Drop impact in the regime of film boiling : transient evolution of the heat transfer and the vapor film thickness

International audienceWhen 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, a model is proposed for the growth of the vapor film and the heat transfer at the impact. 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 carried out with ethanol droplets impacting an overheated sapphire surface (typically above 250°C). Two optical measurement techniques, based on laser-induced fluorescence imaging and infrared thermography, are associated to temporally and spatially characterize the heat transfer as well as the thickness of the vapor film