EXPERIMENTAL AND THEORETICAL INVESTIGATION OF PARTIALLY PREMIXED DIFFUSION FLAMES AT EXTINCTION.

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 Linan 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 Damkoehler numbers at quenching for the partially premixed to the unpremixed case does not depend on any additional parameters. The predicted increasing sensitivity of partially premixed flamelets to flame stretch when compared to initially unpremixed flows is verified.