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

Abstract

Pulsating instability of near-limit C₃H₈, CH₄, and H₂ 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 Liñán (Liñán, 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.

Keywords:

• diffusive-thermal effects
• Lewis number
• extinction
• pulsation
• instability
• nonpremixed flames
• laminar flames

Marcos Chaos was a Siemens-Westinghouse Power Corporation Doctoral Fellow. R.-H. Chen was partially supported by Siemens-Westinghouse Power Corporation, Orlando, FL. The assistance of Xing Xing and Dr. Young-Kil Yu in the early phase of the study is also acknowledged.

Notes

^a At both heat-loss and blowout limits.

^b Flames with α >1 for the C₃H₈–O₂–He system (and Le _eff = Le _F) could not be established during the experiment (see Results and Discussion).

^c These flames exhibited cellular instability as Le _eff in general is less than 0.8 approximately (Chen et al., 1992) and as shown in Figure 3f .

^d Effective Lewis number, L _e, as calculated from Kim and Lee (1999) is given here for comparison; note that A _F=1/α.

^e Adiabatic flame temperature calculated using CHEMKIN (Kee et al., 2000).

^f Maximum measured flame temperature.