Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities

Fuel-lean flammability limits and burning velocities in a closed vessel were measured for methaneair mixtures burning at earth gravity (one-g) and zero-gravity (zero-g) at initial pressures of 50–1500 Torr. The zero-g flammability limit was found to be between the one-g upward and one-g downward flammability limits. For sublimit mixtures burning at zero-g, an extinguishment phenomenon unlike any found at one-g was observed. For fast burning mixtures (Su > 15 cm/s), burning velocities were identical at one-g and zero-g. For slower burning but still flammable mixtures, only the zero-g observations could be interpreted to obtain burning velocity data because at one-g natural convection caused severe flame front distortion. Zero-g burning velocities for these mixtures were in good agreement with existing models of laminar flame propagation and with extrapolation of current and prior one-g data. The main conclusions are that the one-g upward flammability limit occurs at a mixture which has a burning velocity which is so low that flame propagation is impractical, that the one-g downward flammability limit is related to the inability of the flame front to propagate downward against buoyant forces, and that near-limit flame propagation at zero-g is mostly independent of the experimental apparatus. Because of the unusual nature of the extinguishment process for sublimit mixtures burning at zero-g, further experiments are required to determine the cause of the zero-g flammability limit.