Premixed Generated Manifolds for the Computation of Technical Combustion Systems

Abstract

Large eddy simulations (LES) show a good prediction accuracy at a decent computational cost for the simulation of combustion processes in complex geometries. However, the large grids required make the direct solution of detailed reaction kinetics impracticable. Therefore, the chemical reactions can be tabulated in a pre-processing step using detailed chemistry with one-dimensional laminar steady flamelets. These flamelets can be either non-premixed or premixed and are stored based on controlling variables like mixture fraction and reaction progress parameter, for example. In this work, a progress variable approach (PVA) using premixed flamelets was adopted to generate a manifold defined by mixture fraction and reaction progress variable. Since the computation of the flamelets is only feasible between flammability limits, the data outside these limits has to be extrapolated to obtain the complete manifold for all chemical states. The extrapolation influences the stability of the LES and its prediction quality and so four different extrapolation schemes were studied. A probability density function (PDF) model was applied to account for subgrid scale variances. Two methods of modeling the joint PDF of mixture fraction and progress variable in terms of their statistical dependence were investigated. Some results of a bluff body configuration comparing the PDF modeling approaches are shown. The results demonstrated that a diffusion flame can be simulated with both the progress variable approach based on premixed flamelets and classic non-premixed flamelets without progress variable.