Cassiano Tecchio (STMF, ISAS, CEA)

Near-wall phenomena during bubble growth at nucleate boiling

Both experimental and theoretical studies of the near-wall phenomena at single bubble growth in nucleate pool boiling of water are reported. Our focus is on the physics of the micro-metric thin film of liquid (microlayer) that is formed between the heating wall and the bubble. Advanced high speed and high resolution optical techniques are employed. Synchronous and simultaneous measurements of the microlayer thickness, wall temperature and bubble macroscopic shape are performed by white light interferometry, infrared thermography and side-wise shadowgraphy, respectively. The heating to trigger the bubble is provided by an infrared laser. The wall heat flux is numerically reconstructed by using the experimental wall temperature data. It is shown that the microlayer can be seen as the Landau–Levich film deposited by the bubble foot edge during its receding. According to the numerical simulations, the microlayer consists of two regions: a dewetting ridge near the contact line, and a longer and flatter bumped part. It is shown that the ridge cannot be measured by interferometry because of its intrinsic limitation on the interface slope. The ridge growth is linked to the contact line receding. The simulated dynamics of both the bumped part and the contact line agrees with the experiment. We also reveal a temporal rise of the thermal resistance of the liquid–vapor interface during the microlayer evaporation, which corresponds to a decrease of the accommodation coefficient. We attribute it to the progressive accumulation of impurities at the interface during evaporation. The contribution of microlayer evaporation to the overall bubble growth is about 18%.