Assessment of experimental observables for local extinction through unsteady laminar flame calculations

Unsteady premixed and non-premixed counterflow laminar flame simulations were conducted in order to investigate extinction effects on observables commonly used in turbulent combustion. CH4 and n-C12H26 were the fuels studied, with air as the oxidizer at pressures of 1, 5, and 10 bar. It was determined that CH2O persists, compared to all other reactive species, during the extinction transient for both fuels and at all conditions, as the loss of OH concentration removes the dominant CH2O consumption pathway. The persistence of CH2O concentration is duplicated similarly in CH4 and n-C12H26 premixed flames. For non-premixed flames, the results indicate that the peak CH2O concentration reduction for n-C12H26 flames is milder compared to CH4 flames. Increasing the pressure causes an extension of reactivity, resulting in greater CH2O production and thus a delayed decay during the extinction transient. In addition, a change in the magnitude of the applied scalar dissipation rate for the non-premixed flames did not alter the trends of CH2O during extinction. Thus, caution is suggested when using CH2O in turbulent combustion experiments as a marker of the preheat zone thickness, given that increased levels of CH2O could be a result of multiple local extinction events. In addition, the product of OH and CH2O was found to scale well with the heat release rate for CH4 and n-C12H26 flames at multiple pressures. Finally, the CH* and OH* chemiluminescence was examined. CH* was found to extinguish slightly before the other species and more importantly, that once its concentration is reduced to a negligible level, the flame is on its way to extinction with no chance of recovery. OH* was determined to scale well with heat release at both 1 and 10 bar for both fuels and type of flames.