This implies that the phase-separation is mainly ruled by the droplet temperature under microgravity. On the other hand, the droplet temperature at the microexplosion decreases with an increase in the initial water content. This dependence on the initial water content is qualitatively almost the same as the result in the previous work [15]. The droplet temperature at the microexplosion is apparently higher than the one at the end of the phase-separation. It should be noted that the droplet temperature at the microexplosion for all the initial water contents is above the boiling temperature and below the superheat limit (543 K) of water at the atmospheric pressure.
Figure 7 shows the time histories of the estimated volumes of water and the base fuel in the droplet after the end of the phase-separation. The abscissa represents the lapse of time from the start of droplet-heating. Vf, Vw and Vd0 correspond to the volumes of the base fuel and water, and the initial droplet volume, respectively. Vw was defined as the volume of the internal milky-white droplet, which was determined from the droplet image assuming the internal droplet to be a spheroid. Vf was obtained by subtracting Vw from Vf0. In this calculation, the volume of the quartz fiber was also considered. It is evident that the volume of the base fuel decreases with time while the water volume is kept almost constant to the initial value. There was the same tendency for the initial water contents of 0.2 and 0.3. This implies that selective evaporation of the base fuel occurs after the phase-separation. This is expected since the base fuel layer envelopes the internal water-based droplet after the phase-separation, as shown in Fig. 4 (b).
3.2. Onset Rete of Disruptive Microexplosion
Experimentally obtained data of the waiting time for onset of disruptive microexplosion of emulsions, which is defined as the time interval between start of heating or ignition and instant of disruptive microexplosion, scatters largely. In the practical point of view, therefore, the onset rate should be introduced to discuss the disruptive microexplosion by assuming its occurrence to be a random process. In the present study, the onset rate of the disruptive microexplosion was estimated by using the statistical analysis [13, 15] to examine the relationship between the phase-separation and occurrence of the disruptive microexplosion. Firstly, the glass capillary technique was adopted to clarify the onset rate of disruptive microexplosion of emulsions. This technique has been successfully utilized in the study of bubble nucleation [16].
The onset rate of explosive evaporation of an emulsion in a glass capillary of the outer diameter 4 mm and the inner diameter 2.6 mm was measured. The oil-in-water emulsion of n-dodecane was tested. The emulsion contained in the glass capillary was immersed in an oil bath, which was kept at high temperatures ranging from 423 K to 473 K. The initial water content cw was varied from 0.1 to 0.4. More than 50 capillaries were used for each experimental condition. The onset rate of explosive evaporation was statistically determined by using the Weibull distribution [17]. The onset rate is defined as
J = f(t)/[1-F(t)] (1)
where F is distribution function and f is the probability density function. The latter is defined as f(t) = (dF(t)/dt). t is the elapsed time from the instant when the emulsion is heated up to the oil bath temperature.
Fig. 7 Time histories of base fuel and water volumes in emulsion droplet.
Fig. 8 Effect of superheat degree on onset rate of microexplosion in capillary.
Fig. 9 Effect of emulsion volume on onset rate of microcxplosion in capillary.
