Effect of steady and periodic strain on unsteady flamelet combustion

Abstract

An unsteady, finite-rate kinetics, strained, flat-flame model is used to investigate the effects of unsteady dynamics on the burning rate and non-equilibrium processes including ignition, quenching and extinction in non-premixed combustion in the flamelet limit. Unsteady computations are performed for a steady and a periodic strain. Results show that for a shortduration (spark) ignition, a steady positive strain rate enhances the burning rate while exponentially increasing the ignition delay time. Increasing the strain rate leads to sudden quenching of the reaction, with no potential for re-ignition, as kinetic rates can no longer support combustion. The quenching strain is increased and ignition delay is substantially reduced when the ignition source is a long-duration “pilot-flame.” These results indicate that the burning rate depends strongly on the strain history, when flow and chemical time scales are close. An important example of strain history, which resembles that produced by a turbulent flow, is an oscillating strain. We find that flamelet combustion exhibits strong sensitivity to strain field oscillations. For high amplitude oscillation, the flame is quenched as the total strain exceeds the steady quenching strain, even though the mean strain may be well below the quenching strain. Partial quenching and re-ignition may replace permanent quenching if the frequency of oscillations is increased. When compressive strains prevail, partial extinction is observed. The flamelet re-ignites as positive strains induce reactants' fluxes towards the hot extinct reaction zone. quenching, partial quenching, extinction and re-ignition result in large variations in the unsteady burning rate and in a mean burning rate which is smaller than the steady burning rate computed using the mean strain.