Comparative study on thermal performance of Fe ₃ O ₄ ferrofluids with cylindrical and spherical nanoparticles: Chain‐like structure and heat transfer characteristics

Based on the analogy approach between permittivity and thermal conductivity, an equation was derived to consider the effect of spherical and cylindrical magnetic Fe 3 O 4 nanoparticles shape factor on the effective thermal conductivity of a Fe 3 O 4 ferrofluid caused by chain‐like structure (FHD effect). Following this, a comparative study was done between the thermal performance of Fe 3 O 4 ferrofluids, containing spherical and cylindrical Fe 3 O 4 magnetic nanoparticles, when flowed as a hot flow in a mini channel under different magnetic field intensities, nanoparticles concentration and flow conditions. Finally, RSM optimization was recruited to investigate the optimum conditions. The results revealed that cylindrical shape Fe 3 O 4 magnetic nanoparticles can form chain‐like structure easier than spherical nanoparticles. Consequently, cylindrical magnetic Fe 3 O 4 nanoparticles enhance the effective thermal conductivity of ferrofluid more than spherical shape nanoparticles do. Also, for both nanoparticle shapes there is a saturation magnetic field for FHD effect such that as the magnetic field intensity increases, the effective thermal conductivity of ferrofluid initially rises sharply to a maximum value and then remains almost constant. For both nanoparticle shapes an increase in nanoparticle concentration raises the effective thermal conductivity. However, in a fixed concentration of nanoparticles, cylindrical shape nanoparticles have a higher impact on the enhancement of effective thermal conductivity than spherical shape. Finally, comparison of the overall heat transfer coefficients (OHTC) in a mini‐channel with Fe 3 O 4 –water ferrofluid as the hot flow indicates that the magnetic field enhances OHTC for both spherical and cylindrical Fe 3 O 4 nanoparticles, with a greater effect observed for the cylindrical shape.