Experimental and numerical investigation of two flame stabilization regimes observed in a dual swirl H2-air coaxial injector

This study investigates H2-air flames obtained with a laboratory scale coaxial dual-swirl injector in which fuel and oxidizer are injected separately. Two flame archetypes are observed experimentally for the same global equivalence ratio ϕg ≈ 0.45 and two different thermal powers: a flame anchored to the injector (≈ 4 kW) and an aerodynamically stabilized flame exhibiting a characteristic V-shape (≈ 10 kW). Large Eddy Simulations (LES) allow to retrieve both regimes and are used to investigate these two stabilization modes. The numerical setup is first validated against isothermal and reactive Particle Image Velocimetry measurements and OH* chemiluminescence images. The mean velocity field of both operating conditions reveals the existence of a strong inner recirculation zone (IRZ) that, penetrating inside the injector nozzle, leads to a radial divergence of the central hydrogen jet, which ultimately favors one stabilization regime over the other. The first flame anchors on the hydrogen injector lip and it develops along the mixing layer between H2 and air swirling jets. The lifted flame, instead, stabilizes in the inner shear layer between the IRZ and the exiting swirling jet of fresh gases, burning over a wide range of equivalence ratios. LES also unveil the flame structures typical of each flame: the anchored one is entirely controlled by diffusion, while the lifted flame is characterized by a first partially premixed branch and a second diffusion front. Finally, high-speed camera and LES are used to analyze the unsteady transition from lifted to anchored flames.