Two-Phase Closed-Loop Thermosyphon Filled with a Dielectric Liquid for Electronics Cooling Applications

This paper investigates the thermal performance of a small two-phase closed-loop thermosyphon (TPCLT), with an inner tube diameter of 4.8 mm, filled with a dielectric liquid and intended for use in server cooling applications. The TPCLT is filled with a working dielectric fluid free of non-condensable gases, the Novec™ 7000 from the 3M corporation. We have studied two porous-coated copper heat spreaders , both with a surface area of 42 mm by 48 mm exposed to the liquid. The heat spreaders were attached to a 2.54 cm by 2.54 cm high-power heater. One of the heat spreaders was covered by a novel multi-scale electroplated porous coating (MuSEP). The other had a boiling enhancement coating that is commercially available from the 3M corporation. We studied the effect of the filling ratio (FR: liquid volume divided by the volume of the system) and of the cooling power on the system's thermal performance. Among the three tested FR (32%, 42%, and 52%), the MuSEP coating showed its best performance at a FR of 42%. The maximum applied heater power for the MuSEP coated evaporator was (315 ± 8.8) W at a heater temperature below 80°C (heater power density of 49 W/cm
²
). The superior performance of the MuSEP coating was evident as the 3M boiling enhancement coating(BEC) could only remove (254 ±7.1) W at a heater temperature below 80°C. At a FR of 32%, the 3M BEC coating reached a dryout condition at (204 ±5.7) W, while the MuSEP coating kept the heater temperature at 64°C without dryout at the same power. Since we had a substantially subcooled liquid at the evaporator inlets with a liquid temperature in downcomers kept between 21 and 24°C, no pressure related instabilities were observed in the TPCLT loop that deteriorated its performances. The condenser thermal resistance was approximately 50% of the total heater-to-ambient thermal resistance in the TPCLT at (315 ±8.8) W, showing that condenser optimization is key to achieving better performance and higher power levels.