LES Study of stabilization mechanism in lifted ethanol spray flames

Abstract

This work presents a detailed numerical investigation of dilute spray lifted flames experimentally studied by O’Loughlin and Masri at the University of Sydney (O’Loughlin and Masri, Combust. Flame 2011), with a focus on understanding the mechanism that governs flame stabilization under the influence of sprays. Two ethanol flames with varied spray mass loading and degree of partial-premixing at the inlet are simulated by large eddy simulation (LES) combined with the dynamically thickened flame (DTF) model with finite-rate chemistry. Chemical explosive mode analysis (CEMA) is employed which helps identify the prominent role of an entrainment-based ignition process along the shear layer; this process dominates the flame anchoring of the downstream partially-premixed reaction zone. Thus, a higher spray loading can intensify the entrainment, leading to a faster formation of ignition kernels. Furthermore, in present flames, the stabilization point appears to be sustained in a purely gaseous zone with the most reactive mixture fraction and low scalar dissipation rate. Results show that the premixed flamelet can adequately represent flame structures at the leading-edge that are primarily controlled by the inlet partial-premixing, whereas the upstream evolving pre-ignition mixtures are fit for the diffusion flamelet. Moreover, both flamelet configurations fail in capturing downstream structures with a direct spray - flame interaction because the larger droplets produce a less homogeneous mixture with a higher scalar dissipation rate. This is more evident in the lean spray flame.