Fignre 8 shows the relationship between the superheat degree of water in the emulsion ΔTS and the onset rate J for the initial volume 5x10-8 m3 and the initial water content 0.2. The superheat degree corresponds to the difference between the emulsion temperature and the normal boiling temperature of water. The onset rate increases exponentially with an increase in the emulsion temperature. Figure 9 shows the volumetric effect of emulsion on the onset rate for the superheat degree 60 K and the initial water content 0.2. It is clearly seen that the onset rate increases with the volume. Figure 10 shows the relationship between the initial water content and the onset rate for the initial volume 5x10-8 m3 and the superheat degree 60 K. The onset rate also increases with the initial water content. From these results, an empirical equation for the onset rate is derived as
J = K Vcw exp (A ΔT* ), (2)
where K and A are the constants. All the data can be correlated with Eq. (2), as shown in Fig. 11.
This equation can be applied to the disruptive microexplosion of oil-in-water emulsion droplets, since the water volume in the droplets is also kept almost constant before the microexplosion, as shown in Fig. 7. As an example, the dependence of the onset rate on the superheat degree for a suspended emulsion droplet of the initial diameter 1.0 mm and the initial water content 0.2 is shown in Fig. 12. These data were previously obtained from more than 30 experiments conducted both under the normal gravity and under microgravity realized in a small drop tower at Osaka Prefecture University [13, 15]. It is evident that all the data are correlated to one straight line . The constants of Eq. (2) for the disruptive microexplosion of emulsion droplets were determined from the data in Fig. 12, and the onset rate of the microexplosion in the case of the present experimental condition was calculated. Figure 13 shows the relationship between the estimated onset rate and the droplet temperature for the initial droplet diameter 2.5 mm and the initial water content 0.1. The temperatures at the start and the end of the phase-separation, and the temperature at the microexplosion are also indicated. It is obvious that the onset rate at the start of the phase-separation is negligibly low, and the onset rate at the end is still as low as 5 % of the onset rate at the microexplosion. This implies that most of the disruptive microexplosion would occur after the phase-separation.
The process of the disruptive microexplosion of oil-in-water emulsion droplets would be, therefore, explained as follows; The separation of the base fuel and water occurs inside the emulsion droplet and an internal water-based droplet is formed at the center of the primary droplet in the initial stage of droplet-heating or droplet-burning. The internal droplet is superheated with the lapse of time, resulting in sudden and vigorous boiling of the internal droplet, that is, the disruptive microexplosion of the droplet.
Fig. 10 Effect of water content on onset rate of microcxplosion in capillary.
Fig. 11 Correlated onset rate of microexplosion in capillary.
Fig. 12 Dependence of onset rate of droplet microexplosion on superheat degree.
