Abstract

Axially-staged combustion systems offer both enhanced operability and fuel flexibility for gas turbines, allowing stable operation and low emissions across a wide range of engine loads. The sequential combustion concept, where the first combustion stage is supported by a standard lean premixed flame, and the second stage relies on an autoignition-dominated flame forms the focus of the present contribution. Within the present state of the art, the still-oxygen-rich exhaust gases from the first stage are mixed with second-stage fuel within a sequential burner. The flow then exits into a combustor at a sudden expansion where the autoignition-initiated flame stabilizes. The purpose of the sudden expansion is to anchor the flame, as otherwise, minor perturbations in, for example, the inlet temperature, results in large excursions in the flame location. Within the presently investigated concept, rather than relying on a sudden expansion, the flame is anchored by establishing a positive static temperature gradient within the burner. The advantage of such a concept is that it potentially allows for very small combustor residence times and can be easily incorporated into an integrated combustor-nozzle guide vane. The concept does however present significant challenges, which are investigated within then present contribution. A critical challenge is that, in order to set up the static temperature gradient, the flow has to be accelerated to a relatively high Mach number, ca. 0.7, and then decelerated in a diffusing section where the flame is located. Achieving fuel/air premixing and combustion, while achieving acceptable pressure drops is not trivial at the high velocities. Additionally, the dynamic stability of the concept is not clear and needs to be investigated. Within the present work, compressible CFD is used to investigate the pressure drop characteristics within the system. It is demonstrated that for the system a total pressure drop of < 6% can be achieved. To realize this, the premixing section includes multi-point fuel injection coupled with mixing devices. The arrangement is designed to both limit excessive pressure losses by focusing losses within regions of the flow where they contribute effectively to fuel/air mixing as well as locating the flame where Rayleigh losses are acceptable. The dynamic behaviour of the system is studied by way of 2D fully premixed CFD. Investigation of the flame response to harmonic perturbations in inlet temperature shows that the flame transfer function (FTF) is characterized by amplitude growing, in line with the concept of auto-ignition at low Mach number, linearly with frequency. The rate of growth with frequency of the FTF amplitude is rather high reaching up to sixty times the imposed relative fluctuation of inlet temperature at a frequency of 600Hz. This rapid growth is in line with the behaviour of autoignition at low Mach number. A substantial difference with the low Mach number concept is given by upstream travelling acoustic waves generated by the flame that, going through high Mach number locations, can affect, in respect of the conservation of entropy transported by convection, the upstream temperature distribution and therefore auto-ignition itself.

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