Spherical turbulent flame propagation limits of ammonia–hydrogen–oxygen–nitrogen pre-mixtures by intense near-isotropic turbulence in a constant volume vessel

Ammonia, as a promising hydrogen energy carrier and carbon-free fuel, offers significant potential for decarbonizing the energy sector and enabling carbon-free energy generation. The co-combustion of ammonia and hydrogen has emerged as an effective strategy to overcome the challenges of low burning velocity and flame temperature associated with pure ammonia flames. However, the turbulent combustion features of ammonia–hydrogen premixtures remain insufficiently understood. To the best of the authors’ knowledge, this study is the first to investigate the spherical turbulent flame extinction phenomena and mechanism of ammonia–hydrogen–oxygen–nitrogen premixtures. Experiments were performed in a custom-designed, fan-stirred constant-volume chamber to investigate the turbulent flame propagation limits in ammonia–hydrogen–oxygen–nitrogen premixtures under near-homogeneous and near-isotropic turbulence conditions. To facilitate the observation of turbulent flame extinction and gain insight into the underlying mechanisms, experiments were conducted with an oxygen concentration of 0.12 in the oxidizer (oxygen and nitrogen) and hydrogen mole fractions of 0, 0.4, 0.45, and 0.5 in the fuel mixture (ammonia and hydrogen) across various equivalence ratios. Turbulent flame propagation probability maps were constructed to identify and clarify the turbulent flame propagation limits in the mixtures. The results indicate that the turbulent flame propagation limits of ammonia–hydrogen–oxygen–nitrogen premixtures expand with increasing hydrogen mole fraction in the fuel mixture. Notably, the ammonia–hydrogen–oxygen–nitrogen mixture with an equivalence ratio of 0.9 demonstrated the ability to propagate at the highest turbulence intensity, despite the laminar burning velocity peaking at stoichiometric conditions. This phenomenon is considered to be caused by the diffusional–thermal instability occurring at the turbulent flame front. Furthermore, the turbulent Karlovitz number at the turbulent flame propagation limits monotonically decreases with the increase of Markstein number. Comparison analysis with previous studies on ammonia–air [Ichimura et al., 2019], ammonia–methane–air, and methane–air [Hashimoto et al., 2021] combustion revealed that, under fuel-lean conditions, ammonia–hydrogen flames could propagate at higher turbulent Karlovitz numbers for the same Markstein number. This behavior indicates an enhanced resistance to local extinction, highlighting the critical role of diffusional–thermal instability and preferential diffusion effects in turbulent premixed flame dynamics. The present findings provide new physical insights into the turbulent combustion characteristics of ammonia–hydrogen mixtures and contribute to the development and validation of predictive numerical models for their application in low-carbon energy systems. • First experimental measurements of turbulent flame propagation limits in NH 3 –H 2 flames. • Adding H 2 to NH 3 flames substantially expands the turbulent flame propagation limits. • Turbulent Karlovitz numbers at the limits decrease with increasing Markstein number. • Lean NH 3 –H 2 flames can withstand higher turbulent Karlovitz numbers than pure NH 3 flames. • Diffusional–thermal instability and preferential diffusion strongly influence turbulent combustion of NH 3 –H 2 .