Heat transfer for Leidenfrost drops bouncing onto a hot surface

International audienceWhen droplets impinge onto a hot wall, different regimes can be observed depending on the wall temperature and Weber number. In the case of interest, the temperature of the wall is more than the Leidenfrost temperature and the Weber number based on normal velocity of the droplet is less than the threshold value leading to the splashing regime (We < 80). We particularly focused on the perfect bouncing regime (We < 30) for which an impinging droplet levitates on a thin layer of its own vapour. This vapour is instantaneously created between the base of the deforming droplet and the heated surface so that direct contact with the hot solid is avoided. For these low Weber numbers, the droplet surface energy is high enough compared to its kinetic energy to permit the rebound, so that the droplet recovers its initial shape without breaking up after the bounce. Although the generated vapour insulates the droplet, some heat is exchanged with the wall during the interaction (i.e. during the resident time). In this paper, we report on experimental measurements of heat transfer due to droplet impact in the Leidenfrost regime. The energy released by the wall and measured using an inverse conduction method leads to an estimation of the heat transfer coefficient during impact in the Leidenfrost regime. For that, the time evolution of the droplet base surface is estimated using a simple modelling validated on experimental data. Finally, the energy measured is compared to existing models