Stabilization criteria of laminar lifted propane flames and their transition to turbulent lifted flames under extended pressures

Abstract

Recently, the mechanism of a laminar lifted flame (LLF) was clearly explained using an effective Schmidt number. It was found that a stable LLF can be formed within a triangular stabilization regime (TSR) in the domain of normalized lift-off height and Reynolds number. However, the experimental pressure range in the previous study was limited to three-times (1–3 bar) because there was an early transition to turbulent flow at elevated pressures. This study examined the effects of pressure on the LLF characteristics of propane, within an extended pressure range of 12.5-times (0.2–2.5 bar) with larger fuel tubes. This allowed more precise and abundant experimental results to be obtained for flame structures, and the general TSR for all LLFs was confirmed. In addition, variations in the effective Schmidt numbers and effective diffusivities were estimated for extended pressures, and the correlation between the fuel jet velocities and the lift-off heights was explained. Conclusively, a more general relationship between the edge flame speed (EFS) and the effective fuel concentration gradient (EFCG) was obtained under extended pressures. The existence of a minimum EFS at the maximum EFCG was confirmed. Furthermore, the non-monotonic variation in the lift-off height was explained, and the variations in the flame structure during the transition from an LLF to a turbulent lifted flame were investigated in more detail.