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

Recently, experiments carried out with high-resolution measurement techniques showed the formation of a thin liquid microlayer (~µm) underneath a growing bubble in nucleate boiling. However, a deep understanding of the heat transfer enhancement induced by this microlayer is still lacking. In this work, we investigate the heat transfer characteristics of the microlayer in the early stage of nucleate boiling by using direct numerical simulations with the PHASTA solver. The microlayer formation and evaporation during the bubble growth driven by the local temperature gradient are simulated and fully resolved by very fine boundary layer meshes and the level-set method. We obtain the microlayer evolution comparable to recent experimental observations for the first time. The detailed microlayer dynamics indicates that the microlayer formation in the early stage of nucleate boiling can be considered a quasi-steady process without contact line motion. Furthermore, we find that the microlayer thickness is not determined by hydrodynamic effects, thus suggesting a rather constant microlayer heat transfer under different hydrodynamic conditions in nucleate boiling. Here, the local heat flux in the microlayer exceeds 20 MW/m2 near the contact line within 0.6 ms after the bubble inception, and the overall heat transfer from the microlayer evaporation contributes over 70% to the bubble growth. This value emphasizes the significance of microlayer evaporation in the modeling of nucleate boiling heat transfer.