Effects of magnetic field on mixed convection heat transfer and entropy generation of Cu-water nanofluid in a trapezoidal enclosure

In this study، the effects of magnetic field on the flow field، heat transfer and entropy generation of Cu-water nanofluid mixed convection in a trapezoidal enclosure have been investigated، numerically. The side walls of the cavity are insulated، the top lid is cold and moving toward right or left and bottom wall is hot and the side walls angle from the horizon is 45˚. The results showed that with imposing the magnetic field and enhancing it، the nanofluid convection and the strength of flow decrease and the flow tends toward natural convection and finally toward pure conduction. For this reason، for all of the considered Reynolds numbers and volume fractions، by increasing the Hartmann number the average Nusselt number decreases. Furthermore، for any case with constant Reynolds and Hartmann numbers by increasing the volume fraction of nanoparticles the maximum stream function decreases. For all of the studied cases، entropy generation due to friction is negligible and the total entropy generation is mainly due to irreversibility associated with heat transfer and variation of the total entropy generation with Hartmann number is similar to that of the average Nusselt number. With change in lid movement direction at Reynolds number of 30 the average Nusselt number and total entropy generation are changed، but at Reynolds number of 1000 it has a negligible effect.