Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Heat and mass transfers at the impact of a droplet onto a hot solid surface are investigated experimentally. Millimetersizedwater droplets impinges onto a perfectly flat sapphire surface heated at 600°C. The temperature of the liquidinside the droplet is measured using the two-color laser-induced fluorescence (2cLIF) technique. Water is seededwith a temperature-sensitive fluorescent dye, while a nanosecond pulsed laser is used for the excitation of the fluorescence.The ratio of fluorescence signal detected in two appropriate spectral bands allows to determine the liquidtemperature. One advantage of this non-intrusive optical technique is that it eliminates adverse effects associatedwith signal variations caused by droplet shape during its impact. In parallel, the temperature of the solid surfaceis characterized using infrared thermography. The latter measurements are made possible by the deposition of ananosize coating of titanium aluminium nitride (TiAlN) on the upper surface of the sapphire window. Thanks to thehigh frame rate of the IR camera, the time evolution of the heat flux distribution at the solid surface can be reconstructed. A comparison of IR and 2cLIF techniques enable to correlate the heating of the liquid with the cooling ofthe wall. This reveals that most of the heat removed from the solid surface is devoted to the heating of the liquid,the energy used for liquid vaporization being significantly lower.DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4636