Effects of radiative heat loss on the extinction of counterflow premixed H₂-air flames

Radiation heat loss has an important impact on near-limit flames. It has been shown that radiation heat loss can make stretched CH₄-air flames extinguish at a lower stretch rate. Numerical calculations of counterflow premixed H₂-air flames were conducted using an accurate description of the chemical kinetics and transport properties. Radiation heat loss was considered. The results show that in addition to the stretch extinction limit, radiation heat loss also allows the lower and middle equivalence ratio counterflow premixed H₂-air flames to extinguish at a lower stretch rate. For middle equivalence ratio counterflow H₂-air flames, the closed temperature profiles become distorted O-shaped curves due to the lower Lewis number, being different from those of CH₄-air flames. For higher equivalence ratio counterflow premixed H₂-air flames, there are two stable flame branches-a normal flame branch and a weak flame branch. When the equivalence ratio is greater than a critical value, the closed temperature profile curve of every equivalence ratio flame opens and the normal flame curve can be extended to zero stretch rate. The calculation of the concentration limit of one-dimensional planar premixed H₂-air flames was also conducted. The results show that the critical equivalence ratio corresponds to the concentration limit of the one-dimensional planar premixed H₂-air flame. The extension of the flammable region due to the stretch is amplified for the counterflow H₂-air flame because of its much lower Lewis number than that of the counterflow CH₄-air flame.