A single-tank molten-salt heat-exchanger storage system is promising for small-scale industrial heat supply, yet transient natural convection and heat transfer in closed tanks remain insufficiently understood. This study develops a physical model and performs numerical simulations of a top-heated single-tank sensible thermal storage unit using a realistic post-discharge, non-uniform initial temperature field. During charging, an upward plume forms near the heating rod, with heat concentrated around the rod and weak flow in remote regions. Two large-scale circulation cells separated by an inclined thermocline are observed, and the interface shifts downward over time. To address short storage duration, a segmented-heating strategy is proposed by varying the heating-section height. Results show that heater height strongly governs flow and storage performance: compared with full-length heating, 2/3-, 1/2-, and 1/3-length configurations extend storage duration by 93%, 100%, and 103.9%, respectively. Lowering the heating zone toward the tank bottom effectively prolongs storage and improves thermal efficiency.