Effects of inert gas addition on the characteristics of spherically expanding hydrogen-methane-air premixed flames in closed combustion vessels

In the serious accident at Fukushima Daiichi Nuclear Power Station, the presence of steam together with flammable organic compounds affected the hydrogen explosion. To investigate the effects of addition of inert gas, i.e. steam or nitrogen, on the explosion characteristics, we conducted the experiments of spherically expanding hydrogen-methane-air premixed flames in closed combustion vessels. Two types of vessels were used, and expanding flames were caught by Schlieren method. The flame propagation velocity depending on the flame radius was obtained by analyzing Schlieren images. When the flame radius was sufficiently small, smooth surface was found. The addition of inert gas to hydrogen-methane-air premixtures caused the decrease of propagation velocity of unstretched flame. When the flame radius was large, on the other hand, cellular surface generated by intrinsic instability was found. In this range, the flame acceleration was confirmed, which was induced by the evolution of cellular surface. We obtained the parameters of flame acceleration model and predicted the flame propagation velocity depending on the flame radius. The increment coefficient normalized by the propagation velocity of unstretched flame became larger at low equivalence ratios, which was due to stronger diffusive-thermal instability. Under the same equivalence ratio, the inert gas addition caused the increase of normalized increment coefficient. This denoted that the inert gas addition promoted the instability of premixed flames, which was due to the reduction of the effective Lewis number. The maximum pressure in a combustion vessel became lower in the case of inert gas addition. Moreover, the maximum pressure of experiments was lower than that of calculations under the adiabatic conditions, because of heat loss during premixed combustion. The obtained results were valuable knowledge to elucidate the hydrogen explosion at Fukushima Daiichi Nuclear Power Station.