Effects of inert-gas addition on the dynamic behavior and propagation characteristics of spherically expanding hydrogen-air flames

To investigate the effects of inert-gas addition on the dynamic behavior and propagation characteristics of spherically expanding hydrogen-air flames, the experiments of premixed combustion were performed in a closed chamber. The dynamic behavior of premixed flames was caught by high-speed Schlieren imaging, and the flame radius and propagation velocity were measured by analyzing the Schlieren photography. When the flame radius was sufficiently small, smooth flame surface was observed, where the flame stretch affected strongly the propagation velocity. From the correlation between the propagation velocity and flame stretch rate, we estimated the propagation velocity of unstretched flame and the Markstein length including thermal-expansion effects. When the flame radius was large, on the other hand, cellular surface induced by intrinsic instability was observed, and then the flame acceleration was confirmed. As the results, the critical flame radius corresponding to the occurrence of flame acceleration and the increment coefficient of propagation velocity were obtained. It was found that the increment coefficient became larger at low equivalence ratios, which was because the diffusive-thermal instability became stronger. Under the conditions of high concentration of inert gas, the increment coefficient of propagation velocity became smaller. This was because the burning velocity became lower by increasing the inert-gas concentration. Moreover, we obtained the increment coefficient normalized by the propagation velocity of unstretched flame. The normalized increment coefficient increased as the inert-gas concentration became higher, which indicated that the addition of inert gas promoted the instability of hydrogen flames. Based on the dynamic behavior and propagation characteristics of premixed flames, the parameters of flame acceleration model depending on the inert-gas concentration were obtained, and then the flame propagation velocity was predicted.