Numerical Investigation of the Mechanisms of Local Extinction Using Flame Kernel-Vortex interactions.

Abstract

The response of premixed flames to unsteady stretch is studied via kernel-vortex interactions. In this configuration a spark ignited kernel interacts with a vortex pair of variable strength. Both detailed and simple chemistry approaches are explored. In the detailed chemistry effort a dilute Hydrogen-air mixture is used. The vortex causes significant distortion of the kernel topography. Two distinct regimes; "Breakthrough" and "Extinction" are observed. A continuous increase in flame area and volumetric reaction rate values are observed throughout interactions in the breakthrough regime. However, corresponding consumption speed values are lower than 1-D laminar flame speed values. Detailed chemistry analysis of downstream interaction at the leading edge is carried out. During intermediate stages of the interaction, the mixture in between the interacting flames shows rich burning conditions. As the interaction proceeds the pool of products expands against the counter velocity gradient imposed by the vortex. The decrease in the temperature causes a steady decrease in the rates of reaction of the chain branching reactions causing. The behavior of various reaction layers is dictated to a large extent by their arrangement across the region of interaction. A simple two-step global reaction mechanism is formulated for lean methane combustion. These simple chemistry computations are carried out in an axis-symmetric configuration in a spherical frame of reference. Four distinct regimes of interaction: 1) the no-effect regime, 2) the wrinkling regime 3) the break-through regime, and the 4) global extinction regime are observed. Interactions in the no-effect regime show only minor deviations from unperturbed kernel values. Vortices in the wrinkling regime impose substantial stretch on the kernel causing major deviations from unperturbed kernel values. A sharp drop in the flame surface area and the integrated reaction rate is observed during breakthrough. The primary mechanism governing global extinction is downstream flame-flame interaction. A turbulent combustion diagram was derived for kernel-vortex interactions. Predominance of the breakthrough regime was observed. The turbulent combustion diagram represents an important contribution of this work.