Analysis of D2-law in case of Leidenfrost drop evaporation

An experimental method to measure the heat transfer between a hot surface and a drop under Leidenfrost regime is reported. The phenomenon concerns large water drops with initial mass of ∼1 g that evaporate under ambient pressure. The test facility allows measurement of the instantaneous mass change of the evaporating drop located on surface at constant temperature. Both thermal and video cameras register the size of a drop for further calculation of its bottom area. On the basis of the energy balance, the evaporative heat flux is calculated. Its power dependence on the drop orthogonal projection area on the heating surface is found. In the linear range of D2-law, an analytical solution of the energy balance is given and its validity is discussed. The improvement in accuracy by the second order polynomial approximation for drop area is suggested and consequently D2-law in the form of quadratic function. But in this case, the numerical calculation is necessary. The main factors of measurement uncertainty are also discussed. It is pointed out that the electronic scales accuracy and the resolution of camera are the most important factors affecting the accuracy.