Effects of water subdroplet location on the start of puffing/micro-explosion in composite multi-component fuel/water droplets

An earlier reported model for the prediction of the onset of puffing/micro-explosion in composite multi-component water/liquid fuel droplets is generalised to consider the shifting of the water subdroplet relative to the centre of the fuel droplet. The droplet heating and evaporation are described within the Abramzon and Sirignano model. The equations of heat conduction in the droplet and component diffusion inside the fuel shell are solved numerically assuming that the composition and temperature are uniform over the droplet surface but vary with time. The change in the droplet size due to thermal swelling is considered. The verification of the new model is performed by comparing its predictions with those of the previously developed numerical code, based on the analytical solutions to the heat transfer and component diffusion equations, and used at each timestep of the calculations, for the case of a perfectly centred water subdroplet. The coincidence of the results supports both approaches to the problem. The timing of puffing/micro-explosion is then evaluated for droplets of two kerosene surrogates for various positions of the water subdroplet. It is pointed out that shifts of the water subdroplet by less than 20% lead to a reduction in the time to puffing/micro-explosion of less than 5%. This justifies the applicability of the previously developed model that was based on the assumption that a water subdroplet is located exactly in the centre of the fuel droplet. The times to puffing/micro-explosion predicted by the model are validated using the in-house experimental data for kerosene surrogate droplets (SU1: n-decane, iso-octane and methylbenzene; SU12: iso-octane and methylbenzene).