Enhancing convective heat transfer in enclosures is vital for advanced cooling and energy-efficient thermal systems. Hybrid nanofluids, combining multiple nanoparticles, offer enhanced thermal conductivity and flow control compared to conventional fluids. This study investigates natural convection of a ZrO2–ZnO/water hybrid nanofluid inside a flower-shaped cavity. The unique cavity geometry, featuring dual circular cores, induces secondary vortices and complex flow structures that strongly influence heat transfer. The governing nonlinear equations for mass, momentum, energy, and concentration were formulated and nondimensionalized. Numerical simulations were performed using the Finite Element Method (FEM) implemented in COMSOL Multiphysics, with grid dependency tests ensuring accuracy. Parametric studies explored the effects of Prandtl (Pr), Grashof (Gr), Rayleigh (Ra), Hartmann (Ha), Reynolds (Re), and Lewis (Le) numbers on velocity, temperature, and concentration fields. The hybrid nanofluid significantly enhanced thermal convection compared to the base fluid, with Nusselt number improvements up to 45%. Critical Ra marked the onset of strong cavity-induced vortices, while increasing Ha suppressed secondary flows due to Lorentz forces. Higher nanoparticle concentration improved thermal conductivity but altered diffusivity, reflected in variations of Pr and Le. Graphical and tabulated results show the interplay of geometry and hybrid nanofluid properties, offering quantitative design insights for cooling and heat exchange systems.
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