence of magnetic field-dependent viscosity ([delta].sub.0 = 0).