Experimental investigation and quasi-steady modeling of nucleate boiling in mini-channel thermosyphons

This study explores the understanding of two-phase cooling in thermosyphons using a quasi-steady state methodology during the boiling process. A thermosyphon is built as a passive loop to dissipate heat with an evaporator and a condenser. The evaporator is made of multiple mini channels (hydraulic diameter of 1.54 mm, length of 260 mm with 7 x 2 internal ports for a Confinement number of 0.55) with HFO-1336mzz(E) as the working fluid. Different heat loads (500 - 4000 W) are generated directly on the external contact surface of the evaporator to create all the different stages of boiling, from monophasic regime to steady nucleate boiling with the Onset of Nucleate Boiling transition. The temperature evolution inside the evaporator is measured at different heights and compared with a theoretical assumption of a quasi-steady state. A characteristic time depending on critical factors such as thermal mass is determined to model the temperature during a generated heat load. A good agreement between experimental measurements and the quasi-steady model is shown. Thus, this study emphasizes tracking the temperature evolution over time within the system. This dynamic perspective offers a nuanced understanding of the system's response to varying heat inputs, transient phases, and the onset of boiling. This characterized local behavior provides an original insight into the boiling appearing inside mini- channels. It is shown that boiling is initiated by nucleation at a few specific sites and then propagates up to a critical length due to high vapor generation, introducing a thermal lag during the boiling incipience.