Triggering of detonation upon flame interaction with an expansion wave

The transition of a deflagration wave into an abruptly expanding part of a plane channel, where a quasi-steady supersonic underexpanded jet of an unburned gas is formed, is studied for a propane–oxygen mixture using schlieren pictures. Two explosion-initiation modes (weak and strong) are registered. In the first case, almost instantaneous onset of the detonation wave occurs when the flame front enters the expanding section; the initial velocity of this wave is approximately 1.5 times the Chapman–Jouguet detonation velocity $(D_{\mathrm{CJ}})$ and then decreases to a value corresponding to self-sustaining detonation. In the second case, the front velocity gradually increases from $\approx$0,4 $D_{\mathrm{CJ}}$ to $\approx$1,0 $D_{\mathrm{CJ}}$. It is established that the starting pulse triggering the transformation of turbulent combustion to explosion and detonation regimes is generated by interaction of the flame front with expansion waves, which are elements of the structure of the initial section of the jet.