1. Two-phase gravity loops

A two-phase gravity loop is a sealed enclosure containing a two-phase fluid in liquid-vapor equilibrium. It comprises an evaporator, a condenser and two connecting tubes (figure 1 ). The liquid evaporates in the heated zone, while the vapor condenses in the cooled zone. As the density of the fluid in the liquid-rich downstream tube is greater than that in the vapor-rich upstream tube, an Archimedean force is created between the two fluid volumes. If the condenser is positioned above the evaporator, this force allows the fluid to circulate in the loop under the driving action of gravity.