Extinction predictors in turbulent sprays

A Direct Numerical Simulation (DNS) study has been conducted into droplets evaporating and burning under the influence of a spark. Specifically, underlying physical causes behind configurations globally extinguishing or failing to ignite have been investigated. It has been found that the global mixture fraction mean and standard deviation need to be sufficiently large (i.e. sufficient fuel must have evaporated to be available for mixing) in order for a flame to be sustained, with the standard deviation a more universal measure of success. In addition, a predictor to extinction has been identified: successful flames have a substantially large region containing hot products with low scalar dissipation rate. This stable region forms a kernel that is able to supply sufficient heat to promote flame propagation without depleting too quickly. In contrast, flows that fail to ignite never form a region containing hot products, while flows that are about to globally extinguish do not have a region with low scalar dissipation within the hot products. This predictor was also observed in the equivalent partially-premixed gaseous configuration. These indicators were found to be independent of droplet size, droplet number density and turbulent intensity and observed both while the spark was active and after the spark effect had completely dissipated. Further work will aim to quantify the scale of this effect to obtain a reliable measure of when the predictor of no low scalar dissipation amongst hot products is encountered. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.