Comparison of detonation initiation by hot jet and obstacles studied via chemiluminescence imaging at 200 kHz

The hot jet and obstacle have been demonstrated as two main approaches that can initiate the fast and reliable detonation for pulse detonation engines (PDEs). To compare the capabilities and limitations of the two approaches for detonation initiation, a series of experiments were performed under different operating conditions with the two methods, and the chemiluminescence images of flame were recorded at 200 kHz. The results demonstrate four different initiation modes of detonation by the hot jet method: the direct initiation, local explosion induced re-initiation, flame acceleration in boundary layer induced re-initiation and shock reflection induced re-initiation. With the obstacle initiation, only deflagration to detonation transition (DDT) mode was observed. The following quantitative comparisons of flame tip velocity show that: 1) the flame always accelerates faster in hot jet initiations than in the obstacle initiations at the beginning of the flame acceleration process; 2) the detonation was reliably induced in the obstacle initiations while the failure rate of detonation increases with the increase of nitrogen dilution in mixtures in hot jet initiations. At last, this work suggests that the combination of two approaches can enhance both the flame acceleration and the success rate of detonation initiation.