Ultrathin metaliner for dual suppression of broadband flow-induced noise and tonal thermoacoustic oscillations

Abstract

To simultaneously address broadband combustion noise and tonal thermoacoustic oscillations, two distinct but sometimes coexisting noise sources in combustors, this study proposes an ultrathin acoustic metaliner capable of attenuating both. The effectiveness of this metaliner's proof-of-concept design is demonstrated in a prototypical thermoacoustic system: a Rijke tube driven by an electrical heater. The metaliner consists of two key components: azimuthally repeated porous metasurface units with a phase-gradient structure and neck-embedded Helmholtz resonators. The porous phase-gradient metasurfaces convert incident broadband noise into surface waves propagating along the liner, thereby enhancing sound dissipation compared with uniform porous materials. Meanwhile, the embedded Helmholtz resonators provide strong acoustic absorption of discrete thermoacoustic modes by substantially reducing their growth rates. Experiments show that the metaliner achieves simultaneous suppression of broadband noise and thermoacoustic oscillations, reducing the sound pressure level of high-frequency broadband noise by up to 36 dB and that of low-frequency tonal modes by up to 124.9 dB. Furthermore, the metaliner maintains effective noise reduction across a range of natural frequencies, flow rates, and additive white Gaussian noise intensities, demonstrating robust performance. In addition, the proposed design features a compact geometry (total thickness of only 2 cm), high flexibility and extensibility (multiple layers of neck-embedded Helmholtz resonators can be customized and integrated into the metaliner to target several low-frequency modes without altering the metaliner's overall size), and the unique ability to suppress both broadband and tonal noise sources simultaneously. This work introduces, for the first time, the concept of acoustic metasurfaces for noise control in thermoacoustic systems, and shows particular promise for hydrogen-fueled combustors, where the characteristic frequency band shifts to higher values at which metasurfaces are especially effective.