The numerical prediction of the acoustic response of liquid-fuelled, swirl-stabilised flames

This thesis sets out and tests several different approaches to predicting and understanding the acoustic response of an industrially representative liquid fuelled, swirl-stabilised, lean-burn fuel injector using numerical simulations. This work is important as it contributes to the design of fuel injectors with a low susceptibility for thermoacoustic instabilities or ‘rumble’.The flame transfer function (FTF), a transfer function relating the mass flow rate through the fuel injector and heat release rate of the combustor, has been chosen as the best way to describe the flame response as it can be used in conjunction with a acoustic field solver to predict the stability of a combustion system. The FTF of a chosen injector geometry has been predicted using conventional compressible methods and a novel incompressible method which has been shown to be consistent with the compressible method at two frequencies of forcing. This is in contrast with mass flow forced incompressible simulations that fail to reproduce the correct downstream flow field fluctuations. The single phase flow field and acoustic response of the injector has also been predicted and compared to experiments with good agreement.The injector hydrodynamic response has also been investigated along with how hydrodynamics, acoustics, the fuel spray and heat release are related. Acoustic forcing can be seen to actively alter the strength of large scale fluid structures, the mean pressure field and the mass flow rates through the different injector passages. The fuel spray may also couple with these structures causing additional local changes to the mixture fraction field and heat release rates. The effects of fuel spray SMD (Sauter Mean Diamter) and fuel spray injection velocity have on the flame have also been tested showing that the fuel spray atomisation, which can also be affected by acoustic forcing, may play a significant role in combustion instabilities.Several novel numerical methods have been developed and are also discussed in detail includ- ing methods relating to the reproduction of acoustic forcing in incompressible simulations and the reproduction of turbulent fields at inlets. Several innovative post-processing techniques have been employed to identify the relationship between large scale flow structures, the fuel spray and combustion.Modifications of the original injector geometry have been proposed to reduce the sensitivity of the injector to instabilities. These include better atomisation and mixing, better placement of swirl vanes, better aerodynamic design and improved hydrodynamic stability.</div

An efficient method to reproduce the effects of acoustic forcing on gas turbine fuel injectors in incompressible simulations

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

PODFS Inlet Turbulence Generator

Supporting software for:Treleaven, Garmory, Page, (2020). Application of the PODFS method to inlet turbulence generated using the digital filter technique in Journal of Computational Physics. DOI: 10.1016/j.jcp.2020.109541</div

The effects of turbulence on jet stability and the flame transfer function in a lean-burn combustor

Large eddy simulations show that the penetration of the central jet in a multipassage lean burn and liquid fuelled combustor is dependent on the turbulence levels in the three air-flow passages of the injector. These simulations are performed using an incompressible method where an unsteady boundary condition is applied to the inlets of a truncated domain which only includes the domain downstream of the fuel injector using the recently developed Proper Orthogonal Decomposition Fourier Series method. The fluctuating inlets are built from a combination of compressible URANS data and incompressible LES data. This incompressible method is shown to be consistent with fully compressible simulations whilst requiring only one third of the computing time. Neglecting the turbulence generated in the passages results in the incorrect penetration of the central jet, resulting in a flame transfer function with a similar gain but with a different phase. Furthermore, large scale helical modes, previously detected in non-reacting simulations of a similar burner geometry are seen to be imprinted onto the liquid fuel spray, mixture fraction and heat release fields. This shows that coupling between hydrodynamic instabilities and thermoacoustic instabilities in liquid fuelled engines may be more significant than suggested by previous studies of gas fuelled engines