Large Eddy Simulation of a Gas Turbine Model Combustor

The current design of gas-turbine (GT) systems is driven by the need for increased powerdensities, improved fuel-efficiencies, and reduced life cycle costs and environmental impact. Computational techniques have the potential for providing valuable information for the design of GT combustion systems, if adequate models are available. Over recent years, remarkable progress has been made in the development of high-fidelity combustion models and numerical techniques for turbulent reacting flows. In particular, the LES technique has been demonstrated to provide considerably improved predictions for scalar mixing processes compared to Reynolds-averaged Navier-Stokes (RANS) approaches. This improved predictive capability is attributed to the fact that in LES the energy-containing and large-scale coherent structures are fully resolved, and only effects of numerically unresolved turbulent scales require modeling. These small scales, however, are more homogeneous so that more universal closure models can be utilized. Over recent years, different LES combustion models have been developed, including level-set formulations, conditional moment closure models, thickened flamelet models, transported PDF methods, and flamelet-based combustion models. However, these models have been largely developed and validated in the context of canonical and geometrically unconfined flame-configurations, such as jetflames or simple dump-combustors. Furthermore, LES-calculations in complex burner-configuration that are relevant to realistic gas-turbine combustor and operating conditions have so far not been fully utilized. This shortcoming can be attributed to the following reasons: (i) Absence of highfidelity computational models that can accurately describe the turbulent combustion processes and coupling between turbulence, reaction chemistry, and scalar mixing; (ii) Lack of experimental data to enable comprehensive model-validation; (iii) Geometric complexity and construction of geometry-conform meshes for complex combustor geometries; (iv) Highly transient combustion regime, topologic asymmetry, and flow-field sensitivity and solution-dependence on grid-resolution and numerical accuracy; and (v) Computational complexity and necessary requirements for accurately resolving relevant spatio-temporal scales. Apart from very few exceptions, LES-calculations of gas-turbine combustors have so far been performed under drastically simplified conditions, limited or no comparison with experimental data, and by employing significant simplifications in the description of the combustion model (i.e., utilizing one-step reaction chemistry, ambient operating conditions, and restriction to gaseous fuel combustion).