Topology-optimized multi-layer PCM heat sink for 8-hour passive thermal management of high-density CPU superchips

As transistor densities surge and boost frequencies intensify transient CPU heat fluxes, passive thermal management with phase change materials (PCMs) offers a zero-power alternative. However, the low thermal conductivity of PCMs demands high-conductivity fins, and no existing study has employed topology optimization to design a multi-layer stacked PCM heat sink for long-duration cooling. The present study's novelty is a four-row PCM heat sink with aluminum fins generated by density-based topology optimization (SIMP, 20% solid fraction, Helmholtz filtering) under steady operating conditions. The optimized geometry is simulated via a transient enthalpy-porosity CFD model resolving conjugate heat transfer, laminar natural convection, and phase change, validated against experimental data (RMSE=2.44 K, NRMSE = 3.26%). Over eight hours of continuous CPU load, sequential melting occurs: the lowest row fully melts in 66 min, the second in 275 min, while the third reaches a liquid fraction of 0.54 and the top remains solid. The heat-sink base temperature stabilizes at 355.72 K (≈82.6 °C) after 8 h, which lies near the upper limit of typical commercial CPU operating temperatures, with a difference of only 3.51 K, compared to 388.36 K for a conventional single-layer radial-fin heat sink after just 1 h—a 42.4 K reduction, reflecting the combined effect of the multi-layer configuration and the topology-optimized fin geometry. Latent heat (2.6 kJ per row) dominates the lower blocks, while natural convection is suppressed to micrometre-per-second velocities.