Abstract
Sequential combustion constitutes a major technological step-change for gas turbines applications. This design provides higher operational flexibility, lower emissions, and higher efficiency compared to today's conventional architectures. Like any constant pressure combustion system, sequential combustors can undergo thermoacoustic instabilities. These instabilities potentially lead to high-amplitude acoustic limit cycles, which shorten the engine components' lifetime, and therefore, reduce their reliability and availability. In the case of a sequential system, the two flames are mutually coupled via acoustic and entropy waves. This additional interstages interaction markedly complicates the already challenging problem of thermoacoustic instabilities. As a result, new and unexplored system dynamics are possible. In this work, experimental data from our generic sequential combustor are presented. The system exhibits many different distinctive dynamics, as a function of the operation parameters and of the combustor arrangement. This paper investigates a particular bifurcation, where two thermoacoustic modes synchronize their self-sustained oscillations over a range of operating conditions. A low-order model of this thermoacoustic bifurcation is proposed. This consists of two coupled stochastically driven nonlinear oscillators and is able to reproduce the peculiar dynamics associated with this synchronization phenomenon. The model aids in understanding what the physical mechanisms that play a key role in the unsteady combustor physics are. In particular, it highlights the role of entropy waves, which are a significant driver of thermoacoustic instabilities in this sequential setup. This research helps to lay the foundations for understanding the thermoacoustic instabilities in sequential combustion systems.
Highlights
In recent years, the global energy market has undergone significant changes
The work reported how the system loses its stability for a range of values of the second stage thermal power, exhibiting a large-amplitude acoustic limit cycle
We highlighted the particular dynamics associated with this instability: two modes are active in the combustor and they interact with each other, giving rise to a synchronized bi-modal oscillation
Summary
The global energy market has undergone significant changes. Both governments and customers are asking for efficient and non-polluting energy converters, and these requests are having an impact on the global energy mix. The second stage flame stabilizes in the sequential chamber, which has a square section of 88×88mm and quartz walls to provide optical access. Thermoacoustic instabilities originate due to a feedback interaction between acoustic pressure and the unsteady heat release rate from the flame. In the case of sequential combustion, the complexity of this problem is further increased In this architecture, the two flames are mutually coupled, and they both contribute to the overall system stability. The second flame is sensitive to temperature fluctuations, which can be generated from the first stage when the combustion is unsteady. This is especially the case in the technically premixed configuration, where coherent equivalence ratio oscillations can be part of the thermoacoustic feedback. A low-order model of the two thermoacoustic modes, which is able to mimic and explain the observed dynamics is proposed
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