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/