Ammonia, which does not emit carbon dioxide even when it is burned, is expected as a new carbon-free fuel to replace coal and natural gas used in thermal power plant. However, the turbulent flame propagation characteristics have yet to be extensively investigated. This study aimed to clarify the extinction limits of ammonia/air flame in turbulent fields. To achieve this aim, the spherical flame propagation experiments using a fan-stirred constant volume vessel were conducted. The results revealed the unique feature of extinction limit of ammonia in a turbulent field. The ammonia/air mixture with a 0.9 equivalence ratio can propagate at the highest turbulence intensity even though the laminar burning velocity reaches a maximum around an equivalence ratio of 1.1. The fuel-lean mixture can propagate at high turbulence intensity because of the effect of Lewis number. For a lean ammonia/air mixture that has a Lewis number smaller than unity, the local burning velocity increases by the diffusional-thermal instability. On the other hand, the local burning velocity in rich ammonia/air mixtures with a Lewis number larger than unity did not increase in the turbulent field, and the flame was easily extinguished. Because of the diffusional-thermal instability, the turbulence Karlovitz number at the flame extinction limit increases as the Markstein number decreases. The obtained findings from this study can contribute to the optimal design of gas turbines fueled by ammonia as well as the safety use of ammonia.
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