Effects of temperature difference and magnetic field intensity on heat transfer patterns of nanofluids

The numerical study is to investigate a complex interaction between heat conduction and natural convection of water-Al2O3 nanofluids in a cavity affected by an external magnetic field. Effects of the governing parameters, including Hartmann number (Ha), nanoparticle volume fraction (NVF), and temperature difference of two side walls Delta T, on heat transfer behaviors are systematically examined. Based on various magnetic field intensities, three different heat transfer patterns can be observed and named Mode-A (Optimum case), Mode-B (Normal switching), and Mode-C (Monotonic increase), respectively. For weak magnetic fields, the dimensionless Nusselt number shows a maximal value at an optimal NVF where heat convection dominates, called Mode-A. As the magnetic field increases beyond the critical Ha number (Hacr), the heat transfer mechanism changes from a convection-dominated mode to a conduction-dominated one (Mode-C) because of the significant suppression of flow velocity. Moreover, we further explore the Delta T effect on heat transfer modes and find that increasing Delta T improves the heat conduction of nanofluids, resulting in the occurrence of Mode-C. The conditions for heat transfer transition (Mode-B) are also evaluated, and a trend line of Hacr is drawn to distinguish these three heat transfer patterns.