Influence of inert gas addition on propagation indices of methane–air deflagrations

Abstract

The work examines the characteristic indices of laminar deflagrations propagating in methane–air gaseous mixtures diluted by several inert gases: He, Ar, N2 or CO2. Experiments were performed in two spherical vessels of different volumes with central ignition, at ambient initial conditions. Mixtures with variable methane concentrations (6–12vol%) and variable inert concentrations (5–40vol%) were studied, in order to outline the inert influence on the most important and accessible safety-related parameters: the peak explosion pressure, the maximum rate of pressure rise (or the related property, i.e. the deflagration index) and the explosion time (the time necessary to reach the peak explosion pressure). Among the studied inert additives, CO2 is the most efficient, followed by N2, Ar and He. Inert gas addition to any flammable CH4–air mixture determined the decrease of both experimental and adiabatic explosion pressure and of the maximum rate of pressure rise, along with the increase of the explosion time. Using an equation that describes the heat balance of the isochoric combustion of a fuel–air mixture under non-adiabatic conditions, a correlation between the peak explosion pressure and the mole fraction of inert gas was derived and validated for CH4–air– inert mixtures.