The primary function of a natural circulation loop (i.e. thermosyphon loop) is to transport heat from a source to a sink. Fluid flow in a thermosyphon loop is created by the buoyancy forces that evolve from the density gradients induced by temperature differences in the heating and cooling sections of the loop. An advanced thermosyphon loop consists of the evaporator, where the working liquid boils; and the condenser, where the vapour condenses back to liquid; the riser and the downcomer connect these two exchangers. Heat is transferred as the vaporization heat from the evaporator to the condenser. The thermosyphon is a passive heat transfer device, which makes use of gravity for returning the liquid to the evaporator. Thermosyphons are less expensive than other cooling devices because they feature no pump. There are numerous engineering applications for thermosyphon loops such as, for example, solar water heaters, thermosyphon reboilers, geothermal systems, nuclear power plants, emergency cooling systems in nuclear reactor cores, electrical machine rotor cooling, gas turbine blade cooling, thermal diodes and electronic device cooling. The thermal diode is based on natural circulation of the fluid around the closed-loop thermosyphon (Bielinski & Mikielewicz, 1995, 2001), (Chen, 1998). The closed-loop thermosyphon is also known as a “liquid fin” (Madejski & Mikielewicz, 1971). Many researchers focused their attention on the single-phase loop thermosyphons with conventional tubes, and the toroidal and the rectangular geometry of the loop. For example, Zvirin (Zvirin, 1981) presented results of theoretical and experimental studies concerned with natural circulation loops, and modeling methods describing steady state flows, transient and stability characteristics. Greif (Greif, 1988) reviewed basic experimental and theoretical work on natural circulation loops. Misale (Misale et al., 2007) reports an experimental investigations related to rectangular single-phase natural circulation mini-loop. Ramos (Ramos et al., 1985) performed the theoretical study of the steady state flow in the two-phase thermosyphon loop with conventional tube. Vijayan (Vijayan et al., 2005) compared the dynamic behaviour of the single-phase and two-phase thermosyphon loop with conventional tube and the different displacement of heater and cooler. The researcher found that the most stable configuration of the thermosyphon loop with conventional tube is the one with both vertical cooler and heater.