Abstract
<title>Abstract</title> <p>This work presents a numerical investigation of transient natural convection along an isothermal vertical plate under low diffusivity ratio conditions. Particular attention is devoted to fluids characterized by weak thermal diffusivity relative to momentum diffusion, which strongly modifies the evolution of thermal boundary layers and wall heat transfer mechanisms. The governing dimensionless equations based on the Boussinesq approximation are solved using an explicit finite difference formulation with second-order spatial discretization. The study emphasizes the influence of low diffusivity ratios on the thermal boundary-layer structure, transient temperature evolution, and local Nusselt number distribution. Numerical results demonstrate that decreasing thermal diffusivity produces thicker thermal layers, delayed thermal stabilization, and reduced wall heat transfer rates. In contrast, moderate and high Prandtl-number regimes generate sharper thermal gradients and accelerated convergence toward steady-state conditions. Grid-independence analysis and comparison with classical boundary-layer correlations are included to validate the numerical methodology. The present analysis provides physical insight into heat transfer mechanisms in low-diffusivity fluids relevant to liquid-metal cooling systems, thermal energy technologies, and convection-dominated transport processes.</p>