Abstract:
Efficient electronics cooling has always been a perpetual challenge, with the limits of single-phase cooling almost
being reached. Two-phase cooling in the form of pool boiling is an attractive next step, with much research being
devoted to it. While refrigerants operating at lower saturation temperatures are key to achieving effective
cooling, surface modifications have been shown to also affect bubble dynamics and enhance nucleate pool
boiling heat transfer. A simple, easy to implement fabrication method was sought, with the goal of expanding the
knowledge of bubble dynamics. To this end, single bubble growth on structured surfaces that are achievable on a
lathe, with an average roughness of 75 μm and differing indentation angles between 90◦ and 46◦, was studied
numerically using an OpenFOAM multiphase library. Conjugate heat transfer was applied, with heat fluxes
ranging between 7.6 and 28 kW/m2 for pure refrigerants R32 and R1234yf. By comparing the bubble equivalent
diameter with that of a smooth surface at a fixed heat flux, it was found that the bubble growth rates of
structured surfaces were largely independent of indentation angles less than 90◦, but lower than for smooth
surfaces. For structured surfaces, a critical indentation angle of approximately 60◦ was identified which affected
the bubble dynamics. For angles greater than the critical angle the bubble growth time was up to 150 % longer,
which also resulted in larger departure diameters. However, the opposite trend was observed as the indentation
angle was decreased below the critical angle. From a force analysis, it was found that the physical limitation
imposed on the bubble growth was responsible for the critical indentation angle behaviour, with the most acute
angle of 46◦ showing the shortest departure time. Furthermore, the bubble growth from a single cavity corresponded
better with the trends of a smooth surface than a structured surface with comparable indentation angles.
On a structured surface, once the bubble reached the edge of the cavity, its base diameter was limited by the
physical characteristics of the surface. For the single cavity surface, however, bubble growth was uninhibited
beyond the cavity, mimicking a completely smooth surface. The marked difference between results of a fully
structured surface and the single cavity implies that future research will have to take the structural limitations on
bubble growth imposed by a roughened surface into account.
