Experimental investigation of natural convection heat transfer in a Fe 3 O 4 –water ferrofluid within a closed enclosure under fixed and variable magnetic fields

In light of the rising demand for thermal and refrigeration systems with enhanced heat transfer mechanism, natural convention of ferrofluidic under the influence of a magnetic field is of significant importance. Although previous studies have investigated ferrofluids in enclosed cavities, direct experimental comparisons among zero-field, constant magnetic field, and stepwise time-varying magnetic field conditions remain limited, particularly for single-component Fe 3 O 4 –water ferrofluid that simultaneously undergoes under a fixed heat flux at hot wall and a constant cold-wall temperature. The novelty of this study lies in the controlled experimental comparison of ferrofluid concentration, magnetic-field mode and magnetic excitation intensity under identical thermal boundary conditions. In this study, the natural convection behavior of the base fluid and the Water-Fe 3 O 4 Ferrofluid under constant and variable magnetic fields in a close chamber was assessed. For this purpose, an experimental setup was constructed and the thermal performance of the samples was evaluated through measurement of the natural conversation coefficient, analysis of the Nusselt and Rayleigh numbers and assessment of magnetic effect intensity. The results revealed that in the absence of magnetic field, adding nanoparticles and increasing the ferrofluidic concentration resulted in reduction in convective heat transfer coefficient and the Nusselt number. This behavior stemmed from increased viscosity and suppression of buoyancy-driven fluid motion. For the base fluid, both magnetic field modes enhanced heat transfer compared to the zero-field case. The constant magnetic field produced a greater enhancement than the stepwise time-varying field. In contrast, for Fe 3 O 4 -water ferrofluid, applying both magnetic fields resulted in a reduction in heat transfer. Although the time-varying field caused a smaller deterioration than the constant field, under tested conditions, increasing magnetic excitation intensity systematically reduced the convective heat transfer coefficient. It also weakened the Nusselt-number response of the ferrofluid, indicating suppression of buoyancy-driven circulation by magnetic-field-induced flow resistance. Therefore, the results of this study revealed that the thermal performance of ferrofluidic in natural convection is not solely dependent on the presence of a magnetic field, but also it depends on concentration interaction, fluidic viscosity, magnetic type and its intensity. Therefore, the simultaneous optimum selection of magnetic field condition and fluid composition for improving the thermal behavior of these systems is essential.