Abstract: The paper presents experimental studies of nucleate boiling heat transfer from a system of connected horizontal and vertical subsurface tunnels. The experiments were carried out for ethanol at atmospheric pressure. The tunnel external covers were manufactured out of perforated copper foil (holes diameter 0.4 and 0.5 mm), sintered with the mini-fins, formed on the vertical side of the 10 mm high rectangular fins and horizontal inter-fin surface. Visualization studies were conducted with a real sample, formed a square of 27 mm-long side, and with a segment limited by transparent plexiglas plates. The investigations aimed at identifying pool boiling regimes and flow patterns at various heat fluxes for horizontal and vertical tunnels. Passive and active pores were observed for horizontal tunnel. Increased heat flux generated a larger number of active pores in the horizontal tunnel. Pores in the perforated foil of vertical tunnel were inactive they only acted as elements feeding the tunnel with liquid. Vertical tunnel outlets were active within the whole range of investigated heat fluxes. The tunnels "worked" independently, only some fraction of liquid evaporated from the contact area of the horizontal and vertical tunnels.
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