Reduced chemistry for sound generation by planar annihilation in premixed methane/hydrogen flames

Abstract

Sound generation by planar flame annihilation for methane/hydrogen/air premixed flames is investigated using one-dimensional (1D) fully-resolved simulations. The Foundational Fuel Chemistry Model (FFCM) is used to undertake the simulations for different initial pressures and temperatures, for volumetric mixtures of 0 to 100% hydrogen in the fuel mixture. For cases with 25% to 80% of hydrogen in the fuel mixture three stages of annihilation are observed. The first stage exhibits a gradual flame extinction coupled with the generation of a pressure wave with a long wavelength. The second stage features a more rapid extinction as the flame accelerates towards the symmetry axis, producing a sharp pressure drop. The final stage exhibits a gradual decrease in pressure due to slow reactions such as CO oxidation. The contribution of the first stage increases as more hydrogen is added to the fuel. These stages are not distinctive when the fuel is pure hydrogen. The time derivative of the heat release rate is verified to be the dominant source of sound for annihilation events. A correlation linking the sound amplitude from planar annihilation events to key flame parameters is demonstrated to work for the studied fuel blends. The FFCM is then reduced using a modified Directed Relation Graph with Error Propagation and Sensitivity Analysis (DRGEPSA) method with error bounds based on the laminar flame speed, sL. A good agreement between the sL predictions for the reduced and the detailed mechanisms are found. Furthermore, the developed reduced mechanisms in this work are shown to predict the generated sound by flame annihilation accurately.