Abstract
The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.
Highlights
For internal combustion engine, fuel efficiency is critical and emission requirements of NOx, CO2, and soot have become more and more stringent
The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed
It should be mentioned that in our previous paper,[22] our code has been validated for dynamic capillary wave propagation and droplet pinch-off from a ligament. By these and the comparisons performed in this study, it can be said that the current code accurately predicts explosive boiling and interface dynamics
Summary
Fuel efficiency is critical and emission requirements of NOx, CO2, and soot have become more and more stringent. In experiments with a longer time scale (∼O(1 s) for heating) using larger droplets, complete phase separation of the oil and the water was observed before microexplosion.[7] in a realistic fuel spray where the injected fuel does not have a sufficient time for complete phase separation, sub-droplet coalescence occurs only partially.[4,5] It should be noted that the inner-droplet heating is enhanced by the internal circulation due to the relative gas velocity, which disturbs thermocapillary motion of the water sub-droplets.[11,12,13] In such a case, the observed explosion-type breakup (∼10 μs from the beginning of droplet deformation to explosion) of small emulsion droplets (∼20–50 μm) is considered due to explosive boiling of multiple water sub-droplets.[5] Under these considerations, sub-droplet coalescence is not directly considered in this study, and the number and location of water sub-droplets are varied to investigate their effects on microexplosion and puffing.
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