Nucleation inception due to conductive heating in composite droplets: experimental observations and modelling

Nucleation inception of composite droplets at slow to intermediate heating rates is examined both experimentally and theoretically. In the present study, rapid conductive heating of composite droplets of water in Diesel fuel is presented. Composite droplets are formed in such a way that initially water forms the inner sub-droplet and the hydrocarbon fuel forms the shell around it. The composite droplet is placed on a hot substrate until the water reaches the nucleation temperature. At this stage either puffing or micro-explosion are expected to occur. As the composite droplet is heated, the inner water core often comes into contact with the substrate, leading to a more efficient heat transfer and therefore to even faster disintegration. The Onset of Nucleate Boiling temperature (T_ONB), also referred to as the nucleation temperature, pertains to quasi-steady processes, and differs from the saturation temperature (T_sat), which pertains to equilibrium processes. The typical modes of development of the heating and phase change processes leading to puffing or to micro-explosion events are analyzed. First, the composite droplet is being heated due to its contact with the hot substrate. The temperature differences promote vigorous movement of the drop, sometimes to the point of its detachment from the surface, which causes significant temperature fluctuations. Using energy considerations, we came to the conclusion that nucleation occurs at the interface between the two liquids, and the bubble grows into the less volatile component, the Diesel fuel, as long as the mass fraction of the water doesn’t exceed 50%. The location of nucleation affects the dynamics of growth, and the location and time to puffing/micro-explosion; therefore, it is an important parameter, that is often overlooked.