A comparison between turbulent non-premixed jet flames of CH4 and the 50%NH3+50%H2 blend

Abstract

A challenge of application of ammonia in combustion devices is due to its low chemical reactivity. Addition of hydrogen into ammonia can increase drastically the premixed laminar flame speed of the blends. In practical combustion devices, however, turbulent non-premixed jet flames are usually employed, while researches on ammonia jet flames are scarce. In this study, firstly, the premixed laminar flame speeds of CH4 and the 50%NH3+50%H2 blend in volumetric fraction are investigated by the Schlieren imaging. It is confirmed that the laminar flame speeds of the 50%NH3+50%H2 blend are remarkably faster than those of CH4 for equivalence ratios above 1.1, while they are similar for lower equivalence ratios. Then, the Schlieren imaging and planar laser induced fluorescence imaging of hydroxyl of the burning turbulent gas jets are conducted. It is found that the flame propagation speed and OH intensity in the CH4 jets are remarkably higher than those in the 50%NH3+50%H2 jet. To unveil the mechanism, the equivalence ratio and fuel mole fractions along the jet axis are investigated by the laser induced breakdown spectroscopy. The equivalence ratios of the 50%NH3+50%H2 blend are significantly lower than those of CH4. Compared to that of CH4, the volumetric flow rate of 50%NH3+50%H2 blend is higher for the choked flow of the under-expanded jets. Therefore, the lower equivalence ratios are attributed to the higher stoichiometric fuel-air ratio of the 50%NH3+50%H2 blend. Stratification of NH3 and H2 due to their distinct molecule transfer properties is experimentally observed, which probably lowers the flame propagation speed in the jet. These findings are expected to be a valuable reference for development of simulation models and design of practical combustion devices with zero-carbon fuels.