Experimental study on effect of inert gas dilution for flammability of NH3/ O2 mixtures

This study aims to obtain fundamental data and academic knowledge on the dilution effect of flammability by inert gases based on a series of experimental results and numerical simulations. A laser-induced spark was chosen as the ignition source to avoid contact between the initial hot kernel and the electrode. The flammable limit of NH3/O-2/N-2 mixture measured by an inductive spark agreed well with the isothermal curve at 1600 K. Conversely, the flammable range obtained in the present work became narrower than the reference. The critical point of explosion (CPE) was plotted below the stoichiometric line owing to the preferential diffusion effect. Moreover, that the Burgess-Wheeler law was observed to hold true to the entire flammable limit in the case of nitrogen addition. This was also observed until the volume fraction of the added argon became 50 vol%. However, it no longer held true when helium was added because of one-order-larger thermal diffusivity. The dilution effect on the flammable limit was in the order of He > N-2 > Ar, which is closely related to the ignition behavior. The formation of a third lobe, which is often observed in laser-induced spark ignition, was observed in the present experiment. This results in an acceleration effect for ignition on adding argon, owing to its small molar-specific heat; however, contrarily, it causes an energy dispersion effect on adding helium owing to its large thermal diffusivity. The flammable range was influenced by the determination method ("propagation limit" or "ignition limit"), ignition energy, and the provision duration of energy.