Dynamic behavior and flame acceleration of spherically expanding hydrogen flames under the low-pressure conditions

Abstract

To investigate the dynamic behavior and flame acceleration of spherically expanding hydrogen flames under the low-pressure conditions, the deflagration experiments were conducted in a closed combustion chamber. The dynamic behavior of hydrogen flames was observed 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 flame stretching affected strongly the propagation velocity. From the correlation between the flame stretch rate and the propagation velocity, we estimated the Markstein length and Markstein number. When the flame radius was large, on the other hand, cellular surface was observed and the flame acceleration was confirmed. As a result, the critical flame radius corresponding to the occurrence of flame acceleration and the increment coefficient of propagation velocity were obtained. Under the low-pressure conditions, the dynamic behavior of hydrogen flames became weaker and the increment coefficient of propagation velocity became smaller. This was because the intrinsic instability was weakened by increasing the preheat zone thickness. It was also found that the increment coefficient became larger at lower equivalence ratios, which was because the diffusive-thermal instability became stronger. Moreover, the effects of preheat zone thickness on the flame acceleration was elucidated. We obtained the correlation between the normalized increment coefficient of propagation velocity and the preheat zone thickness. Based on the characteristics of dynamic behavior of hydrogen flames, the parameters of flame acceleration model depending on the initial pressure was obtained, and then the flame propagation velocity under the low-pressure conditions were predicted.

Keywords

• premixed combustion
• hydrogen
• spherically expanding flame
• dynamic behavior
• flame acceleration
• low pressure
• intrinsic instability