High-Frequency Combustion Instabilities in Reheat Gas Turbine Combustion Systems
Supervisor | Subject |
|---|---|
| Prof. Dr.-Ing. Thomas Sattelmayer | High-frequency thermoacoustics, flame modulation, auto-ignition stabilized flames, vortex shedding |
Editor | Cooperation/Funding |
| Jonathan McClure, M.Sc. | This project is funded by the DFG, whose support is gratefully acknowledged. |
Motivation
Part-load performance, combined cycle efficiency and low emissions are key drivers in the development of new gas turbine combustion systems. Reheat combustors utilising partial auto-ignition flame stabilisation combined with axial fuel and air staging not only exhibit excellent emission characteristics, but also offer superior performance at part-load conditions. Despite the significance of these staged combustion systems, little information is available on flame stability and dynamics. In particular, instabilities in the high-frequency regime are not well characterized and there is very limited knowledge of the associated driving and damping mechanisms.
Objectives
The goal of this project is to develop a deeper understanding of the driving mechanisms of high-frequency thermoacoustic instabilities in a representative, laboratory-scale combustor. Previous studies at the Chair of Thermodynamics investigated such instabilities in a tubular, swirl-stabilised premixed combustion system and provided insight into the physical mechanisms present. This project aims to bring the level of understanding of high-frequency instabilities in partially auto-ignition stabilized flames to the same level as that for swirl stabilized flames and to provide models for the additional instability driving mechanisms present.
Experimental Approach
The experimental investigations are carried out on the two-stage HTRC (High-Frequency Transverse Mode Reheat Combustor) test rig previously constructed at the Chair of Thermodynamics [1]. The second stage, shown in Fig. 1, is designed to exhibit high-frequency transverse instability modes and features both propagation and auto-ignition flame stabilisation.
Pressure phenomena and flame dynamics are investigated by the simultaneous measurement of the dynamic pressure with OH* and CH* chemiluminescence. This synchronous approach allows phase-locking of the flame images with the pressure time series in order to determine the flame behavior under specific pressure conditions. Additionally, the second stage is optically accessible from all lateral sides, allowing investigation of instability modes in both transversal directions. The aim of these investigations is to identify the contribution of additional driving mechanisms associated with auto-ignition stabilised flames, namely due to (1) vortex shedding, (2) pressure and temperature dependency of auto-ignition and (3) convective/dispersive effects originating from the mixing section.
Results
Two transverse instability modes have been investigated, a marginally unstable T1 mode in the y-direction at 1600Hz and a fully unstable self-excited T1 mode in the z-direction at 3000Hz (see Fig. 2) .
The marginally unstable y-mode exhibits intermittent behavior in the pressure time series which is often observed as a precursor to full instability if the power density is further increased. Acoustically induced vortex shedding is also observed in the chemiluminescence images at the same frequency as the acoustic mode. These vortices appear as alternating patches of higher and lower intensity fluctuation distributed asymmetrically about the centre axis as shown in Fig \ref{fig:T1y_Vortex}. It is yet to be determined if this vortex shedding contributes to the driving or damping of the instability.
In the case of the z-mode, which is fully unstable, there is a clear oscillation of the flame at the frequency of the pressure fluctuations as illustrated in Fig. 3. As the pressure oscillations of the T1 mode in the z-direction are strongest towards the chamber walls, where the flame deformation is greatest, it is therefore likely that this oscillation is driven by pressure modulation of the auto-ignition stabilised flame zone.
References
[1] F. Berger et al, A Novel Reheat Combustor Experiment for the Analysis of High- Frequency Flame Dynamics - Concept and Experimental Validation, ASME Turbo Expo 2018, Oslo, Norway (2018).