Microstructural and vorticity analysis of micropolar nanofluid in pentagonal cavity for industrial thermal and mixing applications

The industrial and technological processes of electronic cooling and micro-heat exchangers and polymer processing and solar thermal devices depend on efficient systems that manage heat and fluid mixing. This study numerically investigates natural convection of carboxymethyl cellulose (CMC)–Al2O3 micropolar nanofluid in a pentagonal cavity using a vertex-centered control volume finite element method. The effects of the Rayleigh number (103≤Ra≤106), the Hartmann number (0≤Ha≤40), the micropolar coupling number (0≤Λ≤4), and nanoparticle volume fraction (0≤φ≤0.05) on flow behavior, heat transfer, entropy generation, and nonlinear dynamics are systematically examined. Performance is evaluated through the average Nusselt number, wall shear stress, couple stress, fluid temperature, entropy generation, and Bejan number. The results indicate that increasing the Rayleigh number strengthens buoyancy-driven convection, producing a transition from steady symmetric flow to asymmetric, periodic, quasi-periodic, and chaotic regimes, while enhancing the average Nusselt number by 172%. Conversely, the induced magnetic field suppresses convection, reducing the average Nusselt number and kinetic energy by 29.6% and 88.7%, respectively. A higher micropolar coupling number weakens microrotation and heat transfer, resulting in a 28.6% reduction in the average Nusselt number and lower entropy generation. Response surface methodology and artificial neural network models accurately predicted heat transfer performance (R2 = 0.9786), providing an efficient optimization framework. These findings provide quantitative insights for optimizing micropolar nanofluid-based thermal systems and offer practical guidance for the design of advanced heat transfer devices.