Experimental investigation and modeling of boundary layer flashback for non-swirling premixed hydrogen/ammonia/air flames

Carbon free fuels such as hydrogen/ammonia blends show a promising potential to become sustainable and renewable fuels for gas turbines and other combustion systems. One interesting aspect about these blends is the possibility to adjust different combustion properties like the laminar burning velocity or ignition delay time by changing the ratio between H₂ and NH₃. Such fuel blends can be produced via partial catalytic decomposition of NH₃. However, such mixtures can lead to flame instabilities such as flashback, especially if the hydrogen content is high. In the present study, the boundary layer flashback of premixed hydrogen/ammonia/air mixtures is investigated experimentally for non-swirling flows at normal temperature (293 K) and normal pressure (101 kPa). A new experimental setup for boundary layer flashback investigation with a fully automated measurement procedure is introduced. With preliminary studies, the influence of various measurement procedures on the flashback limits is firstly investigated. For a broad flashback study, the data of 351 flashback experiments are collected. The ammonia content in H₂/NH₃ fuel mixtures is varied from 0 vol% to 50 vol% in 10 vol% steps. The fuel–air equivalence ratio is ranging from 0.38 to 1.17. As the ammonia content is increasing, the mean flow velocities at flashback are exponentially decreasing. Additionally, theoretical modeling is performed. A model is derived based on the concept of the critical velocity gradient which is able to predict the measured data with high accuracy. For two exemplary cases with H₂/air and 80% H₂/20% NH₃/air mixtures, the process of boundary layer flashback is investigated in detail with low and high speed direct imaging and image post-processing. During the flashback onset of H₂/NH₃/air flames a separate reaction of H₂ followed by the reaction of NH₃ can be observed. Also, a flame-oscillation between fused silica tube and burner head with approximately 10 Hz was observed. Furthermore, indications for an adverse pressure gradient based on the flame propagation speed is seen. Details about the flame structure during the flashback process of H₂/NH₃/air flames are shown. During the upstream flame propagation of H₂/NH₃/air flames, high frequency oscillations with about 830 Hz of the leading flame tip are observed, which are assumed to be related to thermoacoustic instabilities.