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When a drop impinges onto a wall heated above the Leidenfrost temperature, a very thin vapor film is formed at the interface between the liquid and the solid substrate. This vapor layer modifies the impact behavior of the drop and induces a significant decrease in heat transfer.
To study this phenomenon, experiments are carried out with ethanol droplets impacting an overheated sapphire surface (typically above 250°C). Several optical measurement techniques are combined to characterize the heat transfer. A laser-induced fluorescence imaging technique has been specifically developed to provide instantaneous images of the temperature field inside the spreading drop [1]. In addition, the sapphire substrate is covered with a nanosized layer of TiAlN which behaves almost like a black body for radiative transfer. Thanks to the transparency of sapphire, radiations from the TiAlN coating are transmitted to a high-speed IR camera which allows measuring the surface temperature. The resolution of an inverse heat conduction problem in the sapphire substrate, is then used to reconstruct the temporal and spatial evolution of the heat flux on the impacted solid surface, and to obtain the spatial and temporal evolution of the vapor film thickness [2].
Finally, a model is proposed for the growth of the vapor film and the heat transfer at the impact [3]. The main assumptions are: (i) a uniform but time varying thickness of the vapor film, (ii) a quasi-steady Poiseuille flow inside the vapor film, and (iii) a constant wall temperature. Heat energy and momentum balances are employed to obtain an ordinary differential equation describing the evolution of the vapor film thickness during the drop impact. Upon a one-dimensional analysis (nonetheless including some effects due to the complex fluid flow), the local heat flux transferred to the liquid qL can be evaluated. When the initial drop temperature is sufficiently lower than the saturation temperature, qL predominates over the heat flux used for liquid evaporation. This results in a simplified model for the vapor film thickness that we were able to validate against experiments.
[1] W. Chaze, O. Caballina, G. Castanet et F. Lemoine, Experiments in Fluids, Vol. 58, n°8, p. 96, 2017.
[2] W. Chaze, O. Caballina, G. Castanet, J.-F. Pierson, F. Lemoine et D. Maillet, International Journal of Heat and Mass Transfer, Vol. 128, pp. 469–478, 2019.
[3] G. Castanet, W. Chaze, O. Caballina, R. Collignon et F. Lemoine, Physics of Fluids 30, 122109 (2018)
