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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, s L. A good agreement between the s L 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. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a numerical investigation of sound generation by two-dimensional laminar premixed flames. Direct numerical simulation (DNS) is used to study low Mach number flames with different Lewis numbers, namely Le =0.5, 1 and 2, excited by velocity perturbations at the inflow boundary for a range of frequencies. At intermediate forcing frequencies, ‘flame pinch-off’ and ‘flame island burn-out’ are observed as strong, monopolar sound sources for Le =1 and 2. However, these events produce almost no sound for Le =0.5. Dowling’s reformulation of Lighthill’s acoustic analogy is used to investigate the contribution of heat release rate fluctuations to the radiated sound. A comparison of direct calculation of heat release rate fluctuations and estimated values using a flamelet theory suggested by Clavin and Siggia [7], which assumes that heat release fluctuations are due to flame surface area fluctuations, shows good agreement for unity Lewis number when the forcing period is larger than the flame time. For forcing periods smaller than the flame time, flamelet theory does not succeed in describing the effects of heat release fluctuations, as expected. However, even for forcing periods greater than the flame time, the flamelet theory does not capture the observed trend with Lewis number. The pressure fluctuations due to heat release are always over-predicted by flamelet theory for Le =0.5 and under-predicted for Le =2. The flamelet model, intended for application to turbulent flames, assumes a constant consumption speed along the flame surface. The failure of this model in the case of non-unity Lewis number in the present unsteady laminar flames can be therefore due to the large modification of the consumption speed where large surface area fluctuation is present. [Copyright &y& Elsevier]