A study of steady laminar diffusion flames at buoyancy-induced low stretch rates in a spherical porous gas burner

The steady combustion behavior and stability range of methane, ethylene, and propane in a low stretch spherical counterflow burner were investigated. Ethylene had the widest fuel mass flux (${\dot{m}}^{″}$) range of steady blue flame. A decreased buoyant stretch rate (a_b) reduced this steady range. The critical ${\dot{m}}^{″}$ at the extinction limit of the low stretch flames were lower than that of the normal gravity, and ethylene had the lowest critical ${\dot{m}}^{″}$ at the lowest a_b, indicating a wider steady fuel mass flux range for ethylene and a greater fire risk for microgravity low stretch flames. The ratio of the flame horizontal length to the vertical length (l_f/y_f) was quantified by a dimensionless parameter: $P{e}^{-\frac{4}{5}}{\left(\frac{\Delta }{Fr}\right)}^{-\frac{1}{5}}{\left(S+1\right)}^{-\frac{6}{5}}$, which considered the combined effects of burner properties (described by Péclet number Pe), gravity/buoyancy effect (described by expansion parameter $\Delta$ and Froude number Fr) and fuel combustion process (described by chemical reaction characteristic parameter S). The flame temperature T_f for the fuels increased with increasing u_F and a_b, whereas T_f increased slowly when u_F was greater than the sooting limit u_F due to incomplete combustion with appreciable heat loss. The low stretch flame temperatures of the three fuels rank as C₂H₄>C₃H₈>CH₄ because of their different heats of combustion and heat losses. Both the gas-phase and solid-phase heat losses fractions were significant at low stretch rates. Solid-phase heat loss was vital to methane, which largely influenced its periodic flame hole instability near the extinction limit. This flame instability also extended the flammable range of methane at low stretch rates. This study can give instructions for microgravity combustion because a flame is usually initiated at low stretch rates under microgravity.