  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.
