Experimental study of a two-phase thermosyphon with porous graphite foam insert

This paper presents an experimental study of heat transfer in a pool boiling evaporator with porous insert. Porous graphite foams of different structures were tested with FC-72 and HFE-7000 coolants with the objective of maximizing the heat transfer in a pool boiling configuration. A two-phase thermosyphon facility was developed to investigate the system performance using graphite foams of block and fin structures. The effects of foam configuration, working fluid type and coolant filling volume on heater surface temperature and superheat were analyzed. The results showed that coolant filling volume has negligible effect on the cooling performance. On the other hand, the thermosyphon performance is significantly affected by the coolant properties and the configuration of the porous graphite foam. A comparison of the Bond numbers obtained for FC-72 and HFE-7000 indicates that the bubbles have to overcome higher surface tension forces before departing the foam surface in HFE-7000. Meanwhile, the effect of foam configuration on the boiling heat transfer performance implies that a properly designed geometry of porous graphite foam will lead to significant enhancement of the evaporation process in a thermosyphon system.