Sound sources in turbulent hydrogen/methane premixed jet flames

Sound generation by hydrogen/methane premixed flames is examined using a Direct Numerical Simulation (DNS) dataset of three jet premixed flames at a Reynolds number of 10,300, with 10%, 50%, and 80% hydrogen by volume. Dowling’s reformulation of Lighthill’s acoustic analogy is also used to evaluate the contributions of different sound sources.

Hydrogen addition increases high-frequency sound, particularly at Strouhal numbers greater than two where flame annihilation events are strong noise sources. Our analysis of Dowling’s source terms reveals that the dominant contributors to the generated sound are the Lighthill’s stress tensor, the time derivative of the heat release rate, the momentum changes of density inhomogeneity, and boundary noise. The energy transport by mass diffusion also plays a role in the 50% hydrogen case. A strong coupling is observed between the Lighthill’s stress tensor and the momentum changes of density inhomogeneity. Further analysis shows that their combined effect can be approximated by the conventional turbulence-generated noise source term used in non-reacting jets, i.e. $\rho {\partial }^2{u}_i{u}_j/\partial x_i\partial x_j$. Examination of the swirl strength field suggests that turbulent eddies outside the flame primarily contribute to this source term.

Our results suggest that at high Reynolds numbers relevant to practical applications, the contributions of boundary noise and turbulence-generated noise, as well as the time derivative of the heat release rate, should be all considered to fully explain sound generation by premixed flames.