Impact of low-pressure conditions on flame width and volume of buoyant methane laminar diffusion flames

Abstract

In this work, an experimental investigation of the main morphological parameters of buoyant methane laminar diffusion flames, including flame width and flame volume, is carried out under various sub-pressures (20-100 $kPa$). Quantitative models for flame width and volume are derived and compared against experimental results by assuming a cylindrical flame shape. The methane buoyant laminar diffusion flames are produced using a circular burner with an inner diameter of 8 $mm$, and the fire tests are conducted in a low-pressure chamber with inner dimensions of 3 × 2 × 2 $m$. The mass flow rate in the current study ranges from 2.988 $mg/s$ to 8.365 $mg/s$. The experimental results show that air pressure impacts flame appearance. When the pressure is increased, it is noted that the flame transitions from steady flames or tip-flickering flames to sinuous meandering flames or even bulk-flickering flames. Additionally, the flame width decreases with increasing ambient air pressure for steady and tip-flickering flames, and the results show that the flame width is inversely proportional to the 1/3 power on the ambient pressure, $W_f\sim P^{1/3}$. For bulk-flickering flames, the flame width is independent of ambient air pressure, $W_f\sim P^0$. Also, the flame volume decreases with increasing air pressure at relatively small mass flow rates, and the measured flame volume is inversely proportional to the ($0.43-0.55$) power of the ambient pressure. Finally, based on dimensionless analysis, prediction models for flame width and volume are proposed, which are $W_f^{\ast }=0.388{\Phi }^{-0.51}$ and $V_f^{\ast }=2.177\times {10}^{-4}{\Phi }^{-0.85}$. The correlated results successfully unify the current experimental findings.