Theoretical analysis on thermoacoustic dynamic responses of laminar premixed ammonia-hydrogen-air conical flames based on G-equation models

This paper investigates thermoacoustic dynamic responses of ammonia-hydrogen-air flames by theoretical analysis. Utilizing G-equation models, flame transfer functions (FTFs) can be obtained through a linearized analytical solution, while flame describing functions (FDFs) are derived from a fully numerical solution using the level set approach. The study examines linear FTFs under different ammonia enrichment levels (η_NH3), unstretched flame aspect ratios (β), Markstein lengths (L), and equivalence ratios (ϕ), as well as nonlinear FDFs under different normalized incoming flow velocity perturbation amplitudes. Results indicate that increasing η_NH3 enhances the flame speed, accelerating the decline in the FTF gain with frequency and reducing the propensity for thermoacoustic instability. The effect of flame curvature on increasing FTF gains and altering flame shape becomes more pronounced at small β and large L. Consequently, ammonia-hydrogen-air flames exhibit greater stability under stoichiometric conditions, as L approaches 0 across various η_NH3 and β. Finally, a comparative analysis of FDF results indicates that higher velocity perturbation amplitudes effectively reduce the flame height at smaller L. The reduction in flame front wrinkles alters the behavior of FDFs, leading to more pronounced nonlinear effects in the flame response.