Schlieren observations of flame acceleration as precursors of transition to detonation

Deflagration to detonation transition (DDT) is a phenomenon that is difficult to observe experimentally because of the inherent stochastic nature of shock and flame acceleration processes that produce the thermodynamic and gasdynamic conditions required for DDT. The shock tube however provides an attractive means of establishing or modifying these conditions independently of chemical reactivity parameters. In the present paper results are presented from an experimental study of turbulent flame acceleration and eventual transition to detonation following the perturbation of a curved reaction front (spheroidal flame bubble) by a shock wave. Of particular interest is how an initial flame kernel, propagating with a laminar burning velocity of few metres per second or less, can be enhanced by shock processes to the point where a transition to detonation occurs, without the influence of turbulence producing obstacles. This is compared with a more conventional flow, but again generated by a shock wave, through a grid scenario where the flame propagated into a preestablished turbulent field. The events were recorded using spark Schlieren photography as the primary diagnostic technique.