Explosions of methane/air/nanoparticles mixtures: Comparison between carbon black and inert particles

During incomplete combustions or nano-size carbon blacks generation, atmospheres of carbonaceous nanopowders and combustible gases are encountered. These hybrid mixtures exhibit specific explosive behaviors, which can notably be caused by the modification of the initial turbulence level or by changes in oxidation reactions. In order to either support or reject such assertions, various nanoparticles/methane mixtures were tested, some with carbonaceous nanopowders, some with inert nanopowders (alumina). The aim of this work is then to compare the influences of alumina and carbon black nanoparticles insertion on the explosion severity and on the flame velocity of methane. Tests were performed in a 20 L explosion sphere and in a 1 m vertical flame propagation tube. An estimation of the unstretched flame velocity is obtained assuming a linear relationship between the burning velocity and Karlovitz stretch factor. It appears that the use of carbon black nanoparticles increases the explosion overpressure for lean methane mixtures by approximately 10%. Similar behaviors have been observed for hybrid mixtures involving alumina particles for fuel lean conditions. For alumina, non-significant changes are observed for fuel rich mixtures. Moreover, a considerable diminution of the explosion severity was noted for fuel rich mixtures when carbon black nanoparticles are dispersed into the reaction vessel. Regarding the flame propagation test for stoichiometric methane concentration, higher unstretched burning velocities were obtained for carbon black hybrid mixtures compared to alumina mixtures. These results suggest soot or carbonaceous nanopowders not only impact the oxidation kinetics, but also the flame stretching and heat transfer.