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Title: Development of acoustic-enhanced damping configurations for gas turbine combustion systems
Author: Cassell, Mark A.
ISNI:       0000 0004 7970 9804
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2017
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Combustion instabilities can be self-excited through a feedback loop between unsteady heat release and acoustic pressure oscillations. Furthermore, whilst low emission lean burn systems are attractive due to reduced NOx emissions, their application may be restricted due to their increased sensitivity to these instabilities. Combustion instabilities cover a range of frequencies and modes that are dependent upon the engine operating conditions and geometry. However, passive acoustic dampers may potentially be used to control the instabilities by damping the pressure pulsations; although, multiple dampers may be required to cover the entire range of frequencies with sufficient damping. Geometrically large cavities and cooling flows through the dampers result in systems that are challenging to design within the strict geometric and operating envelopes of gas turbine aero-engines. Thus, novel passive damping systems that can offer a consistent acoustic response, but with a reduced space requirement or cooling flow, are highly attractive. One-dimensional analytical models are used and developed for analysing the acoustic performance of Helmholtz resonator-based devices and perforated damping liners. Some devices analysed have multiple resonant frequencies that may be exploited, in addition to having the advantage of a smaller cavity to target the same fundamental resonant frequency as a datum Helmholtz resonator. Experiments were performed with an incident axial mode to validate the analytical models using a long wavelength assumption. However, in a practical engine environment where annular geometries are typically used, higher-order circumferential modes may cut-on. This results in helical waves that have axial and circumferential components. These incident helical waves and mixed mode fields can cause pressure variations along the damper surface that can invalidate the long wavelength assumption. Thus, a helical wave model is developed that is capable of modelling annular and circumferentially segmented perforated damping liners. Using the model unique parameters for damping the acoustic response of a circumferential mode are identified. To validate the model within a mixed mode acoustic field a unique experimental facility has been designed and commissioned. Accompanying decomposition software for the higher-order modes is developed based on the ubiquitous multi-microphone method and an optimised configuration of microphones. The microphone configuration and decomposition technique are analysed to assess reconstruction errors. Using the software for microphone placement the minimum number of microphones and optimised locations can be established for acceptable error levels in an acoustic field of unknown structure. Furthermore, based on experimental data that has been acquired, the software calculates an optimised solution that determines the combination of acoustic modes that are present. The new experimental mixed mode facility has been used to create test data for a range of conditions. It shows an initial frequency increase when mode shapes are generated circumferentially within the cavity, which is also predicted by the helical wave model. An initial comparison to the model is presented, with initial design recommendations made to optimise the liner acoustic damping response. Additionally, asymmetric geometries, which may generate an acoustically coupled response have been experimentally investigated.
Supervisor: Not available Sponsor: EPSRC ; Rolls-Royce plc
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering not elsewhere classified ; Acoustic absorption ; Impedance ; Gas turbine engine ; Combustion system ; Passive damping ; Higher-order acoustic modes ; Helmholtz resonator ; Thermo-acoustic instability ; Acoustic field decomposition