Combustion characteristics of a single decane/ethanol emulsion droplet and a droplet group under puffing conditions

Combustion characteristics of an ethanol-in-decane emulsion droplet and a droplet group under puffing conditions have been investigated by direct numerical simulation (DNS). Before puffing, a quasi-steady flame is formed in the wake of the parent decane droplet. Due to superheating, boiling of ethanol sub-droplets is initiated. Following rapid growth of the vapor bubble, ethanol vapor is unsteadily ejected out and interacts with the downstream wake flame. The local gas mixture fraction is affected by this ejection. The gas-phase temperature and reaction rate also show different characteristics from those of a 1D steady flame. In the dual-fuel system, fuel/air mixing in combustion can be characterized by the scalar dissipation rates (SDRs) due to mixing of decane/air and ethanol/air and cross mixing of decane/ethanol. The transient interaction between the droplet wake flame and the ejected vapor by puffing is evident in the flame S-curves. The interaction is further quantified by the budget analysis in the mixture fraction - SDR space. The contribution of the cross SDR between decane and ethanol to the rate of change of the SDR of the primary fuel decane is initially negative, which is particular to puffing. Later the cross SDR can also become positive. As the mixing continues, the magnitude of the SDRs becomes smaller. When puffing occurs in the transverse direction, the ejected vapor may sweep a region within a few diameters away from the parent decane droplet. If other emulsion droplets are in this region of influence, inter-droplet interactions occur. A multiple-droplet case demonstrates this interaction and implies that such an interaction will occur in an emulsion fuel spray and should be considered in modeling a multi-component emulsion fuel spray in a combustor.