Effect of scale on flame speeds of methane–air

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.

Impact of suspended coal dusts on methane deflagration properties in a large-scale straight duct

Knowledge about flame deflagrations in mixtures of methane and diluted coal dust assists in the prediction of fires and explosions, and in the design of adequate protective systems. This vital lack of information on the role of hybrid mixtures (methane/coal dust) is covered in this work by employing a novel Large-Scale Straight Duct (LSSD) designed specifically for this purpose. The hybrid fuel was injected along the first 8m of the 30m long LSSD. The results revealed that a 30gm^-3 coal dust concentration boosted the flame travel distance, from 6.5m to 28.5m, and increased the over pressure rise profile to 0.135bar. The over pressure rise (OPR), pressure wave velocity, flame intensity and the flame velocity were significantly boosted along the LSSD in the presence of 10gm^-3 or 30gm^-3 coal dust concentrations in the methane flame deflagrations. Finally, the high speed camera showed that the presence of the coal dust enhanced the turbulence in the front flame. Consequently, the pressure wave and flame velocities were both increased when a 10gm^-3 coal dust concentration coexisted with a 9.5% methane concentration in the deflagration.