Further considerations on the determination of laminar flame speeds with the counterflow twin-flame technique

The accuracy of the laminar flame speed determination by using the counterflow twin-flame techniquehas been computationally and experimentally examined in light of the recent understanding that linear extrapolation of the reference upstream velocity to zero strain rate would yield a value higher than that of the laminar flame speed, and that such an overestimate can be reduced by using either lower strain rates and/or larger nozzle separation distances. A systematic evaluation of the above concepts has been conducted and verified for the ultralean hydrogen/air flames, which have relatively large Karlovitz numbers, even for small strain rates, because of their very small laminar flame speeds. Consequently, the significantly higher values of the previous experimentally measured flame speeds, as compared with the independently calculated laminar flame speeds, can now be attributed to the use of nozzle separation distances that were not sufficiently large and/or strain rates that were not sufficiently small. Thus, by using lower strain rates and larger nozzle separation distances, the experimentally and computationally redetermined values of these ultralean hydrogen/air flames agree well with the calculated laminar flame speeds. The laminar flame speeds of methane/air and propane/air mixtures have also been experimentally redetermined over extensive ranges of the equivalence ratio and are found to be slightly lower than the previously reported experimental values.