This work is concerned with the fluid dynamic processes and the associated loss of
acoustic energy produced by circular apertures within noise absorbing perforated walls.
Although applicable to a wide range of engineering applications particular emphasis in
this work is placed on the use of such features within a gas turbine combustion system.
The primary aim for noise absorbers in gas turbine combustion systems is the
elimination of thermo-acoustic instabilities, which are characterised by rapidly rising
pressure amplitudes which are potentially damaging to the combustion system
components. By increasing the amount of acoustic energy being absorbed the
occurrence of thermo-acoustic instabilities can be avoided.
The fundamental acoustic characteristics relating to linear acoustic absorption are
presented. It is shown that changes in orifice geometry, in terms of gas turbine
combustion system representative length-to-diameter ratios, result in changes in the
measured Rayleigh Conductivity. Furthermore in the linear regime the maximum
possible acoustic energy absorption for a given cooling mass flow budget of a
conventional combustor wall will be identified. An investigation into current Rayleigh
Conductivity and aperture impedance (1D) modelling techniques are assessed and the
ranges of validity for these modelling techniques will be identified. Moreover possible
improvements to the modelling techniques are discussed. Within a gas turbine system
absorption can also occur in the non-linear operating regime. Hence the influence of the
orifice geometry upon the optimum non-linear acoustic absorption is also investigated.
Furthermore the performance of non-linear acoustic absorption modelling techniques is
evaluated against the conducted measurements. As the amplitudes within the
combustion system increase the acoustic absorption will transition from the linear to the
non-linear regime. This is important for the design of absorbers or cooling geometries
for gas turbine combustion systems as the propensity for hot gas ingestion increases.
Hence the relevant parameters and phenomena are investigated during the transition
process from linear to non-linear acoustic absorption.
The unsteady velocity field during linear and non-linear acoustic absorption is
captured using particle image velocimetry. A novel analysis technique is developed
which enables the identification of the unsteady flow field associated with the acoustic
absorption. In this way an investigation into the relevant mechanisms within the
unsteady flow fields to describe the acoustic absorption behaviour of the investigated
orifice plates is conducted. This methodology will also help in the development and
optimisation of future damping systems and provide validation for more sophisticated
3D numerical modelling methods.
Finally a set of design tools developed during this work will be discussed which
enable a comprehensive preliminary design of non-resonant and resonant acoustic
absorbers with multiple perforated liners within a gas turbine combustion system. The
tool set is applied to assess the impact of the gas turbine combustion system space
envelope, complex swirling flow fields and the propensity to hot gas ingestion in the
preliminary design stages.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.