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

Abstract 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.