Prediction of Vapor Film Thickness Below a Leidenfrost Drop

In this work, the vapor film thickness below a stagnant Leidenfrost drop at saturation temperature is predicted by performing a balance of the dominant forces acting on the drop. Inclusion of a new momentum force term is proposed. Two assumptions are considered for the radial velocity of vapor at drop–vapor interface. One of them is zero radial velocity at interface and the other is zero shear at interface. The actual scenario is expected to lie between these extremes. This is also supported by the comparison against experimental data on vapor film thickness. The effect of convection in the vapor layer is also modeled and it is shown that the use of ad hoc heat transfer coefficients in the vapor layer to explain difference between experiments and prediction is incorrect. Finally, it is highlighted that accurate prediction of the vapor layer characteristics also requires proper quantification of the shape of the drop–vapor interface.