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Abstract OH-PLIF and PIV are employed to analyze hydrogen/air flames stabilized by a dual swirl injector under globally lean and atmospheric conditions with preheat air temperature up to 673K. The flames exhibit two distinct reaction branches. The first, located in the CRZ, is a diffusion-controlled reaction layer characterized by a relatively large thickness associated with low strain rates. The second branch, stabilized in the shear layer of the swirling jet, is strongly influenced by large coherent structures. Depending on operating conditions, this front may adopt either the form of a fully diffusive strained reaction layer anchored to the hydrogen injector lip or a lifted diffusion front with a leading-edge flame evolving into a partially premixed flame. Flue gas analysis indicates low-NOx emission levels for sufficiently large air injection velocities. Air preheating barely increases NOx emissions at lean operating conditions. The injector operational range is constrained only by the ultra-lean blowout limit reached for global equivalence ratios below 0.02. Furthermore, it demonstrates remarkable resilience to large and rapid drop in fuel flowrate. However, combustion efficiency drops close to the LBO limit due to intermittent fragmentation of the flame wings that progresses further upstream as the equivalence ratio drops. The results demonstrate that fragmentation arises from the combined effects of a temperature drop in the central recirculation zone and flame exposition to high shear stress. Additionally, it is shown that combustion efficiency under ultra-lean conditions improves significantly with an increase in air preheat temperature.
Magnes et al. (Sat,) studied this question.