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Unsteady premixed and non-premixed counterflow laminar flame simulations were conducted in order to investigate extinction effects on observables commonly used in turbulent combustion. CH 4 and n-C 12 H 26 were the fuels studied, with air as the oxidizer at pressures of 1, 5, and 10 bar. It was determined that CH 2 O 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 CH 2 O consumption pathway. The persistence of CH 2 O concentration is duplicated similarly in CH 4 and n-C 12 H 26 premixed flames. For non-premixed flames, the results indicate that the peak CH 2 O concentration reduction for n-C 12 H 26 flames is milder compared to CH 4 flames. Increasing the pressure causes an extension of reactivity, resulting in greater CH 2 O 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 CH 2 O during extinction. Thus, caution is suggested when using CH 2 O in turbulent combustion experiments as a marker of the preheat zone thickness, given that increased levels of CH 2 O could be a result of multiple local extinction events. In addition, the product of OH and CH 2 O was found to scale well with the heat release rate for CH 4 and n-C 12 H 26 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.

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An experimental and numerical investigation of fuel and hydrodynamic effects is performed on piloted premixed jet flames. The investigation is carried out at a constant laminar flame speed, varying heat losses, jet Reynolds number, fuel molecular weight, and fuel chemical classification. Large Eddy Simulations are performed in an attempt to reproduce the behaviors observed experimentally. Simulations are compared against well-characterized boundary conditions, well-resolved two-dimensional velocity fields from particle image velocimetry, and line-ofsight CH* profiles. Experimental results indicate that small amounts of heat losses may play a significant role on the jet reactivity as the flame heights scale with the heat loss from the jet. However, differences between flames with different fuels can still be seen in the absence of heat losses and these differences are magnified at higher Reynolds numbers. Particularly, methane flames are consistently taller and ethylene flames consistently shorter while other fuels present approximately the same flame height. LES reproduce the experimentally observed trends in global flame heights (effects of heat losses and Reynolds number) but some of the differences between fuels are not captured.

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