Effect of channel geometry on the flame acceleration and transition to detonation in acetylene-oxygen-nitrogen mixtures

The study of unsteady combustion modes and, in particular, the deflagration-to-detonation transition, is of particular importance for the aerospace industry. Knowledge of the basic mechanisms, as well as the criteria for the development of detonation, helps to control this process in the framework of its applications for novel propulsion devices and safety issues. In this paper, a series of calculations are carried out in order to distinguish the role of channel geometry (planar or circular) in the flame dynamics at all the stages of acceleration prior to the deflagration-to-detonation transition. The combustion of a stoichiometric acetylene-oxygen mixture diluted with nitrogen is considered. It is established that in channels with a circular cross -section, the flame front is stretched and the combustion process evolves up to the transition to detonation, in accordance with the experiment. At the same time, in planar channels (slits), the primary flame acceleration leads to the establishment of a quasi-stationary mode. The peculiarities of the compression and rarefaction waves evolution in the cylindrical and planar geometry are responsible for such a difference between the flame dynamics in channels of different geometry. The detonation onset takes place via the formation of a "chocked flame"structure characterized by permanent joint compression and acceleration of the combustion. As well as in experiments the deflagration-to-detonation transition occurs exactly at the flame front.