In this paper, the effect of porosity and variable magnetic field (VMF) on the natural convection heat transfer of Fe 3 O 4 -water nanofluid is studied in a wavy channel including hot tubes using the finite element method by FlexPDE commercial code. The upper and lower surfaces of the channel are wavy shaped and in low temperature and three hot tubes were located in the channel by equal distances. In this study, the effects of five main parameters (Darcy number (Da), Hartmann Number (Ha), nanoparticles volume fraction (ϕ), Rayleigh number (Ra) and magnetic source location (a)) on the local and average Nusselt numbers of the tube center and the wavy wall are investigated. As a main outcome, results confirm that the nanoparticles volume fraction plays the most important role on the Nusselt number among other parameters.

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