Turbulent flame propagation limits of ammonia/methane/air premixed mixture in a constant volume vessel

Ammonia is one of promising energy carriers that can be directly used as carbon-neutral fuel for combustion applications. However, because of the low-burning velocity of ammonia, it is challenging to introduce ammonia to practical combustors those are designed for general hydrocarbon fuels. One of ways to enhance the combustibility of ammonia is by mixing it with other hydrocarbon fuels, such as methane, with a burning velocity that is much higher than that of ammonia. In this study, flame propagation experiments of ammonia/methane/air were conducted using a fan-stirred constant volume vessel to clarify the effect of methane addition to ammonia on the turbulent flame propagation limit. The flame propagation maps were constructed from the experimental results, and the flame propagation limits were elucidated. Results demonstrated that the flame propagation limits were extended with an increase in mixing a fraction of methane to ammonia. The ammonia/methane/air mixture with a 0.9 equivalence ratio can propagate at the highest turbulence intensity, even though the peak of the laminar burning velocity is at the fuel-rich side or stoichiometric condition because of the diffusional-thermal instability of the flame surface. The Markstein number of the mixture obtained in this study successfully expressed the strength of the diffusional-thermal instability effect on the flame propagation capability. The turbulence Karlovitz number at the flame propagation limit monotonically increases with a decreasing Markstein number.