Lift-off Heights and Visible Lengths of Vertical Turbulent Jet Diffusion Flames in Still Air

Abstract

The lift-off heights and visible-flame lengths of jet diffusion flames in still air have been determined for hydrogen, propane, methane and ethylene. The flame lift-off height varies linearly with the jet exit velocity and is independent of the burner diameter for a given gas. The results support the assumption that if the burner exit flow is choked the burner can be approximated by an equivalent convergent-divergent nozzle at whose exit the flow has expanded to ambient pressure. The data for different gases can be collapsed onto a single curve if they are plotted in terms of the appropriate non-dimensional groupings. These results and previous results for blow-out stability suggest that diffusion flames blow out when the base is lifted to between 0.65 and 0.75 times the height at which stoichiometric concentration is reached at the jet axis. It can be deduced from the experimental results that, at the base of the flame. the ratio of turbulent burning velocity to laminar burning velocity varies as the square root of the local turbulence Reynolds number based on the integral length scale. The predicted correlation for the turbulent burning velocity agrees well with the experimental data presented in the literature. The flame length results for different gases and burner diameters can be collapsed onto a single curve if plotted in terms of the non-dimensional groupings suggested by Becker and Liang (Combust. Flame. 32, p. 115, 1978). The results near the forced convection limit are in line with the theoretical work presented by Becker and Liang but disagree with their final recommendation. Away from the forced convection limit, the flame length correlation is similar to that proposed by Becker and Liang.