In this study, free convection heat transfer of the magnetic nanofluid ( water) in enclosed vessle under magnetic field was investigated, experimentally. The effect of nanofluid with different concentrations (0-0.7vol.%) on free convective heat transfer coefficient in different Rayleigh numbers (10 6 Ra 10 8 ) was examined. Results showed that heat transfer coefficient increases with increasing in concentration of nanofluids and reaches to an optimum at0.05vol.%. It was also found that increasing in Rayleigh number causes to enhancement of heat transfer rate. Applying the fixed magnetic field results in decreament of the heat transfer coefficient. With increasing in Rayleigh number the effect of magnetic field o heat transfer coefficient is decreased. Free convection heat transfer coefficient under constant magnetic field (410 gauss) at 0.05vol.% in two dirrection was examined; parallel and perpendicular to the hot and cold surface. Reduction effect on heat transfer coefficient in parallel mode is lower than that in perpendicular mode. Also, it is seen changes of direction of magnetic field by replacing the poles of the magnet in one direction, there is no change in the free convection heat transfer coefficient. Influence of variable magnetic field on free convection heat transfer was compared with fixed magnetic field. Results showed that the effect of fixed magnetic field on decreament of heat transfer is more than that for variable magnetic filed.. Free convection heat transfer coefficient at different angles of inclosed vessle based on direction of hot surface (0, 30, 90 and 120 degrees), in presence and absence of the magnetic field were investigated. Increasing in vessele angle results in decreasing of heat transfer coefficient. Also, it was seen the effect of magnetic filed on heat transfer in lower inclination of vessel is more profound. KEY WORDS:F ree convection heat transfer, magnetic nanofluid , magnetic field, Nusselt number, Rayleigh number
