PREMIXED COMBUSTION IN A PERIODIC-FLOW FIELD .2. THE IMPORTANCE OF FLAME EXTINCTION BY FLUID DYNAMIC STRAIN

A numerical model has been developed to study the importance of flame extinction by fluid dynamic strain that occurs during premixed combust ion in an oscillating flow field. This is the second of a pair of pape rs that reports on the fundamental mechanisms that ensure the correct phase relation between the energy release and the resonant pressure wa ves, which is required by Rayleigh's criterion for stable oscillations . The axisymmetric model is based on the vortex dynamics method combin ed with a flame sheet algorithm. The model is used to demonstrate that flame extinction by fluid dynamic stretch plays a dominant role in de laying reaction until later in the cycle. Without the extinction mecha nism significant energy release occurs during the peak of injection, a t a time when experimental observation shows it proceeds through a min imum. Additionally, the predicted rate of energy release decreases as the reactant injection rate decreases, but the experimental observatio n shows an increase in the energy release rate at this cycle time. By including extinction due to stretch in the flame model the onset of ig nition of the fresh reactants is significantly delayed, giving much be tter agreement with the experimental data. Results are presented that show that the effect due to flame extinction by stretch dominates the effects due to either the expansion of the reactants as they burn or t he shape of the injection profile. These results also show that althou gh reactants injected during one cycle may still be burning during the next cycle, they are not the source of ignition for the subsequent cy cles. This implies that the ignition mechanism is either thermal or ra dical, or both. Although the flow field studied here is from a pulse c ombustor, the conclusions are applicable to combustor configurations w here a large vortical structure is formed during injection of reactant s.