Local extinction and reignition in nonpremixed turbulent CO/H 2 /N 2 jet flames

In practical combustors, high-intensity mixing is often desirable in order to minimize combustor volume and also minimize the production of certain pollutants. However, high-intensity mixing may also lead to local quenching and, potentially, a failure to stabilize the flame. Two crucial issues are what fraction of the reactive mixture is quenched and how rapidly these regions reignite. To address this question over a range of conditions at relatively high Reynolds numbers, the one-dimensional turbulence (ODT) model is adopted. Results are reported for a piloted planar temporal CO/H 2 /N 2 jet flame configuration at a nozzle Reynolds number of 15,000 and three global mixing rates, U 0 / d 0 =8.3 2 10 3 s m 1 , 1.3 2 10 4 s m 1 , and 2 2 10 4 s m 1 . The fraction of extinguished stoichiometric regions increases with the global mixing rate as expected from laminar flamelet theory. The maximum fraction of extinguished regions, measured as a volume fraction of stoichiometric mixture, is 5%, 17%, and 40%, respectively, for the three mixing rates. The fraction of extinguished regions is significantly greater than the probability that the scalar dissipation rate exceeds the critical value for extinction, indicating that the reignition process is slow relative to the extinction process. The rate of reignition is found to scale roughly with the global mixing rate; this means that reignition is more rapid for conditions favorable to extinction.

Keywords:

• Turbulent-jet Diffusion Flames
• One-dimensional Turbulence
• Extinction
• Reignition
• Syngas Flames