<p>Natural convection heat transfer within enclosures is essential for numerous engineering applications, including thermal management in electronics, energy systems, and other industrial processes. Despite its importance, optimizing heat transfer efficiency in enclosures, particularly with complex geometries and added elements such as fins, remains a challenge. The existing studies largely focus on steady-state conditions, leaving gaps in understanding how transient effects evolve and influence performance. This study addresses these limitations by numerically investigating the impact of symmetrical fins on transient natural convection and heat transfer in a closed triangular cavity, with a Prandtl number of 7.01. Using computational fluid dynamics (CFD) simulations at a fixed Rayleigh number of 1.67 × 10⁶, the research characterizes the transient flow and temperature field development. The findings reveal that the flow evolution progresses through three stages: initial, transitional, and quasi-steady. In the initial stage, heat transfer is primarily governed by thermal conduction. In the transitional phase, plume formations and intrusion flows boost heat transfer by up to 54% compared to finless cavities. The fins also delay asymmetric flow, maintaining symmetry for longer. In the quasi-steady state, fins reduce oscillations in flow and stabilize heat transfer patterns by clarifying primary and secondary flow frequencies. This study contributes valuable insights into transient natural convection in complex geometries, offering a theoretical framework for enhancing heat transfer performance in industrial systems, where dynamic thermal control is critical.</p>