Effect of blockage ratios on the characteristics of methane/air explosion suppressed by BC powder.

To investigate the effect of blockage ratios on the explosion suppression by powder suppressant, an experimental study was performed to suppress the methane-air explosion in a 5L duct with different blockage ratios and various concentrations of BC dry powder. The results indicate that flames experienced both the spherical and finger-shaped stages. Furthermore, the smoothness of flame front initially decreased and then increased. Flame propagation velocities were higher with larger blockage ratios except for φ = 1. The maximum peak overpressure (MPP) with the blockage ratio was slightly increased till φ reached 0.7 then surged sharply. The MPP decreased as the powder concentration increased. The maximum drop rate in the MPP being 34.8%-59.9%, depending on powder concentrations, occurred at the blockage ratio between 0.4 and 0.6. The result is ascribed to the competition between the suppression augmentation by the higher venting-generated turbulence and the suppression attenuation by the shorter residence time of the particle. However, the drop rate was relatively less promoted by increasing the concentration from 80 g/m3 to 240 g/m3. The inhibitor at higher concentration was less effective. An inhibition mechanism is explained by analogy to droplet group combustion, in which the decomposition regime of NaHCO3 differs at different concentrations.