Experimental studies of autoignition events in unsteady hydrogen-air flames

An experimental study is presented of unsteady N2-in-H2 jet flames in a co-flow of hot combustion products from lean premixed hydrogen combustion for investigation of the statistical likelihood of autoignition events in the mixing region. The unsteady jet flame is characterized by rapid ignition followed by a gradual blowout of the flame. Audio recordings and Schlieren imaging high speed videos are used in investigating the unsteady flame. The frequency of the blowout re-ignition event is investigated as a function of nitrogen dilution mole fraction (YN2=0.180-0.566), co-flow equivalence ratio (Φcf=0.20-0.27) and jet velocity (Vjet=300-500 m/s). The results from the audio recordings and Schlieren imaging indicate that autoignition dominates the re-ignition. The frequency of ignition increase with increasing nitrogen dilution until a maximum is reached after which it decreases with further nitrogen dilution. For increasing equivalence ratios a higher nitrogen dilution is needed in the jet for the flame to become unsteady. The effect of the nitrogen dilution is explained primarily through a reduction in reaction rates and increased jet momentum. Furthermore, the results suggest that the re-ignition rates are controlled by both chemistry and turbulent mixing. The results from the audio recordings and the Schlieren imaging videos correspond well which validates the use of audio recordings as a diagnostic for studying of unsteady hydrogen jet flames.