The Influence of Preferential Diffusion On the Blow-Off of Laminar Aerated Burner Flames

Abstract

The blow-off of a laminar aerated burner flame in which Hydrogen is present cannot be characterised by the flame stretch correlation proposed by Reed. A literature review has indicated that the preferential diffusion of Hydrogen towards the point of stability in the region of maximum primary flame front curvature at the base of the flame may cause this anomaly. The influence of the introduction of Hydrogen into the Propane-Air system on blow-off has been studied and correlations of critical boundary velocity gradient and burning velocity against fuel concentration obtained for eight different Propane/Hydrogen ratios. It has been proposed that the degree to which preferential diffusion will occur may be characterised by a dimensionless group termed the preferential diffusion factor. This has been correlated against critical flame stretch factor for the Hydrogen-Propane-Air system and hence an equation has been developed which may be used to predict critical flame stretch factors for aerated systems from a knowledge of fuel concentration and preferential diffusion factor. Physical and transport properties of the Hydrogen-Propane-Air system have been varied by the replacement of the Nitrogen present in the Air by Helium and Argon. These data, together with that for the Hydrogen-Methane-Air system from the literature, have been analysed using the preferential diffusion factor approach and have revealed considerable agreement between predicted and experimental critical flame stretch factor-fuel concentration relationships. Hydrogen concentration isopleths have been studied in the stabilising region of three laminar Hydrogen-Propane-Air flames with 60% Hydrogen present in the fuel gas mixture. Isopleths plotted from this investigation are consistent with the preferential diffusion mechanism. Similar studies of the Ethylene-Air system have been made at all stages of the investigation for comparison purposes.