A visual study of phase-change heat transfer in a two-dimensional porous structure with a partial heating boundary

A visual study on the phase-change behaviors in a vertical two-dimensional porous structure made of staggered miniature silver–copper circular cylinders has been carried out. Subcooled water was pumped into the porous structure from its bottom due to the capillary action developed in the vicinity of a grooved heating block placed on the top of the porous structure. Using a high-speed video imaging system, both pore-scale bubble-growth behaviors and continuum-scale distributions of two-phase zone in the porous structure were observed. Photographic results reveal that for a small or moderate heat flux, isolated bubbles formed, grew, and collapsed in the pores in a cyclic manner with a nearly constant frequency. In the macroscopic view, it is found that the periodic downflows of dispersed bubbles and upflows of the liquid phase in the porous structure led to a quasi-steady liquid–vapor two-phase zone. As the imposed heat flux was increased, both the frequency of the bubble growth–collapse cycle and the number of isolated bubbles increased while in the macroscopic view, the two-phase zone expanded laterally but shrank vertically. When the imposed heat flux was sufficiently high, a vapor film was observed beneath the heated fin. These visual observations explain heat transfer measurements: with an increase of the imposed heat flux, the heat transfer coefficient increases to a maximum value and then rapidly decreases afterwards.