Momentum- and buoyancy-driven laminar methane diffusion flame shapes and radiation characteristics at sub-atmospheric pressures

• Buoyancy and momentum controlled laminar methane diffusion flame in low pressure.
• Dimensionless flame height y/C generally decreases when CRi increases.
• Radiant fraction increases at p0.2.
• Strouhal and Froude numbers posses as St ∝ Fr−0.48.
• B-driven flames flicker slower than M-driven ones.

Buoyancy and momentum are two major driving forces that affect the behavior of diffusion flames, which have not been fully interpreted in sub-atmospheric environments. In this work, a theoretical model based on the cylindrical flame shape is proposed incorporating with the flame width and height in order to predict the steady flame height with Richardson number. The theoretical equation of dimensionless non-flickering flame height y/C was deduced, and the value of y/C was shown to slightly increase for relatively small CRi and decrease significantly with increasing CRi. To verify this model, buoyancy (B)- and momentum (M)-driven methane laminar diffusion flames with the mass fuel flow rate in the range of 2.99–23.9 × 10−6 kg/s were investigated at 0.45–1.00 atm. The flow regimes are dominated by the secondary buoyancy acceleration and initial fuel axial velocity, respectively. Experimental results show that first, for steady flames y/C decreases with increasing CRi, which is consistent with the prediction of the model. The value of y/C decreases with increasing air pressure linearly at −0.046, −0.068, and −0.077 slopes for three different CRi levels. Second, radiant fraction of B-driven flames is generally bigger than that of M-driven ones due to longer soot residence time. The radiant fraction increases with increasing air pressure for both B- and M-driven flames at nearly p0.2. Third, for flame oscillation, Strouhal and fuel Froude numbers have the following relationship: St ∝ Fr−0.48, f ∝ uf,00.04/d0.52, i.e., B-driven flames flicker slower than M-driven ones. Considering the effect of air pressure, f ∝ p1/3−β (β ≈ 0.30 for B-driven flames and β ≈ 0.19 for M-driven ones), thereby indicating that flickering frequency increases with increasing air pressure, and the increasing rate of flickering frequency of M-driven flames is higher than that of B-driven ones.