Experimental investigation on the thermal performance of high temperature two-phase loop thermosyphon

High-temperature two-phase loop thermosyphons (TPLTs) have the potential to be applied in high-temperature thermochemical conversions, concentrated solar energy, and nuclear energy. However, there has been limited investigation on the thermal performance of TPLTs so far. This work experimentally examines the thermal performance of TPLTs using alkali metals (NaK alloy and pure Na) as working fluids, aiming for internal energy recovery of a double-riser reactor. The effects of filling ratios (FRs), heating temperature, and altitude difference between the evaporator and condenser are investigated regarding start-up characteristics and steady-state performance. The experimental results show that the TPLT with NaK as the working fluid can successfully complete the start-up when the heating temperature is set to above 650 degrees C under the test conditions. However, the TPLT using pure Na as the working fluid can't successfully complete the start-up from a frozen state without a preheating operation. According to the variations in thermal resistance, an FR of 38% in the TPLT allows for the best heat-transfer performance under steady state when compared to the conditions that FRs are equal to 25% and 50%. Additionally, placing the condenser slightly higher than the evaporator is beneficial for improving the steady-state performance. However, if the condenser is lower than the evaporator, its steady-state performance is remarkably degraded.