With the ongoing miniaturization of electronic devices and energy systems, along with the trend toward higher heat fluxes, demand for compact two-phase closed thermosyphons (TPCTs) has been steadily increasing. Due to the small diameters of compact TPCTs, confinement effects that hinder internal two-phase flow may occur, potentially disrupting stable TPCT operation. In general, the onset of confinement in TPCTs can lead to a severe degradation in thermal performance. Our research has primarily focused on analyzing flow instability phenomena and heat transfer characteristics of TPCTs across a wide range of geometries (inner diameters of 5–25 mm) and working fluids (water, acetone, ethanol, and HFE-7100). As a key outcome, this study is the first to demonstrate that stable operation can be achieved when both the confinement number (Co) and the Froude number (Fr) are less than 0.3.

Geyser boiling is a well-known instability in two-phase closed thermosyphons (TPCTs), characterized by the rapid growth of vapor bubbles that displace the liquid column and generate impulsive impacts at the upper section. Such behavior can shorten the service life of TPCTs and induce fatigue failure, underscoring the need for a more accurate understanding of its onset conditions and underlying mechanisms. Accordingly, this study analyzes Geyser boiling dynamics through flow visualization. It quantitatively measures the impact force of the displaced liquid during Geyser boiling events by installing a load cell at the end of the condenser section of the TPCT. The results indicate that Geyser boiling occurs more readily under high filling ratio conditions (FR > 75%), and that the cumulative loading resulting from repetitive impacts can have a significant influence on equipment fatigue and operational stability. Furthermore, this work provides practical design guidelines for avoiding Geyser boiling and ensuring structural durability during the TPCT design stage.

A heat-pipe heat exchanger (HPHX) is a thermal exchange device that efficiently transfers heat by employing an array of heat pipes. Operating on the principle of phase change of the working fluid, heat-pipe heat exchangers exhibit exceptionally high thermal performance, enabling superior energy efficiency. Moreover, owing to the intrinsic isothermal characteristics within a heat pipe, the temperature gradient between the evaporator and condenser sections remains relatively small, which imparts excellent thermal robustness and durability when deployed as a heat exchanger. Building upon these advantages, our research focuses on developing fabrication-friendly strategies to maximize the performance of gas-to-liquid and gas-to-air heat-pipe heat exchangers for the recovery of industrial waste heat. In particular, our efforts are primarily directed toward modifying the internal metallic surfaces of the evaporator section to reduce thermal resistance and further enhance thermal performance.

The wrap-around loop heat pipe heat exchanger can be utilized as an energy-saving device in HVAC systems. When a wrap-around loop heat pipe heat exchanger is installed in a configuration that wraps around a chiller, the wrap-around loop heat pipe heat exchanger transfers heat from the upstream side to the downstream side after the chiller. Consequently, it eliminates the need for electric reheaters to compensate for the temperature drop downstream of the chiller in conventional HVAC systems, making it a highly energy-efficient heat exchange solution. In our laboratory, studies are being conducted to analyze the system-level suitability and operational characteristics of the heat exchanger as a function of the working fluid employed (water, ethanol, acetone, and R-1233zd(E)). The results indicate that among the working fluids tested, water exhibits the best overall performance, while R-1233zd(E) also demonstrates relatively favorable performance.

A geothermal thermosyphon is a passive phase-change heat transfer device that utilizes geothermal energy to enable snow melting and prevent re-freezing without external power input. Research and development are currently underway to ensure effective operation not only during winter conditions but throughout all seasons. In this laboratory, the snow-melting performance of the geothermal thermosyphon is being experimentally validated under conditions that simulate real-world snow removal and anti-icing environments.