An experimental study of pulsating instability in near-limit laminar nonpremixed flames

Pulsating instability of near-limit C3H8, CH4, and H-2 nonpremixed flames was experimentally investigated. The criterion of an effective Lewis number (Le(eff)) (Chen, R.-H., Mitchell, G.B., and Ronney, P.D., Proc. Combust. Instit., vol. 24, pp. 213-221, 1992) based on the premixed burning regime described by Linan (Linan, A., Acta Astronaut., vol. 1, pp. 1007-1039, 1974), which successfully explained the cellular instability of near-limit nonpremixed flames with Le(eff) < 1, is extended for pulsating instability in this study. In this criterion, near-extinction diffusion flames exhibit fuel-lean or-rich burning and the Lewis number of the deficient species is Le(eff). Pulsating instability of nonpremixed flames was found to occur near extinction limits and for Le(eff) sufficiently greater than 1. Pulsating modes were observed near both the heat-loss and blowout limits. Present experimental results suggest that Le(eff) greater than or equal to 2.6 for pulsating instability to occur in near-limit flames. Flames with Le(eff) as large as 1.8 did not exhibit pulsating instability. A criterion recently proposed by Kim and Lee (Kim, J.S. and Lee, S.R., Combust. Theory Model., vol. 3, pp. 123-146, 1999), where a Lewis number (L-e) is a weighted average of the fuel and oxidizer Lewis numbers with the mixture strength (A(F)) as the weighting factor, is also examined. The theoretical criterion is found to be supported by the experimental results. For all values of A(F), no pulsation was observed for L-e < 2.0. For A(F) --> 1, pulsation was not observed for L-e as large as 2.21, while for A(F) sufficiently greater than 1 pulsating instability was observed for L-e > 2.0. This suggests that larger values of L-e are necessary for pulsating instability to occur in near-limit flames for A(F)-->1, in agreement with the theoretical prediction.