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.