Stabilization criteria of laminar lifted flames in a non-premixed jet through experiments at elevated pressures

Abstract

The stabilization mechanism of a laminar lifted flame in a non-premixed jet remains an essential issue in combustion theory. Recently, effective Schmidt numbers and effective diffusivities were evaluated from experimental results, and the relationship between the fuel jet velocities and lift-off heights could be converted to a theoretical relationship between the fuel concentration gradient and the flame propagation velocity. In this study, the lift-off heights of propane and butane flames were measured at elevated pressures, and the results were used to determine the effective Schmidt numbers and the effective diffusivities. Based on the experimental results, a triangle regime for stable laminar lifted flames was introduced, and its three limiting mechanisms were explained. First, the minimum lift-off height was determined using the momentum core. Second, the maximum lift-off height was determined by elimination of the stoichiometric condition. Third, the sudden extinction of a lifted flame was explained by the critical Reynolds number. Furthermore, when the tube diameter was larger and the pressure was higher, an additional flame stabilization mode having transient phenomena to a turbulent lifted flame was found. Within the flame stabilization triangle, an improved mechanism of a lifted flame was introduced. The effective Schmidt number increases from a smaller mass diffusion of fuel to a larger one of air as the lift-off height increases. In addition, the flow redirection at the flame base increased the effective Schmidt number even more to reach the local maximum value. A revised fuel concentration gradient that can be used even at elevated pressures was proposed, and the existence of maximum values was confirmed. Finally, the relationship between the revised fuel concentration gradient and the flame propagation velocity (or the edge flame speed) was revealed.