Nucleation and bubble growth during puffing and micro-explosions in composite droplets

Heating of droplets composed of water and fuel is known to lead to internal nucleation and bubble growth that can eventually lead to their puffing and to micro-explosions. The time to puffing/micro-explosions includes times
spent on: heating (time), bubble growth. In the present paper, we examine the effect of different aspects of bubble growth on the puffing and micro-explosions. Specifically, we address the effects of nucleation temperature and
the relative positions of the inner water sub-droplet and the bubble within it. The nucleation temperature of the water sub-droplet is higher than its normal boiling temperature yet lower than its spinodal temperature in most realistic cases. The degree of superheating and the nucleation time depend on the heating rate and the nucleation site density. Higher nucleation temperatures imply larger driving force for the bubble growth. Bubble growth
rate is dominated by the degree of superheating, while growth time is dominated by both the degree of superheating and the location of the bubble with respect to the inner and outer interfaces of the composite droplet. It is found that the inertial bubble growth regime is dominant for micron-sized droplets, and thus sensitivity to the modelling of the inertial regime can be of crucial importance to the evaluation of the breakup time for the droplets. The model for puffing and micro-explosion presented in the paper considers an isolated bubble growing
at the water/fuel interface at various degrees of superheating, and for a wide range of Jakob numbers. This analysis allows us to assess the sensitivity of bubble growth time to the initial bubble location, and to generalise the previously developed model of the phenomenon taking into account the effect of finite time of bubble growth during the development of puffing/micro-explosion.