Enhancing thermal performance in phase change material through nanoparticles and magnetic field in an octagonal cavity with plus-shaped fins

Abstract Enhancing thermal performance in phase change materials (PCMs) is critical for advancing thermal energy storage systems. Passive strategies, such as optimizing geometry and using nanoparticles, offer promising ways to enhance heat transfer and energy efficiency. This study examines a flow of non-Newtonian Casson nanofluid synthesized by sodium sulfate decahydrate PCM, water, borax stabilizer, and aluminum oxide (Al₂O₃) nanoparticles subjected to an external magnetic field in an optimized octagonal cavity with plus-shaped fin. Octagonal cavity is heated from below; the remaining walls of the enclosure are thermally insulated. The governing equations are solved numerically using the finite-element method (FEM). Simulations explored the effects of the Casson parameter $\beta $, Rayleigh number $Ra$, and Hartmann number $Ha$ Casson on flow structure, Nusselt number $( {Nu} )$, and mass Sherwood number $( {Sh} ).\ $ Results show $\beta $ and $Ha$ have competing influences. Lower $\beta $ enhanced convection, raising the mean Nusselt number by ∼55% versus large $\beta $, while high $Ha$ suppressed flow and heat transfer. $Ra$ was the dominant factor and increasing $Ra$ shifted the system to convection-dominated regime, strengthening vortices, and significantly improving thermal $( {Nu} )$ and solute $( {Sh} )$ transfer.