Experimental and theoretical investigation of partially premixed diffusion flames at extinction

Abstract

The structure and the mechanism of extinction of partially premixed diffusion flames is analyzed on the basis of a model that uses a one-step irreversible reaction with a large activation energy. Close to extinction the inner flame structure is essentially that of the Liñán diffusion flame regime with modifications of the boundary conditions due to partial premixing. Extinction conditions are derived for small fuel-to-air mass ratios. In this limit the ratio of the Damköhler numbers at quenching for the partially premixed to the unpremixed case does not depend on any additional parameters. Experiments on flat partially premixed diffusion flames are performed in an opposed flow burner between two ducts. A fuel stream of diluted methane and an oxidizer stream of diluted air was prepared. The diluent in both streams was nitrogen. Both streams were partially premixed by the other in such a way that the stoichiometric fuel-to-oxidizer mass ratio was the same as in the corresponding unpremixed diffusion flame. As predicted by the theory two nonequilibrium flame structures are observed. The ratio of the velocity gradients at extinction for the partially premixed to the unpremixed flames was compared with the theoretical results. In particular, the predicted increasing sensitivity of partially premixed flamelets to flame stretch when compared to initially unpremixed flows is verified.