Combined effects of sub-cooling and operating pressure on the performance of a two-chamber thermosyphon

Heat dissipation rates at chip level are projected to reach 50-100 W/cm² for future high performance electronic systems. Liquid cooling with phase change has been shown to be a very efficient thermal management technique for such high heat dissipation rates. Past work on fluorocarbon-based liquid immersion cooling has shown the advantage of using enhanced structures to reduce boiling incipience excursion and raise the critical heat flux (CHF). Thermosyphons using these enhanced structures are an alternative to liquid immersion and are suitable for point cooling applications, where very compact evaporators are needed. This study investigates the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon, which has shown very high heat transfer rates (up to 100 W/cm² with wall superheat of 27.8°C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa). Experiments were initiated at room temperature, and hence the sub-cooling corresponded to the difference in liquid saturation temperature at the starting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures at a given heat flux. The system performance was evaluated by defining a surface-to-ambient resistance. Results show that a partial vacuum at all heat fluxes results in better performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results obtained were similar to the baseline case (larger evaporator)