Pool boiling for extended surfaces with narrow tunnels – Visualization and a simplified model

The paper presents experimental investigations and theoretical analysis of boiling heat transfer for a system of connected narrow horizontal and vertical tunnels. These extended surfaces, named narrow tunnel structures (NTSs), can be used to cool electronic elements. The experiments were carried out with water, ethanol and R-123 at atmospheric pressure. The tunnel external covers were manufactured out of 0.1 mm thick perforated copper foil (hole diameters 0.3, 0.4 and 0.5 mm), sintered with the mini-fins formed on the vertical side of 10 mm high rectangular fins and the horizontal inter-fin surface. Visualization studies were conducted with a real sample as well as with a transparent structured model of joined narrow tunnels with perforated walls. The visualization investigations aimed to identify nucleation sites and flow patterns, and determine the bubble departure diameter and frequency. On the basis of his previous studies and the existing analytical boiling models, the author proposed a simplified model for narrow tunnel structures. The heat fluxes for evaporation within the tunnels and convection on the external extended surfaces were determined based on the calculated bubble parameters (diameter, nucleation sites density, frequency). The predicted total heat flux, when compared to the experimental results, showed satisfying agreement for boiling water in medium and high heat flux ranges.