Hybrid Thermosyphon/Pulsating Heat Pipe for Ground and Space Applications: A novel two-phase passive heat transfer device

Pulsating heat pipes (PHP) are very promising passive heat transfer devices, simply made of a capillary tube and characterized by high thermal performance and extraordinary space adaptability. One of the main advantages with respect to Thermosyphons (TS) is that PHPs can work also without gravity assistance, making such technology interesting also for space applications. Nevertheless, the global heat power input that they can absorb is limited due to the capillary dimensions of the tube. The actual literature shows that it would be theoretically possible to build a hybrid TS/PHP with an Inner Diameter larger than the capillary limit, evaluated in normal gravity conditions, that works indeed as a loop thermosyphon on ground and switches to the typical PHP slug/plug operation when microgravity occurs. The aim of the present work is to prove the feasibility of such hybrid two-phase passive heat transfer device concept by means of a complete multi-parametric experimental campaign. Therefore, during the first year of the doctoral thesis, a fully equipped hybrid TS/PHP experiment is designed and built at the Thermal Physics Laboratories of the University of Bergamo. Then, such device is tested both on ground and in hyper-micro gravity conditions during the 61th and the 63th ESA Parabolic Flight Campaign. A thorough thermo-hydraulic characterization is performed on ground, varying important parameters such as the heat power input, the inclination angle, the ambient temperature and the heating elements position. It is found that a strategic arrangement of multiple heaters may be used in order to enhance the flow motion and consequently the thermal performance. In micro-gravity, parabolic flight tests point out a PHP working mode. The sudden absence of buoyancy force activates an oscillating slug/plug flow regime, typical of the PHP operation, allowing the device to work in any orientation. Although the present work demonstrates the feasibility of the TS/PHP concept, and that a strategic position of the heating elements promotes the two-phase flow motion, further tests in prolonged micro-gravity conditions (i.e. onboard a sub-orbital flight or, even better, the International Space) can point out the effective heat transfer performance in weightlessness conditions.