The results of comparative analysis and tests of ammonia loop heat pipes with cylindrical and flat evaporators

Comparative analysis and tests of ammonia loop heat pipes (LHPs) with a flat disk-shaped evaporator 40 mm in diameter and a cylindrical evaporator 10 mm in diameter equipped with a 40 mm × 40 mm × 12.2 mm copper interface were conducted. Both evaporators had the same heated surface 12.56 cm2 in size. The length and the diameter of the vapor line and the condenser were also equal. Tests of LHPs were conducted at +90°, 0°, −90° angles with respect to gravity and identical condenser cooling conditions at +20 °C heat sink temperature. For the cylindrical evaporator, the maximum heat load of 400 W (31.8 W/cm2) was reached at a junction temperature of 86.5 °C. For the flat evaporator, the maximum value of heat load was 320 W (25.5 W/cm2) at a junction temperature of 91 °C. For the cylindrical evaporator, the minimum value of thermal resistance of 0.064 °C/W was reached at maximum heat load. For the flat evaporator, it was 0.067 °C/W at 180 W heat load. As orientation was shifted, the value of the maximum heat load fluctuated by no more than 20-25% for both devices, while the value of the minimum thermal resistance fluctuated by no more than 4% for the cylindrical evaporator and no more than 12% for the flat one.