On the structure of a stretched/compressed laminar flamelet—Influence of preferential diffusion

Abstract

The local Lewis number on the fuel side of a H 2/air diffusion flame is knwn to be lower than unity. When a high-Damkohler-number hydrogen flame is stretched, the flame temperature changes—not because of incomplete chemical reactions, but due to the preferential diffusion resulting from the nonunity Lewis number. These changes are examined using a Computational Fluid Dynamics with Chemical reactions (CFDC) code that is third-order accurate. Detailed chemical kinetics including NO reactions is used to investigate the effects of stretch on the flame structure. A low-speed H 2/air diffusion flame having a fuel-jet velocity of 3.26 m/s is subjected to stretch using two vortices—one located on the fuel side of the flame and the other on the air side. Both vortices create positive (stretch) and negative (compression) flame-stretch factors when interacting with the flame. The temperature of the positively stretched flamelet was found to decrease when the vortex is located on the air side and to increase when it is on the fuel side. Similarly, compression of the flamelet resulting from the air-side vortex increases the flame temperature, whereas fuel-side vortex decreases the flame temperature. Among the eleven species considered in this model, production of NO in the flame zone appears to be the most sensitive to flame stretching or compression. Concentration of NO is found to be maximum in the compressed flamelet formed during the outside vortex-flame interaction. Finally, a comparison of the mean-NO-concentration profiles obtained by averaging over several cycles of outside vortex-flame interactions and the time-averaged measurements yielded favorable agreement.