Abstract:The objective of this work is to model underexpanded turbulent sonic jets. A pressure-based computational fluid dynamics methodology has been employed, incorporating extensions to handle high speed flows. A standard two-equation turbulence model is used, with an optional compressibility correction. Comparison with experimental jet centre-line Mach number showed the correct shock cell wavelength but a too rapid decay. The compressibility correction had no effect on the shock cell decay but increased the potential core length to give better agreement with experiment. Calculations for nozzle pressure ratios up to 30 showed the variation of Mach disc location in good agreement with experiment. For nozzle pressure ratios above 6, unsteady solutions were observed, emanating from the intersection of the Mach disc with the shear layer. Experimental work has identified similar large-scale instabilities; the peak mode of the prediction had a Strouhal number of 0.16, close to experimental values.

The introduction of lean premixed combustion technology in industrial gas turbines has led to a number of interesting technical issues. Lean premixed combustors are especially prone to acoustically-coupled combustion oscillations as well as to other problems of flame stability such as flashback. Clearly it is very important to understand the physics that lies behind such behaviour in order to produce robust and comprehensive remedies, and also to underpin the future development of new combustor designs. Simulation of the flow and combustion using Computational Fluid Dynamics (CFD) offers an attractive way forward, provided that the modelling of turbulence and combustion is adequate and that the technique is applicable to real industrial combustor geometries. The paper presents a series of CFD simulations of the Rolls-Royce Trent industrial combustor carried out using the McNEWT unstructured code. The entire combustion chamber geometry is represented including the premixing ducts, the fuel injectors and the discharge nozzle. A modified k-ε model has been used together with an advanced laminar flamelet combustion model that is sensitive to variations in fuel-air mixture stoichiometry. Detailed results have been obtained for the non-reacting flow field, for the mixing of fuel and air and for the combustion process itself at a number of different operating conditions. The study has provided a great deal of useful information on the operation of the combustor and has demonstrated the value of CFD-based combustion analysis in an industrial context.

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