Experimental study on ignition and flame propagation of hydrogen/carbon black hybrid mixtures in a vertical tube

Under incomplete oxidation or carbonaceous nanoparticle formation, hybrid mixture of nano-size carbon powders and combustible gases may arise. These mixtures display specific explosive behaviours. Therefore, it is vital to get insight into the explosion characteristics of these hybrid mixtures before the completion of risk assessments. In this work, the ignition behaviour and flame characteristics of hydrogen and carbon black dust hybrid mixtures were experimentally investigated in a transparent vertical tube with a diameter of 60 mm and a length of 1 m. Clear and intuitive flame images of the hybrid mixture, initiated by a 10-J electrical spark ignition source, were obtained by using a high speed-infrared camera. The results showed that the hydrogen was involved in the combustion reaction in the initial stage of ignition, even with 1 vol H2. However, the flame propagation of hybrid mixture with 1 vol H2 could not be able to self-sustained due to the trace amount of fuel. Moreover, the addition of carbon black dust did not significantly influence the lower flammable limit of the hybrid mixture. Under the condition of low carbon black concentration (50 g/m3), increasing H2 concentration to 7 vol changed the flame structure from a coupled flame front to two separated flame fronts. Furthermore, with increasing carbon black dust concentration, the combustion transitioned from "gas driven" to "dual-driven" under the condition of low hydrogen concentration (less than 7 vol). In contrast to hydrogen, the increase in carbon black concentration had little effect on the flame propagation speed, indicating that the thermodynamics of the hybrid mixtures was mainly controlled by the combustion of H2. These results improve our understanding of the ignition behaviour and flame propagation characteristics of hybrid mixtures containing hydrogen.