Direct experimental determination of laminar flame speeds

The stability of premixed flames at ultralow strain rates was assessed experimentally and numerically in the stagnation flow configuration. Results indicate that there are inherent limitations in establishing weakly strained planar flames, and that the accuracy of the laminar flame speeds obtained through linear extrapolations can, thus, be compromised. In view of these limitations, a new methodology is proposed for the direct experimental determination of laminar flame speeds. It includes the use of the stagnation flow configuration and large separation distances betwenn the nozzle and the stagnation plane, which allow for the establishment of Bunsen-type flames as the flow rate is reduced. The flow velocities are measured by using laser Doppler velocimetry. The proposed technique is based on the principle that whereas the planar, strained flames are positively stretched, the Bunsen flames are negatively stretched. Thus, by achieving a smooth, quasi-steady transition between planar and Bunsen flames, the flames pass through a near-zero strain-rate state. Real-time LDV measurements were obtained at numerous fixed spatial locations in the region within which transition occurs. The minimum velocity obtained in these measurements corresponds to the flame speed at the limit of near-zero stretch and is proposed as a representative value of the true laminar flame speed, S H o . Laminar flame speeds were obtained for atmospheric CH 4 /air, C 2 H 6 /air, and C 3 H 8 /air mixtures and for a wide range of equivalence ratios. The new S n o values were found to be systematically lower than the values that have been determined by using the traditional stagnation flow technique and linear extrapolations to zero strain rate.