Effect of Expansion Ratio on Flame Acceleration During Hydrogen Fueled Gas Explosions

A precise understanding of the flame turbulence induced by cellular instabilities is indispensable to perform an appropriate risk assessment of hydrogen fuelled gas explosion. In this research, Darrieus Landau instability (DL instability), whose effect on gas explosion is  remarkable, was experimentally examined. The DL instability is essentially caused by a volumetric expansion of burned gas at flame front. Therefore, in order to examine the effects of volumetric expansion ratio, the experiments were conducted using H2-O2-N2-Ar gas mixtures of various volumetric expansion ratio conditions by changing N2-Ar ratio. When Ar content ratio is increased, the flame temperature becomes higher and volumetric expansion ratio is increased owing to lower specific heat of Ar. The experiments were conducted in nearly unconfined conditions of laboratory-scale and large-scale. Gas mixtures were filled in a 10 cm diameter soap bubble for the laboratory-scale and in a plastic tent of thin vinyl sheet of 1m3  for the large-scale. The gas mixtures were ignited by an electric spark, and blast wave and flame speed were measured simultaneously by using a pressure sensor and a high-speed video camera. The DL instability owing to volumetric expansion accelerates flame propagation. In addition, the intensity of blast wave was greatly raised depending on flame acceleration, which can be explained by an acoustic theory. The effects of expansion ratio and experimental scales on flame propagation and blast wave were analyzed in detail. These results are quite important to perform an appropriate consequence analysis of accidental explosion of hydrogen.