Buoyancy-driven natural convection produces spatial nonuniformities in local heat transfer. For Newtonian fluids, these effects are often simplified into averaged heat transfer measures in the design of passive and microscale thermal systems. In viscoelastic fluids, however, the coupling between geometry, elasticity, inertia, viscosity, and buoyancy further complicates the local heat transfer behavior. This study numerically investigates the role of viscoelasticity in natural convection under the assumption of temperature-independent fluid material properties confined within rectangular cavities of varying aspect ratios ($\text{AR}\mathrm{s}$) and Rayleigh numbers ($\text{Ra}$). Specifically, using a hybrid lattice Boltzmann method, this work demonstrates the complex interplay of $\text{AR}$ and viscoelastic effects on natural convective heat transfer performance in viscoelastic fluids. Generally, viscoelastic fluids show enhanced heat transfer compared to Newtonian fluids, which becomes more pronounced at high $\text{Ra}$ in nonunity $\text{AR}$ cavities, arising from the coupled effects of geometry and viscoelasticity. Moreover, simultaneously increasing the effective shear-dependent viscosity and elastic stress contributions within the FENE-P model reduces the average heat transfer while redistributing the local heat transfer, decreasing it in some regions and increasing it in others. This study provides new insight into understanding the enhancing and suppressive effects of elasticity on local heat transfer, which is crucial for the optimization of cavity geometries and polymer properties to improve convective performance.

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• Thermal convection
• Non-Newtonian fluids
• Viscoelasticity
• Lattice-Boltzmann methods