Acoustic radiation of thermodiffusively unstable turbulent lean premixed hydrogen-air flames

Abstract

The impact of thermodiffusive effects on combustion noise in turbulent premixed slot jet flames is investigated using Direct Numerical Simulations. Two thermodiffusively unstable lean hydrogen-air flames are compared with a thermodiffusively stable stoichiometric methane-air flame with comparable laminar properties and same turbulence intensity. The hydrogen cases differ in bulk velocity, chosen to match either the turbulent flame brush length or the bulk velocity of the methane case. Thermodiffusive effects are found to strongly alter both the heat release rate fluctuations, which dominate the far-field acoustic radiation, and the flame surface dynamics. A theoretical framework extending the classical flamelet theory to thermodiffusively unstable flames is proposed and validated, relating the flame-generated sound to the time derivative of the flame surface area and to the stretch factor $I_0$. The analysis identifies flame stretch as a key mechanism promoting noise radiation in thermodiffusively unstable flames. Spectral analyses further show that hydrogen flames exhibit stronger low-frequency heat release rate fluctuations and reduced high-frequency content relative to the methane flame. This is shown to be related to the coupled action of turbulence and thermodiffusive instabilities, which enhance large-scale flame motions while attenuating small-scale flame annihilation events. Consequently, hydrogen flames radiate more strongly at low frequencies, near the acoustic peak, and exhibit a steeper high-frequency spectral roll-off. Finally, Spectral Proper Orthogonal Decomposition reveals that hydrogen non-equidiffusion intensifies shear layer instabilities between combustion products and ambient air. These results indicate that thermodiffusive effects influence both direct and indirect combustion noise generation mechanisms in hydrogen flames.

Figures (25)

• Figure 1: Schematic view of the computational domain (not to scale).
• Figure 2: Location in the Borghi--Peters turbulent combustion diagram peters1999turbulent of CH4 and H2 flames of the present study and of DNS in the literature aspden2015turbulencehaghiri2018soundluca2019statisticslapeyre2019trainingbrouzet2021impactrieth2022enhancedcoulon2023directmale2025hydrogen.
• Figure 3: Isosurfaces of progress variable $C=C^*$ coloured by the normalised HRR $\dot{\omega}_T/\dot{\omega}_{T,max}^{1D}$ taken at $t=4\tau$ for the three cases.
• Figure 4: Temporal evolution of the normalised turbulent flame surface $A_T/A_0$, of the normalised turbulent flame consumption speed $S_T/S_L^0$, and of the stretch factor $I_0$ for the three cases.
• Figure 5: Averages of the normalised heat release rate $\dot{\omega}_T/\dot{\omega}_{T,max}^{1D}$ conditioned by the progress variable $C$ (white lines) with the corresponding joint PDF (in logarithmic scale) at $t=4\tau$ for the three cases. The results for the corresponding 1-D unstretched flames (black dashed lines) are added for reference.