Indirect noise from weakly reacting inhomogeneities

TL;DR

This paper develops a low-order model to predict indirect noise caused by chemically reacting flow inhomogeneities in aircraft engine nozzles, highlighting the influence of chemical reactions and flow parameters on noise generation.

Contribution

It introduces a novel low-order model for reacting inhomogeneities, extending existing non-reactive models to account for chemical reactions and entropy perturbations.

Findings

Hydrogen inhomogeneities produce larger indirect noise than methane.

Reaction rates and frequency significantly affect wave phase and magnitude.

The model applies to both subsonic and supersonic flows.

Abstract

Indirect noise is a significant contributor to aircraft engine noise, which needs to be minimized in the design of aircraft engines. Indirect noise is caused by the acceleration of flow inhomogeneities through a nozzle. High-fidelity simulations showed that some flow inhomogeneities can be chemically reacting when they leave the combustor and enter the nozzle (Giusti et al., 2019). The state-of-art models, however, are limited to chemically non-reacting (frozen) flows. In this work, first, we propose a low-order model to predict indirect noise in nozzle flows with reacting inhomogeneities. Second, we identify the physical sources of sound, which generate indirect noise via two physical mechanisms: (i) chemical reaction generates compositional perturbations, thereby adding to compositional noise; and (ii) exothermic reaction generates entropy perturbations. Third, we numerically compute the nozzle transfer functions for different frequency ranges (Helmholtz numbers) and reaction rates (Damk\"{o}hler numbers) in subsonic flows with hydrogen and methane inhomogeneities. Fourth, we extend the model to supersonic flows. We find that hydrogen inhomogeneities have a larger impact to indirect noise than methane inhomogeneities. Both the Damk\"{o}hler number and the Helmholtz number markedly influence the phase and magnitude of the transmitted and reflected waves, which affect the sound generation and thermoacoustic stability. This work provides a physics-based low-order model, which can open new opportunities for predicting noise emissions and instabilities in aeronautical gas turbines with multi-physics flows.