Study on dynamic characteristics of hydrogen micromix flame under self-excited oscillation

Numerical study on the dynamic characteristics of a single-nozzle hydrogen micromix flame was conducted using Large Eddy Simulation (LES). A specific chemical mechanism and an acoustic boundary condition were selected. And the turbulent combustion model was modified to consider the differential diffusion effect. The obtained results show qualitatively and quantitatively good agreement with experimental data and numerical studies by others. By analyzing the mean and transient distributions of characteristic parameters, the flow and flame structures of hydrogen micromix flames were investigated. Spectral analysis and spectral proper orthogonal decomposition (SPOD) were employed to examine the frequency characteristics and unsteady behavior of the flame under self-excited oscillations, while the periodic variations of flame temperature, pressure, and OH radical concentration were further analyzed. The results indicate that the hydrogen micromix combustion flame propagates downstream along the edge of the outer recirculation zone (ORZ), and the flame stretching and wrinkling during propagation are primarily dominated by the shear layer between the recirculation zones. Flame stabilization mainly relies on auto-ignition of the hydrogen-air mixture at the step, while the entrainment of hot combustion gases by the ORZ supplies thermal energy to ignite the upstream unburned mixture. Under self-excited oscillation, the flow field exhibits a typical first-order standing wave pattern in pressure fluctuations, with two distinct characteristic frequencies observed in the pressure spectrum, the low frequency f ₀ is primarily induced by periodic variations of large-scale turbulent reverse structures downstream of the nozzle, while the higher frequency f ₁ is associated with the periodic vortex shedding at the shear layer between the IRZ and ORZ.