Determination of laminar flame speeds from nozzle-generated counterflow flames

Abstract

The accuracy of the counterflow, twin-flame technique for the determination of laminar flame speeds was examined analytically and numerically. The analysis was conducted by using multiple-expansion, high activation energy asymptotics while the numerical simulation incorporated detailed chemistry and transport. In both approaches the solutions were obtained in a finite domain and with plug flow boundary conditions in order to better simulate the actual experiments. Results show that linear extrapolation of the minimum velocity to zero stretch over-estimates the true laminar flame speed. This over-estimate, however, can be reduced by using smaller ratios of the flame thickness to the nozzle separation distance. Numerical results indicate that for typical paraffin/air mixtures, nozzle separation distances of the order of 14 to 22 mm yield laminar flame speeds accurate to within the uncertainty range of the experiment. The results obtained herein thus provide further support for the viability of the counterflow technique, when the influence of the nozzle separation distance is properly accounted for. An alternate technique for the determination of laminar flame speeds, based on the variation of flow velocity at a constant temperature near the upstream boundary of the flame with stretch, suggest that the over-estimation by linear extrapolating to zero stretch is smaller compared to the minimum velocity approach.