A radical index for the determination of the chemical kinetic contribution to diffusion flame extinction of large hydrocarbon fuels

The extinction limits of diffusion flames have been measured experimentally and computed numerically for fuels of three different molecular structures pertinent to surrogate fuel formulation: n-alkanes, alkyl benzenes, and iso-octane. The focus of this study is to isolate the thermal and mass transport effects from chemical kinetic contributions to diffusion flame extinction, allowing for a universal correlation of extinction limit to molecular structure. A scaling analysis has been performed and reveals that the thermal and mass transport effects on the extinction limit can be normalized by consideration of the enthalpy flux to the flame via the diffusion process. The transport-weighted enthalpy is defined as the product of the enthalpy of combustion per unit mole of fuel and the inverse of the square root of fuel molecular weight. The chemical kinetic contribution provided by the specific fuel chemistry has thus been elucidated for tested individual component and multi-component surrogate fuels. A chemical kinetic flux analysis for n-decane flames shows that the production/consumption rates of the hydroxyl (OH) radical govern the heat release rate in these flames and therefore play significant roles in defining the extinction limit. The rate of OH formation has been defined by considering the OH concentration, flame thickness, and flow strain rate. A fuel-specific radical index has been introduced as a concept to represent and quantify the kinetic contribution to the extinction limit owing to the fuel-specific chemistry. A relative radical index scale, centered on the radical index of a series of n-alkanes which are observed and fundamentally explained to be common, is established. A universal correlation of the observed extinction limits of all tested fuels has been obtained through a combined metric of radical index and transport-weighted enthalpy. Finally, evidence as to the validity of the fundamental arguments presented is provided by the success of the universal correlation in predicting the extinction limits of the multi-component mixtures typical of surrogate fuels.