The Influence of Accelerating Entropy Inhomogeneities on Combustor Thermoacoustics

Abstract

The growing global concern over environmental emissions such as nitrogen oxides and noise sets challenging problems for aero-propulsion engineers. Acoustic waves generated by unsteady combustion not only contribute towards the overall noise transmission, but may also cause thermoacoustic instability in combustors, particularly those designed for low NOx emissions. Combustion noise is generated by unsteady combustion – either by the direct generation of acoustic waves or indirectly by the creation of entropy waves. Entropy waves by themselves are silent, but when accelerated, such as through the combustor exit, they create further acoustic waves known as entropy noise. This thesis aims to study transmitted and reflected combustion noise. Current predictions for noise transmission often assume that the wavelengths of the flow perturbations are large compared to the combustor length, known as the compact assumption. We will develop predictions for finite-length combustors accurate to first-order in frequency. The effect of the interaction between an oscillating shock wave with combustion noise is also studied analytically. The predictions agree with data from numerical simulations. Combustion acoustics reflected at the combustor exit may go on to interfere with the combustion process, setting up a feedback mechanism that may lead to thermoacoustic instability. A modified combustor model is presented to study the effect of dissipation and dispersion of entropy waves on the instability, and it was found that the extent of dissipation or dispersion not only plays a significant role on whether instability occurs, but also determines the dominant frequency of oscillations. Furthermore, analytical and numerical investigations suggest that entropy waves are convected with the flow undissipated, and that modelling improvements may be made to take entropy dispersion into account. The findings in this work provide better tools to understand indirect combustion acoustics and to analyse their importance in both transmitted combustion noise and the thermoacoustic instability experienced by low NOx combustors.