Heat Transfer and Flow Dynamics for Natural Convection in Fe3O4/H2O Nanofluid

Fe3O4/H2O nanofluid attracts many researchers’ attention due to its considerable potential for practical applications. This work is focused on the study of heat transfer efficiency in Fe3O4/H2O nanofluids with nanoparticles (NPs) of mean diameter dNPs in the nanosized range (13–50 nm) at volume fractions up to 2%. The Rayleigh–Bénard problem of free convection between plane-parallel plates corresponding to Rayleigh numbers 103–107 is numerically solved. It was shown that the addition of up to 2% of NPs with a diameter of 13 nm can increase the Prandtl number by up to 60% compared to pure water. A map of flow regimes is constructed, indicating the emerging convective patterns. It is demonstrated that as the volume fraction of NPs increases, the Prandtl number grows and the transition to more chaotic patterns with Rayleigh number slows down. It is observed that at a Rayleigh number of 104, the heat flux through the nanofluid layer decreases by up to 25% relative to pure water. Conversely, at Ra ≈ 105, the heat flux through the nanofluid layer increases by up to 18% when using a 2% volume fraction of 13 nm diameter NPs.