Abstract: Both the dispersion pressure and ignition delay time have a significant influence on the spatial distribution of the powder inhibitor, thus on the efficiency of explosion inhibition. To study the effect of dispersion condition of given mass of NaHCO₃ on inhibiting 9.5% methane-air mixture explosion, a series of experiments were conducted in a self-built 5 L duct having a capability of changing both the dispersion pressure and the ignition delay time. Based on the powder dispersion state visualized by a high-speed camera and the explosion inhibition efficiency, conclusions can be drawn as follows. First, the dispersion pressure and the ignition delay time should be changed with the mass of NaHCO₃. An inappropriate dispersion pressure cannot enable the NaHCO₃ powder to distribute evenly in the duct. Specially, the larger the mass of NaHCO₃ is,the higher the dispersion pressure should be. Moreover, under the same dispersion pressure condition, there is a positive correlation between the ignition delay time required for achieving the even powder dispersion and the mass of the powder. Second, the inhibitor with the excellent efficiency can always prolong the flame propagation time by hundreds of milliseconds. Therefore, the NaHCO₃ powder would always be spatially uneven at the initial stage of the flame propagation owing to the gravitational sedimentation, provided that the ignition was activated at the moment when the powder achieved the best spatially-even dispersion. Consequently, to evaluate the efficiency of the inhibitor reasonably when the powder dispersion system and the ignition system are located at the bottom of the duct, the ignition delay time should be chosen during the rising stage of the powder dispersion at which the powder is about to fill fully the duct.

Key words: dispersion pressure, ignition delay time, mass, methane, NaHCO₃, explosion suppression