Interplay Between Unburned Emissions and NOx Emissions From a Dual Swirl Hydrogen Air Injector

OH planar laser induced fluorescence and particle image velocimetry are employed to analyze the structure of hydrogen/air flames stabilized by a dual swirl injection system under globally lean and atmospheric conditions with preheat air temperature varied from ambient up to 673 K. The flames exhibit two distinct reaction branches. The first, located in the central recirculation zone, 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 at high air injection velocities. Flue gas analysis indicates NOx emission levels, typically below 10 ppm at 15% O₂, 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 lean blowout limit due to intermittent fragmentation of the flame wings that progresses further upstream as the equivalence ratio drops. It is demonstrated that this fragmentation results from the combined drop in temperature of the central recirculation zone and the flame wings subjected to high shear stress. It is also shown that combustion efficiency at ultra lean conditions rapidly increases as the air preheat temperature increases.