Physics of puffing and microexplosion of emulsion fuel droplets

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.This article has been made available through the Brunel Open Access Publishing Fund

Puffing-enhanced fuel/air mixing of an evaporating n-decane/ethanol emulsion droplet and a droplet group under convective heating

Pu ng of a decane/ethanol emulsion droplet and a droplet group under convective heating and its e ects on fuel/air mixing are investigated by direct numerical simulation (DNS) that resolves all the liquid/gas and liquid/liquid interfaces. With distinct di erences in the boiling point between decane and ethanol, the embedded ethanol subdroplets can be superheated and boil explosively. Pu ng, i.e. ejection of ethanol vapour, occurs from inside the parent decane droplet, causing secondary breakup of the droplet. The ejected ethanol vapour mixes with the outer gas mixture composed of air and vapour of the primary fuel decane, and its e ects on fuel/air mixing can be characterised by the scalar dissipation rates (SDRs). For the primary fuel SDR, the cross-scalar di usion due to ethanol vapour pu ng plays a dominant role in enhancing the micromixing. When the vapour ejection direction is inclined toward the wake direction, the wake is elongated, but the shape of the stoichiometric mixture fraction iso-surface is not changed much, indicating a limited e ect on droplet grouping in a spray. On the other hand, when the ejection direction is inclined toward the transverse direction, the stoichiometric surface is pushed further away in the transverse direction and its topology is changed by the pu ng. The trajectories of ejected ethanol vapour pockets can be predicted by the correlation obtained for a jet in cross ow, and the vapour pockets may reach a few diameters away from the droplet. Therefore, in a multiple-droplet con guration, the transverse ethanol vapour ejection due to pu ng may transiently change the droplet grouping characteristics. In simulation cases with multiple droplets, the interaction changing the droplet grouping due to pu ng has been con rmed, especially for droplets in the mostupstream position in a spray. This implies that pu ng should be accurately included in the mixing and combustion modelling of such a biofuel-blended diesel spray process.Financial support from the Engineering and Physical Sciences Research Council (EPSRC), grant No. EP/J018023/