Mixing and combustion characteristics of turbulent non-premixed zero- and low-carbon fuel gas jets

Ammonia (NH3), hydrogen (H2) and methane (CH4) as zero- or low-carbon fuels are considered as highly promising alternative fuels. Turbulent non-premixed jet combustion is a prospective way in practical combustion devices for ammonia, hydrogen and methane, but the relevant experimental researches are rare, while such information is critical for the development and validation of numerical models. In this paper, the laser diagnostic studies are carried out in a constant volume combustion chamber to investigate the characteristics of the 50% NH3+50%H2 blend jet with or without ignition in comparison to H2, CH4 and NH3 jets. Firstly, the morphological developments and concentration distributions of the four gas jets without ignition are compared. Then, the OH or NO distributions in the non-premixed gas jet flames are clarified by the planar laser-induced fluorescence techniques. The generation and consumption processes of OH and NO are elucidated by the one-dimensional simulation. The results show that the four jets exhibit a similar morphological development with the nearly same jet angles, except for a little slower tip penetration of 50%NH3+50%H2 blend. The equivalence ratio of the 50%NH3+50%H2 blend and NH3 jets along the jet axis are lower than those in the CH4 and H2 jets due to the difference in the flow rate and stoichiometric number. The H2 jet exhibits the highest combustion intensity, featuring faster flame propagation, the shortest lift-off length and an apparently higher OH radical intensity, while the NH3 jet fails to develop a stable flame. The mixture in the 50%NH3+50%H2 blend jet is too lean, resulting in a weaker combustion intensity than the H2 and CH4 jets. However, the NO intensity in the 50% NH3+50%H2 jet flame is remarkably stronger than those of the H2 and CH4 jet, due to the much higher amount of fuel-type NO converted from NH3 than that of thermal NO. A self-reducing (SR) zone can be observed where OH is produced and then consumed in the 50%NH3+50%H2 blend, H2 and CH4 jet flames. However, the NO SR zone is only observable in the 50%NH3+50%H2 blend jet flames, probably owing to the consumption by the NH2 and NH radicals.