Kinetic Similarity between Extinction Strain Rate and Laminar Flame Speed

Abstract

Extinction strain rate (ESR) and laminar flame speed (LFS) are fundamental properties of a fuel/air mixture that are often utilized as scaling parameters in turbulent combustion. While LFS at atmospheric and elevated pressures are extensively investigated, experimental measurements of ESR with counterflow premixed flames are very limited for flame instability often occurs near extinction, especially at high pressures. Due to the scarcity of ESR measurements, most combustion kinetic models are mainly validated and optimized against LFS. However, it is questionable whether the controlling reactions are the same for ESR and LFS such that those models are also valid for predicting ESR. This work quantifies the kinetic similarities between ESR and LFS by analyzing their kinetic sensitivity directions. The direction is represented by a unit vector composed of the normalized sensitivity of ESR or LFS to the rate constant for each elemental reaction. Consequently, the similarity between the two directions is measured by the inner product of the corresponding unit vectors. The sensitivity directions of ESR and LFS are found parallel for various fuels, equivalence ratios, and pressures. Furthermore, sensitivity directions at various strain rates are also similar for the maximum temperature, local temperature at various locations in the flame coordinate, and ESR in counterflow premixed flames. These findings suggest that LFS and ESR are similarly effective as the target for constraining and optimizing rate constants in kinetic models. In addition, the independence of the sensitivity directions on the strain rate also enables us to perform uncertainty quantification for turbulent flames with a wide range of strain rates based on the kinetic sensitivity of ESR and LFS.