In a closed loop, working fluid is partially evaporated at a lower elevation (in the evaporator). By buoyancy, the two-phase mixture reaches the condenser at a higher elevation through the riser. It is cooled down and changes into liquid again, before finally draining into the evaporator through the downcomer. Because the two-phase mixture’s density in the riser is lower than that of the liquid in the downcomer, gravity sustains the flow, removing the need for mechanical drivers.
Loop Thermosyphons (LTS) gravity-driven cooling requires elevation between evaporator and condenser
Passive two-phase cooling refers to the cooling of components using a working fluid that undergoes phase change and has self-sustained motion driven by the application/extraction of heat.