Experimental analysis and theoretical lift-off criterion for H2/air flames stabilized on a dual swirl injector

Abstract

Stabilization mechanisms of partially premixed H2/air flames on a coaxial dual swirl injector are investigated at atmospheric conditions. Hydrogen is injected through a central duct, and the air by the outer annular channel. Both channels are swirled and two stabilization modes are observed depending on the geometrical configuration of the injector and on the operating conditions. In certain regimes, the H2/air flame stabilizes on the injector lips as a diffusion flame. For other operating conditions, the flame is lifted from the injector and burns mainly in partially premixed regime leading to limited NOx emissions. PIV measurements in cold flow conditions and direct observations of the flame indicate that the flame stabilization mode is mainly controlled by the inner hydrogen swirl level, the injector recess and the hydrogen velocity. For a given air flowrate, a minimum hydrogen velocity to lift the flame is determined for each combination of inner swirl level and injector recess. Assuming the flame close to the injector lips behaves like an edge flame, a model for flame stabilization based on the triple flame speed and the location of the stoichiometric mixture fraction line is built. According to this model, the flame is anchored to the injector if the triple flame can propagate to the inner injector lips, i.e., if the velocity along the stoichiometric line is lower than the triple flame speed. The model is tested using hydrogen diluted with argon and air diluted with nitrogen. Two cases producing predicted opposite trends are verified. First, the stoichiometric line is moved in the direction of lower velocity zone keeping the triple flame speed constant in order to anchor a lifted flame. Next, the stoichiometric line is kept constant and the triple flame speed is reduced in order to lift an anchored flame. The mechanisms driving flame stabilization are discussed.