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

TL;DR

This study investigates how thermodiffusive effects influence combustion noise in turbulent lean hydrogen-air flames using DNS, proposing a theoretical framework and analyzing spectral and flow dynamics differences compared to methane flames.

Contribution

It introduces an extended flamelet theory for thermodiffusively unstable flames and links flame stretch to noise radiation, highlighting the impact of thermodiffusive effects on combustion noise mechanisms.

Findings

Thermodiffusive effects significantly alter heat release fluctuations and flame surface dynamics.

Hydrogen flames show stronger low-frequency fluctuations and reduced high-frequency content.

Flame stretch is identified as a key mechanism promoting noise radiation.

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.