Dynamical vapour pocket of an impacting Leidenfrost droplet: Evaporation and scaling relations

In this paper, the impact of a three-dimensional droplet falling on a hot surface above the Leidenfrost temperature is investigated by a volume-of-fluid based Navier-Stokes solver for liquid-vapour flows with phase change. The applicability of the direct numerical simulation (DNS) to capture the vapour film under an evaporating droplet is validated by favourable comparisons with static measurement results in experiments. Using this DNS method, the oscillation of the vapour pocket of a saturated Leidenfrost drop is studied in the rebounding stage, which provides important information about the transient heat transfer. By probing the evaporation and energy characteristics as well as the heat flow in the vapour layer, we reveal that the strong surface energy dominates the droplet morphology, and the bottom interface oscillation is mainly ascribed to the evaporation and lubrication effects. The effects of the governing dimensionless numbers indicate that increasing We (Weber number, inertia to surface tension) or Ja (Jakob number, sensible heat to latent heat) will intensify the bottom interface oscillation. In addition, the maximum spread factor of the impacting Leidenfrost drops has the scaling of similar to We(1/4), according well with recent experiments. Our results further show that for rebounding drops with a small We, the non-dimensional maximal vapour pocket width and oscillation time interval are linear functions of We(1/2) and We(3/4), respectively. These relations facilitate the flow physics for dynamical Leidenfrost events and therefore, will guide the applications in the future.

Automated summary

This study investigates the impact of a three-dimensional droplet falling on a hot surface above the Leidenfrost temperature. The findings demonstrate that the droplet morphology is dominated by strong surface energy, and the oscillation of the bottom interface is mainly caused by evaporation and lubrication effects. Increasing the Weber number and Jakob number intensifies the oscillation of the bottom interface. Furthermore, the maximum spread factor of the impacting Leidenfrost drops follows a power-law relationship with the Weber number. For rebounding drops with a small Weber number, the non-dimensional maximal vapor pocket width and oscillation time interval are linear functions of the Weber number to the power of 1/2 and 3/4, respectively.