Two-phase closed thermosyphon system with microstructured boiling surfaces: Steady-state and transient regimes analysis

This work presents the experimental evaluation of a Two-Phase Closed Thermosyphon (TPCT, 32 mm inner diameter and 125 mm total length) using HFE-7100 dielectric fluid, equipped with novel laser-fabricated microstructured evaporator surfaces and a see-through adiabatic section for flow visualization. A complete redesign enabled reliable testing of seven surface geometries: smooth, anodized, and microstructured with micro-holes, micro-waves, and micro-pillars with feature depths of 100 µm and 300 µm, and center-to-center distances of approximately 120 µm and 370 µm, respectively. Steady-state tests dissipating power values up to 110 W at an optimized filling ratio of 30% showed a constant condenser resistance of about 0.11 K/W for power values above 30 W, regardless of evaporator surface, and a maximum heat transfer coefficient (HTC) of 2.54 W/cm 2 .K at 110 W, about 3.5 × larger than the plain surface. Critical boiling conditions were not reached for any of the surfaces due to the mechanical fragility of the see-through section of the TPCT. The surface that achieved the best performance was the W100 followed closely by the H100 surface. Additionally, it was observed that condenser performance influences bubble dynamics and boiling resistance of the device. Transient 100 W step tests revealed that the abruptness of heat delivery can influence positively or negatively HTC after stationarity is reached; surface H100 showed a 40% HTC improvement in relation to the steady-state tests. These findings demonstrate that microstructures significantly enhance both steady and dynamic TPCT cooling performance, offering a plausible alternative for high heat flux thermal management of electronic components.