Visible flame heights of laminar coflow diffusion flames

Results of experiments are reported for overventilated buoyancy-controlled flames of various fuel mixtures in normal atmospheric air. The fuels studied were C2H6, C2H4, C2H2 and CH4, as well as fixed hydrogen-hydrocarbon mixtures of the first three, to give C2H6+H-2 C2H4+H-2 C2H2+H-2 and C2H2+2H(2). The focus of the work was to test the concept of flame height correspondence between pure fuels and "synthetic" fuels, synthetic methane being C2H6+H-2 synthetic ethylene C2H2+H-2 and synthetic ethane C2H4+H-2 and C2H2+2H(2). Adiabatic flame temperatures T-af were controlled by addition of N-2 to the fuels, and comparisons were made at fixed T-af. In accordance with theory, excellent flame-height agreements were obtained for ethane and the two synthetic ethanes. Agreements were somewhat poorer for synthetic ethylene and especially poor for synthetic methane. The former discrepancy is associated with differing sooting tendencies of the different fuels, while the latter is readily attributable to influences of molecular diffusion coefficients. The reasons for these results and their relationships with previous flame-height and soot-height results are discussed.