Stabilization of Low-NO_x Hydrogen Flames on a Dual-Swirl Coaxial Injector

Hydrogen-fueled carbon-free energy is a growing prospect for the future of several industries, including the aeronautical and energy generation sectors. However, the transition to hydrogen comes with several challenges: flame stabilization due to the unique combustion properties of hydrogen compared to conventional fuels and control of nitrogen oxides (NO_x) production due to the higher combustion temperature. In addition, the high flammability of hydrogen poses an increased risk of flashback in premixed configurations commonly used to generate low-NO_x flames. As a consequence, research into strategies for low-NO_x hydrogen combustion is developing, and several technologies are being considered for industrial use. Among these technologies, we consider here a dual swirl coaxial injector operated in nonpremixed conditions. This setup enables the stabilization of multiple types of flame structures that are parametrically investigated for their NO_x emissions. The flame structure is observed using OH* chemiluminescence images collected using an intensified CCD camera equipped with a bandpass filter. Exhaust gas species mole fractions of NO, NO₂, and O₂ are measured using NDIR gas detectors and a paramagnetic sensor. This investigation reveals that stabilization of hydrogen-air flames on the double swirl injector is possible and that varying the flow parameters induces changes in structure that affect NO_x emissions, independently of swirl level, residence time, and temperature that also play a role. The study is carried out at atmospheric pressure.