Influence of scalar dissipation on flame success in turbulent sprays with spark ignition

Abstract

A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c were predominantly restricted to the highest reaction progress variable (c>0.98), with zero probability within the range 0.95<c<0.98 and low probability within 0.9<c<0.95. In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95<c<0.98. This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9<c<1, there are volumes of significant size where the value of c is uniform. Note that a successful flame must have a volume of substantial size with c=1. From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.