Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS


MUSCLES Berichtzusammenfassung

Project ID: G4RD-CT-2002-00644
Gefördert unter: FP5-GROWTH
Land: United Kingdom

4.2 Combustor flow field sensitivity to acoustic oscillations

The main objective of this experimental programme is to identify aerodynamic features within a gas turbine combustion system that may be sensitive to pressure fluctuations generated by the presence of heat release. Such features could provide a feedback mechanism and thereby magnify oscillations, associated with the unsteady heat release process, at certain frequencies. Measurements have therefore been undertaken at nominally atmospheric pressure to examine various parts of the combustor aerodynamic flow field and assess its sensitivity to acoustic excitation over a range of frequencies (50Hz ~ 1500Hz). The acoustic excitation is provided by loudspeakers to enable the non-reacting flow to be studied, thereby allowing decoupling of the combustion and airflow instabilities. The work has been undertaken in 2 phases namely; (i) Phase 1 in which both axial and radial low emission fuel injectors are studied in isolation and (ii) Phase 2 where a sector rig is used to study the effect of excitation on the overall combustor flow field.

The phase 1 measurements were initially with single phase flow (i.e. airflow only) and utilised an axial and radial fuel injectors of comparable effective areas. Analysis of the experimental data enabled the response of each injector flow field to the acoustic excitation to be characterised over the range of excitation. At low frequencies the injectors behave in a quasi-steady way in which the acoustically generated change in static pressure, at the injector exit plane, alters the injector pressure drop and hence the mass flow passing through it. As the excitation frequency is increased, though, the response of the injector flow field generally decreases relative to that of the quasi-steady case. However, it should be noted that superimposed on this distribution are (i) localised maximum and minima and (ii) different parts of the injector flow field appear to respond differently to the excitation. Further two phase flow measurements were undertaken in which liquid fuel was simulated by the introduction of water into the fuel injector. Laser light sheet imaging techniques were then used to visualise the fuel sheet and subsequent break up downstream of the injector. Phase averaging of the images enabled the effect of excitation to be identified relative to the random fluctuations in the fuel spray. The observed periodic fluctuations of the fuel spray at various frequencies appears to correlate with the flow field response measured in the single phase measurements. However, further measurements would be required to confirm this.

In stage 2 measurements have been made within an aero-style gas turbine combustion system incoporating a pre-diffuser, dump cavity and flame tube. In certain regions of the combustion system naturally occurring flow field instabilities were observed. When the acoustic pressure fluctuations are applied at the frequency of the naturally occurring instabilities a large flow field response is observed for modest excitation levels. Measurements also indicated how these coherent structures can be convected downstream and influence the flame tube and internal flow field along with other regions of the combustor

The experimental programme has characterised the response of various combustor flow field features to acoustic excitation. This characterisation, along with an understanding of the mechanisms responsible for the observed characteristics, will be of significant benefit to the designer of modern low emission gas turbine systems. These are particularly prone to combustion instabilities due to the desire to operate at lean conditions in order to maximise the emissions performance. In addition to improved modelling of these instabilities components can be designed such that, where possible, there response characteristics do not match that of the preferred frequencies at which instabilities are observed in a specific combustor geometry. In this way the potential feedback between fluctuations in the combustor flow field and the unsteady heat release process can be minimised. Hence low emission systems can be designed with both good emissions performance and acceptable levels of fluctuating pressures within the combustion system.

It is the intention to submit the above results in various publications with the consent of the MUSCLES partners and European Union.


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