Effects of Lewis number and ignition energy on the determination of laminar flame speed using propagating spherical flames

The trajectories of outwardly propagating spherical flames initiated by an external energy deposition are studied theoretically, numerically, and experimentally by using hydrogen/air mixtures. Emphasis is placed on how to accurately determine the laminar flame speeds experimentally from the time history of the flame fronts for mixtures with different Lewis numbers and ignition energies. The results show that there is a critical flame radius only above which is the linear and non-linear extrapolation for flame speeds valid. It is found that the critical radius depends strongly on the Lewis number. At large Lewis numbers, the critical radius is larger than the minimum flame radius used in the experimental measurements, leading to invalid flame speed extrapolation. The results also show that there is a maximum Karlovitz number beyond which propagating spherical flame does not exist. The maximum Karlovitz number decreases dramatically with the increase of Lewis number. Furthermore, the results show that the ignition energy has a significant impact on the flame trajectories. It is found that the unsteady flame transition causes a flame speed reverse phenomenon near the maximum Karlovitz number with different ignition energies. The occurrence of flame speed reverse greatly narrows the experimental data range for flame speed extrapolation. The strong dependence of flame trajectory on ignition energy and the existence of the flame speed reverse phenomenon are also confirmed by experimental results.