Lifting mechanism of free jet diffusion flames

Abstract

The lifting phenomena of free jet diffusion flames of pure hydrogen and hydrogen-inert gas mixtures stabilized on a thin-walled burner tube were studied in unconfined, quiescent, oxygen-inert gas atmospheres. Nitrogen, argon, and helium were used as the inert gases. A laminar flame base, which exists even in a fully-developed turbulent jet diffusion flame, controls the flame stability. A simplified flame stability model is presented based on the concept that the flame base is the very end of the diffusion flame and it provides a continuous ignition source to propagate towards the combustible zone between the flame base and the burner tip. The local burning velocity parallel to the diffusion flame zone is considered at the flame base to be equilibrated with the entrained stream velocity component opposite to the direction of the burning velocity. Velocity measurements were made in the flame stabilizing region under near-limit conditions by means of the laser-Doppler technique. The measured velocity component parallel to the flame zone at the flame base was found to be directly proportional to the maximum laminar burning velocity obtainable by mixing the jet and external fluids. The result is independent of whether the mixture strength was obtained by varying the hydrogen content of the jet fluid or the oxygen content of the external fluid.