The upper flammability limits (UFL) of hydrogen–air,
methane–air,
ethane–air, n-butane–air, and ethylene–air
were determined experimentally at room temperature (20 °C) and
initial pressure of 1.0, 0.7, 0.5, 0.3, 0.1, and 0.05 atm. Experiments
were conducted in a closed cylindrical stainless steel vessel (i.d.
10.22 cm, length 100 cm) with upward flame propagation. The UFL of
hydrogen was observed to be inversely proportional to the initial
pressure in the range from 1.0 to 0.3 atm and proportional to the
initial pressure from 0.3 to 0.05 atm. In contrast, the UFLs of the
lower alkanes and ethylene decreased with the initial pressure. The
average flame propagation velocities at UFL concentrations of hydrogen,
methane, ethane, n-butane, and ethylene in air at
reduced pressures were also examined. It was found that the flame
propagation velocity of hydrogen was larger than those of the hydrocarbons,
increased when the initial pressure decreased from 1.0 to 0.3 atm,
and then decreased with further decrease of pressure. Flame propagation
velocities at UFL concentrations of the hydrocarbons decreased with
the initial pressure. Finally, based on the behavior of the UFLs and
flame propagation velocities, the relative risk and hazards of ignition
and flame escalation of hydrogen and the light hydrocarbons at subatmospheric
pressures were discussed.
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