Transition from Fast-Flame to Detonation

Abstract

Flame acceleration within a channel is facilitated by repeating obstacles that generate turbulence. As the flame accelerates to supersonic velocities, it generates compression waves that coalesce to form a strong leading shock. This shock-flame complex continues to accelerate until it reaches a maximum velocity equal to the speed of sound in the combustion products; the complex is referred to as a fast-flame. Shock reflection driven onset of detonation may occur, characterized by an instantaneous jump in velocity to the theoretical ‘Chapman-Jouguet’ velocity. A more fundamental problem is the transition from a fast-flame to detonation without the presence of obstacles. 
 In the current study, a 7.6 cm square ‘optical section’ housing a perforated plate was used to study the transition from fast-flame to detonation. A spark- and glow- plug served as ignition sources. The optical section had windows installed on its side and end walls so that Schlieren- and direct- photography could be captured. Soot foils were also utilized to record detonation cell structure. The fast-flame was generated by the passage of a detonation wave through the perforated plate, after which the transition to detonation could be studied. 
 Gaseous fuel-oxygen mixtures of varying degrees of detonation stability over a wide range of pressures were tested. In low-pressure tests, delayed onset of detonation was observed, and detonation initiation sites were located at the channel walls indicating that flame-generated turbulence alone is not sufficient for the transition process. As pressure was increased, onset occurred immediately after the perforated plate at various locations across the channel cross-section, not strictly at the walls. For this abrupt ignition mode, transverse shock collisions played an important role in the transition to detonation.