Impact of surface parameters on confined boiling heat transfer in a Hele-Shaw cell

Confined boiling and bubble dynamics of pure water at atmospheric pressure were experimentally investigated in a Hele-Shaw cell established by two plates with a small spacing. The vapor bubble motion was visualized by means of a high-speed camera for different plate spacing and heating power levels. Four different kind of heating surfaces (copper/alumina and smooth/rough) and one surface with an artificial nucleation site were investigated. The copper and alumina surfaces were polished and grinded in order to investigate the impact of the surface parameters on the confined boiling system. Significant fluctuations of the heat transfer characteristics were observed during low heat flux level boiling. This observation was attributed to an intermittent boiling regime, when confined boiling was interrupted by flooding of the cell. The flooding effect substantially affected the boiling behavior, but it was found that even a single artificial nucleation site was able to stabilize the system at low heat flux level. This effect vanished during moderate heat flux level boiling, and then minor influence of the surface roughness was observed for small spacing values. Micro-layer evaporation was identified as major mechanism for the confined boiling in a Hele-Shaw cell by a computer-based post processing image analysis. Based on the new experimental observations, a theoretical model resting on a modified capillarity number was proposed.