Abstract
Previous studies have highlighted the importance of both air mass flow rate and swirl fluctuations on the unsteady heat release of a swirl stabilised gas turbine combustor. The ability of a simulation to correctly resolve the heat release fluctuations or the flame transfer function (FTF), important for thermoacoustic analysis, is therefore dependent on the ability of the method to correctly include both the swirl number and mass flow rate fluctuations which emerge from the multiple air passages of a typical lean-burn fuel injector. The fuel injector used in this study is industry representative and has a much more complicated geometry than typical premixed, lab-scale burners and the interaction between each flow passage must be captured correctly. This paper compares compressible, acoustically forced, CFD (computational fluid dynamics) simulations with incompressible, mass flow rate forced simulations. Incompressible mass flow rate forcing of the injector, which is an attractive method due to larger timesteps, reduced computational cost and flexibility of choice of combustion model, is shown to be incapable of reproducing the swirl and mass flow fluctuations of the air passages given by the compressible simulation as well as the downstream flow development. This would have significant consequences for any FTF calculated by this method. However, accurate incompressible simulations are shown to be possible through use of a truncated domain with appropriate boundary conditions using data extracted from a donor compressible simulation. A new model is introduced based on the Proper Orthogonal Decomposition and Fourier Series (PODFS) that alleviates several weaknesses of the strong recycling method. The simulation using this method is seen to be significantly computationally cheaper than the compressible simulations. This suggests a methodology where a non-reacting compressible simulation is used to generate PODFS based boundary conditions which can be used in cheaper incompressible reacting FTF calculations. In an industrial context, this improved computational efficiency allows for greater exploration of the design space and improved combustor design.
Highlights
Combustion instabilities continue to be a major hurdle in the development of efficient, low emission, gas turbines [1]
All mass flows are normalised by the total mean mass flow rate from all passages in the compressible simulation
The fluctuations in the incompressible simulation are approximately in line with the bulk mass flow, showing that the mass flow split is constant throughout the forcing
Summary
Combustion instabilities continue to be a major hurdle in the development of efficient, low emission, gas turbines [1]. These instabilities involve interaction between the acoustic field, flow field and combustion. A crucial part of the process is the response of the flame to the unsteady air supply. When approaching the fuel injector, the acoustic wave induces an acceleration of the air within the injector body This acceleration induces vorticity at the exit of the injector leading to a vortex ring that is convected towards the flame. The same axial velocity fluctuations that cause the production of the vortex ring interact with the swirl vanes within the injector creating an azimuthal velocity fluctuation. The swirl number [6] is formally defined as the the ratio between azimuthal momentum flux and axial thrust, this requires both the pressure and axial velocity to be known and as such the pressure term is usually excluded leading to: S=
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