AN EXPERIMENTAL STUDY OF PULSATING INSTABILITY IN NEAR-LIMIT LAMINAR NONPREMIXED FLAMES

摘要

Pulsating instability of near-limit C_3H_8, CH_4, 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) ≥ 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.