Large-Eddy simulation of gas turbine combustors using Flamelet Manifold methods
Lietz, Christopher Fernandez
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The main objective of this work was to develop a large-eddy simulation (LES) based computational tool for application to both premixed and non- premixed combustion of low-Mach number flows in gas turbines. In the recent past, LES methodology has emerged as a viable tool for modeling turbulent combustion. LES is particularly well-suited for the compu- tation of large scale mixing, which provides a firm starting point for the small scale models which describe the reaction processes. Even models developed in the context of Reynolds averaged Navier-Stokes (RANS) exhibit superior results in the LES framework. Although LES is a widespread topic of research, in industrial applications it is often seen as a less attractive option than RANS, which is computationally inexpensive and often returns sufficiently accurate results. However, there are many commonly encountered problems for which RANS is unsuitable. This work is geared towards such instances, with a solver developed for use in unsteady reacting flows on unstructured grids. The work is divided into two sections. First, a robust CFD solver for a generalized incompressible, reacting flow configuration is developed. The computational algorithm, which com- bines elements of the low-Mach number approximation and pressure projection methods with other techniques, is described. Coupled to the flow solver is a combustion model based on the flamelet progress variable approach (FPVA), adapted to current applications. Modifications which promote stability and accuracy in the context of unstructured meshes are also implemented. Second, the LES methodology is used to study three specific problems. The first is a channel geometry with a lean premixed hydrogen mixture, in which the unsteady flashback phenomenon is induced. DNS run in tandem is used for establishing the validity of the LES. The second problem is a swirling gas turbine combustor, which extends the channel flashback study to a more practical application with stratified premixed methane and hydrogen/methane mixtures. Experimental results are used for comparison. Finally, the third problem tests the solver’s abilities further, using a more complex fuel JP-8, Lagrangian fuel droplets, and a complicated geometry. In this last configu- ration, experimental results validate early simulations while later simulations examine the physics of reacting sprays under high centripetal loading.