Uncertainty reduction in laminar flame speed extrapolation from expanding spherical flames

The reduction of extrapolation uncertainty for laminar flame speed is studied using expanding spherical flame measurements under positive Markstein length (Lb>0) conditions. The performance of three extrapolation formulas including one linear and two non-linear formulas were examined under a wide range of experimental conditions of various flame radius ranges. It is found that the ratio of Lb and the flame radius Rf used for extrapolation is the controlling parameter of the extrapolation uncertainty. For conditions with Lewis number close to unity, the extrapolation uncertainty is small due to small value of (Lb/Rf)2. However, for mixtures with supra-unity Lewis number, the (Lb/Rf)2 terms in the expanded form of the extrapolation formulas are large at commonly used Rf, resulting in large uncertainties in the extrapolated laminar flame speeds. The extrapolation uncertainty can be effectively reduced using flame measurements with large flame radius range as demonstrated in a newly built constant-pressure, dual-chamber experimental rig of large dimension.