Transient evolution of the heat transfer and the vapor film thickness at the drop impact in the regime of film boiling

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. A model is proposed for the growth of this vapor layer and its resistance to the heat transfer. The main assumptions are as follows: (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. For droplets injected at a temperature sufficiently lower than the saturation temperature, this equation predicts that the impact velocity has no influence on the thickness of the vapor film. This latter is solely governed by the local heat flux transferred to the liquid, which predominates over the heat flux used for liquid evaporation. An accurate description of the droplet heating is therefore required to complement this model. As an attempt, this description is based upon a one-dimensional analysis, which includes some effects due to the complex fluid flow inside the spreading droplet. Finally, the theoretical model is validated against experiments dealing with millimeter-sized ethanol droplets. Two optical measurement techniques, based on laser-induced fluorescence and infrared thermography, are combined to characterize the heat transfer as well as the thickness of the vapor film