The LEAF concept operated with hydrogen: Flame topology and NOx formation

The development of low NO hydrogen (H) burners is crucial for the sustainability target of the power/propulsion sector. However, the technical difficulty of burning H at low NO emissions is challenging for the combustion community. Recently, the concept of LEan Azimuthal Flame (LEAF) has demonstrated promising results for low NO kerosene/hydrogen combustion by rapidly diluting the reactants with burnt gas and fresh oxidizer. However, there is a lack of understanding of the flame dynamics and the NO formation routes for H LEAF. We therefore carried out a joint experimental and numerical study of the LEAF combustor at atmospheric pressure fueled with H. Experiments are based on OH-planar laser-induced fluorescence and exhaust gas analysis. Numerical results are based on massively parallel Large Eddy Simulation (LES) with an accurate description of the combustion and NO chemistry. This study focuses on understanding the effects of the Air Ratio (AR), which defines the distribution of the air injected from the top and the bottom of the LEAF combustor. The LES results are in excellent agreement with the experimental data in terms of flame topology and exhaust emissions. A dual flame structure is observed when rich premixed gas is injected from the bottom together with air from the top, leading to the coexistence of premixed and non-premixed combustion regimes. An optimum AR is identified to minimize NO emissions. It is attributed to the enhancement of the azimuthal whirling flow by the air injected from the bottom, having higher momentum than pure H injection only.