Evaporation microlayer shape, thickness and heat transfer upon nucleate boiling of water

When a vapour bubble grows over a solid surface upon nucleate boiling, a thin liquid film may remain trapped between the bubble and the wall, and its evaporation provides a significant contribution to heat transfer. Our understanding of the dynamics of this “evaporation microlayer” is still limited, owing to the multiscale nature of a problem spanning from micrometre to millimetre scales, which poses significant experimental and computational challenges. In this work, we use numerical simulations with adaptive mesh refinement (AMR) to model the growth of a single, isolated steam bubble in pool boiling conditions at atmospheric pressure, fully resolving the microlayer with the computational grid. Thanks to the reduced computational cost allowed by AMR, we are able to perform a broad and unprecedented systematic analysis of the effect of the wall superheat, initial bulk temperature, wetting conditions, and interfacial heat transfer resistance, on the shape and thickness of the evaporation microlayer and the corresponding heat transfer. This study shows that the microlayer thickness depends significantly on the speed at which the bubble grows, such that the wall temperature and the initial temperature field within the liquid have primary influence. The contact angle determines the contact line speed and lower contact angles promote microlayer formation, however, far from the contact line, the microlayer shape and thickness are weakly dependent on it. The wall heat transfer in the microlayer region increases with the wall superheat, however, this relationship is less than linear because the bubble grows faster thereby trapping a thicker microlayer.