A thermosiphon is a heat transfer device that utilizes the phase change of a liquid and has a single closed-loop shape in a gravity-dominant field. This can be expressed as a single-step thermosiphon because boiling and condensation occur once per cycle. In contrast, the multistep thermosiphon, introduced for the first time in the field of thermal engineering in this study, is a new heat transfer mechanism in which boiling and condensation occur several times per cycle in a single loop with multiple channels. The new mechanism has a superior heat transfer rate compared to the existing single-step thermosiphon, and the operating pressure of the loop can be lowered. However, as the heat transfer rate increases, the circulation flow in the channel tends to pulsate. This thermohydraulic characteristic was confirmed through theoretical and computational analyses of a two-step thermosiphon.

In this study, an improved concept of an asymmetric two-step thermosiphon was developed that can be applied to heat exchanger design by eliminating pulsating flow while maintaining the advantages of a two-step thermosiphon. The newly proposed heat transfer mechanism, termed the multistep thermosiphon, can be effectively used in the design of heat exchangers in industrial fields. In particular, if the asymmetric two-step thermosiphon is applied to the design of small nuclear reactor containments currently being developed in several countries, there are several advantages associated with the reduction of the containment volume and design pressure.