This study investigates and quantifies the soot free length ( \({L}_{\rm b}\) ) and soot free length fraction ( \(\text{SFLF}\) ) of methane laminar diffusion flames at sub-atmospheric pressures (20–100 \(\text{kPa}\) ), which are rarely reported in literature. Methane-buoyant laminar diffusion flames are produced using a circular aperture burner with an inner diameter of 8 mm, and a series of fire tests is conducted in a hypobaric chamber with internal dimensions of \(3\times 2\times 2\text{ m}\) . The mass flow rates in this study are set to 2.988–8.365 mg s−1. The results indicate that for steady and tip-flickering flames, the total flame length increases with pressure \({L}_{\rm f}\sim {P}^{1/5}\) . However, for bulk-flickering flames, the total flame length is nearly constant with \({L}_{\rm f}\sim {P}^{0}\) . Additionally, both \({L}_{\rm b}\) and \(\text{SFLF}\) decrease with an increasing ambient air pressure. For a given pressure, both the \({L}_{\rm b}\) and \(\text{SFLF}\) decrease with an increasing mass flow rate (or heat release rate). The Reynolds number ( \(\text{Re}\) ), which has been successfully used to characterize the \({L}_{\rm b}\) and the \(\text{SFLF}\) of buoyant turbulent jet flames, fails with buoyant laminar diffusion flames. The dimensionless soot free length correlates well with the nondimensional heat release rate. Moreover, a prediction correlation for the \(\text{SFLF}\) of methane-buoyant laminar diffusion flames is developed, which could effectively unify the experimental data in the current work.
