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

The heat dissipation rates at the chip level are projected to reach the 50- 100 W/cm(2) mark for some future high performance electronic systems. Liqui d cooling with phase change has been demonstrated to be a very efficient te chnique for thermal management of such high heat dissipation rates. Past wo rk on liquid immersion cooling using fluorocarbons has shown the advantage of using enhanced structures to reduce boiling incipience excursion and rai se the critical heat flux (CHF). Thermosyphons, employing these enhanced st ructures 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 perfor mance of an enhanced microstructure based thermosyphon, which has shown ver y high heat transfer rates (up to 100 W/cm(2) with a wall superheat of 27.8 degrees C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa), The ex periments were initiated at room temperature, and hence the sub-cooling cor responded to the difference in the liquid saturation temperature at the sta rting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures, at a given heat flux. The perfo rmance of the system was evaluated by defining a surface-to-ambient resista nce. Results show that a partial vacuum at all heat fluxes results in bette r performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results o btained were similar to the baseline case (larger evaporator).