Q. Liao et Ts. Zhao, A visual study of phase-change heat transfer in a two-dimensional porous structure with a partial heating boundary, INT J HEAT, 43(7), 2000, pp. 1089-1102
A visual study on the phase-change behaviors in a vertical two-dimensional
porous structure made of staggered miniature silver-copper circular cylinde
rs has been carried out. Subcooled water was pumped into the porous structu
re from its bottom due to the capillary action developed in the vicinity of
a grooved hearing block placed on the top of the porous structure. Using a
high-speed video imaging system, both pore-scale bubble-growth behaviors a
nd continuum-scale distributions of two-phase zone in the porous structure
were observed. Photographic results reveal that for a small or moderate hea
t flux, isolated bubbles formed, grew, and collapsed in the pores in a cycl
ic manner with a nearly constant frequency. In the macroscopic view, it is
found that the periodic downflows of dispersed bubbles and upflows of the l
iquid 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 t
he bubble growth-collapse cycle and the number of isolated bubbles increase
d while in the macroscopic view, the two-phase zone expanded laterally but
shrank vertically. When the imposed heat flux was sufficiently high, a vapo
r film was observed beneath the heated fin. These visual observations expla
in heat transfer measurements: with an increase of the imposed heat flux, t
he heat transfer coefficient increases to a maximum value and then rapidly
decreases afterwards. (C) 2000 Elsevier Science Ltd. Ail rights reserved.