Direct numerical simulation of microlayer formation and evaporation underneath a growing bubble driven by the local temperature gradient in nucleate boiling

Abstract

Recently, experiments carried out with high-resolution measurement techniques showed a thin liquid microlayer (∼μm) formation underneath a growing bubble in nucleate boiling. However, a deep understanding of the heat transfer enhancement induced by this microlayer is still lacking. In the present work, we investigate the microlayer dynamics and its effect on microlayer evaporation during the microlayer formation stage. Using direct numerical simulations with the PHASTA solver, a fully resolved microlayer underneath a growing bubble driven by the local temperature gradient in nucleate boiling is reproduced. The simulation results are compared and largely validated against recent experimental observations and Mikic's model. The detailed microlayer dynamics indicates that the microlayer formation can be considered a quasi-steady process. In addition, the hydrodynamic effect shows limited influences on the microlayer thickness, thus suggesting a rather constant amount of microlayer evaporation during the entire microlayer life cycle under different hydrodynamic conditions. Here, the maximum evaporative heat flux of the microlayer occurs near the contact line and exceeds 1 MW/m2. Evaporation of the microlayer can account for ∼50% bubble volume growth after the onset of nucleate boiling. This value emphasizes the significance of microlayer evaporation in nucleate boiling modeling.