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

Abstract

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.

Large Eddy Simulation of a Reacting Kerosene Spray in Hot Vitiated Cross-Flow

“…For this purpose, laminar diffusion flame simulations were performed providing a solution to the temperature and species concentrations in mixture fraction ( ) space at constant scalar dissipation rates (SDR). More details on the approach used for the laminar diffusion flame simulations can be found in Paxton et al (2019). The investigated conditions are equivalent to those found in the vitiated cross-flow (summarised in Table 1) and the fuel vapour is assumed to have a temperature of 475 K.…”

“…It was later shown that despite certain drawbacks, the product of Y OH and Y CH 2 O is also a good indicator for increased chemical activity in multi-component fuels (Nikolaou and Swaminathan 2014). More recently, the correlation was extended to non-premixed flames at elevated pressures (Paxton et al 2019) and to the case of a premixed gaseous jet in hot vitiated cross-flow . The present work confirms that Y OH × Y CH 2 O may also be used to identify the topology of a kerosene spray flame in high-temperature vitiated air conditions.…”