Using Planar Laser Induced Fluorescence to explore the mechanism of the explosive disintegration of water emulsion droplets exposed to intense heating

In this paper, we study the boiling of heated water emulsion droplets in the air flow at a temperature of 20–800°С. The relative volume concentration of the flammable components in the emulsion varies from 10% to 70%. We explore the unsteady temperature fields of droplets using a contactless optical diagnostic technique, Planar Laser Induced Fluorescence, with a cross-correlation system featuring a camera, a laser, a synchronizer, and the ActualFlow software. Rhodamine B acts as a fluorophore. We also use a high-speed video camera (up to 10⁵ fps) and continuous automatic tracking algorithms (Tema Automotive software) to record the rates of heating and evaporation, as well as transformation of droplet surfaces. We demonstrate the unsteady temperature fields of droplets and three modes of their boiling and breakup. These differ in the number and dimensions of the emerging droplets as well as the durations of the main stages. The temperature differentials at the water – flammable component interface are determined corresponding to three boiling and breakup modes. We show that the third mode provides the greatest number of fine droplets (no less than 200) if the heating temperature exceeds 400°С and the concentration of the flammable component is over 66%. The temperature at the phase interface reaches 100 °C–125 °C before disintegration, and the droplet heating times before explosive breakup may vary from 0.1 s to 10 s. Finally, we analyze how the temperature, additive concentration and droplet size affect the conditions and characteristics of these modes.