Opening of the burke-schumann flame tip and the effects of curvature on diffusion flame extinction

Effects of flame curvature on diffusion flame extinction have been studied via the model problem of the opening of the Burke-Schumann flame tip. Asymptotic analysis of the flame structure in the tip region shows that increasing the extent of the flame curvature facilitates near-complete reaction and theraby enhances the burning intensity. Consequently, for unity Lewis number flames, increases in the flow velocity tend to inhibit tip opening because of the corresponding decrease in the radius of the flame tip. Expermental results using nearunity Lewis number acetylene/air flames agree with the predicted flame geometry and its inability to achieve tip opening. Tip opening, however, can be achieved by using a sub-unity Lewis number fuel stream of hydrogen and carbon dioxide, which causes a general lowering of flame temperature in the entire flame tip region. Further experiments then confirm the theoretical result that negative stretch, in the form of compressive flame curvature, promotes burning and thereby retards extinction.