Computational Fluid Dynamics Investigation of Oscillatory Behaviour in Annular Combustors

The drive towards environmentally-friendly gas turbine engines imposes strict design constraints on combustion chambers to minimise harmful pollutant emission. Measures employed to counter this issue may result in oscillatory behaviour, which can cause substantial damage to the engine. Recent work has been undertaken on predicting this behaviour in annular combustors. The complexity of the annular design can give rise to an additional set of intricate oscillations due to the presence of interactions between neighbouring flames. The project is concerned with capturing trends in oscillatory behaviour in the laboratory-scale annular combustion chamber designed by Worth and Dawson. Combustion-driven oscillations are studied by a variety of methods including experimental, analytical and Computational Fluid Dynamics (CFD) approaches. The cost associated with experimental studies and the limitations of most analytical models in representing complex flow phenomena both put CFD forward as a preferred tool for the present work. The aim was to create a modelling approach that maximises efficiency in both computational time and cost, to be fit for use in an industrial context. Three objectives are defined: to develop an efficient numerical methodology to bridge the gap between analytical and high-order CFD investigations; to identify ways in which to reduce computational demands in the CAD and meshing methods; and to compare the results obtained to available reference data. A set of new inlet boundary conditions, a modular CAD geometry and a coarse meshing approach are developed to answer the computational efficiency constraint. The method also considers the use of lower-order turbulence models to reduce computational demands. Cyclic boundary conditions are applied to a single sector of the annular geometry to reduce the size of the domain. The numerical methodology developed is compared to reference data in isothermal, reacting and forced-inlet reacting cases. Its adaptability to various operating conditions is explored by observing trends in flow behaviour with varying inlet velocity and temperature, fuel, equivalence ratio, and forcing amplitude. It is shown at each stage that the modelling approach is capable of representing expected trends. This is achieved at a fraction of the cost compared to full burner configuration studies or higher-order CFD simulations. The thesis provides evidence that the computational methodology developed can be used to describe trends in forced oscillatory behaviour efficiently in the Worth and Dawson annular combustor rig, in the context of the reference cases studied. Additional work is necessary to determine the full scope of applicability of the new methodology.EU/EEC Clean Sky DYNAMO, Rolls-Royce Plc