Numerical study of nano-enhanced PCM and oxytactic microorganisms in a micropolar magneto-bioconvective flow within a finned complex cavity

Purpose This study examines magnetohydrothermal bioconvective heat transfer in a cavity filled with micropolar nano-enhanced phase change materials (NEPCMs) and oxytactic microorganisms, incorporating a rectangular fin. This study aims to quantify how magnetic fields, micropolar effects and microorganism motility collectively influence thermal performance and flux patterns, with applications in thermal energy storage and polymer processing. Design/methodology/approach A Galerkin-based finite element method solves the governing equations for fluid flow, heat transfer and mass transport. The model incorporates buoyancy-driven convection, micropolar fluid dynamics and microorganism motility under varying Hartmann (Ha), Lewis (Le) and Peclet (Pe) numbers. Parametric analyses assess the impact of baffle length, magnetic field strength and diffusion properties on thermal and hydrodynamic performance. Findings Key results include: Increasing rotation rate (Γ) suppresses large-scale circulation by up to 82% at low Ra, with damping weakening as buoyancy dominates; Hartmann numbers >50 reduce flow circulation by 35%–57%, localizing streamlines; Lewis numbers >5 decrease Sherwood numbers by 24.1% because of suppressed mass diffusion; baffle lengths >0.3L improve temperature uniformity by 15%. The Nusselt number shows marginal sensitivity to Pe (0.5% reduction for Pe=0.1→1) but strong dependence on Le (24.1% reduction for Le=1→10). Originality/value This work pioneers the coupled analysis of micropolar nano-enhanced phase change materials, oxytactic microorganisms and non-uniform magnetic fields in finned cavities. The quantitative correlations between Ha, Le, Pe and baffle geometry provide actionable insights for optimizing industrial systems, achieving up to 40% thermal performance gains in energy storage and polymer processing applications.