: In this paper, for the first time, the effects of both constant and alternating magnetic fields on the laminar forced convection heat transfer of a hybrid nanofluid containing tetramethylammonium hydroxide (TMAH) coated Fe3O4 nanoparticles and gum arabic (GA) coated carbon nanotubes (CNTs) flowing through a heated tube were investigated experimentally and Numerically. The experiments were carried out over wide range of parameters such as Reynolds number (548-2190), volume fractions of Fe3O4 (0.5%-0.9%) and carbon nanotube (0.25%-1.35%) nanoparticles, magnetic field strength (300-700 Gauss) and alternating magnetic field frequency (10-50 Hz). The experimental results showed a good consistency with the obtained numerical values. In present study, the experimental observations in the case without magnetic field revealed that by using Fe3O4/CNT hybrid nanofluid, the convective heat transfer has been improved significantly. The maximum enhancement of 62.7% was achieved in the local Nusselt number for hybrid nanofluid containing 0.5 volume percent Fe3O4 and 1.35 volume percent CNT at Reynolds number equals to 2190. Additionally, the results showed that the heat transfer of the studied hybrid nanofluids has been improved in the presence of constant and alternating magnetic fields and the amount of heat transfer increment due to a constant magnetic field was much more significant compared with an alternating magnetic field. Moreover, the effects of magnetic field were more noticeable in the hybrid nanofluids with higher volume concentrations and lower Reynolds number. Eventually, the highest increment of 20.5% in comparison with the case without field was reported in the local Nusselt number for hybrid nanofluid containing 0.5 volume percent Fe3O4 and 1.35 volume percent CNT at Reynolds number equals to 548. Key words: ferrofluid, carbon nanotubes, convective heat transfer, alternating magnetic field
