A Level-set Flamelet Library Model for Premixed Turbulent Combustion

University dissertation from Division of Fluid Mechanics, Department of Energy Sciences, Lund Institute of Technology, Lund University

Abstract: The modelling of premixed turbulent combustion has been studied. Premixed turbulent combustion occurs in a range of applications, such as gas turbines and spark-ignition engines. It is important that the modelling can capture the most relevant details of the combustion in order to minimize the emissions and secure the operability already in the design process of these devices. As the simulation of emission formation requires a detailed chemical mechanism, which often results in high computational effort, the modelling is a major challenge. A level-set flamelet library approach (FLA) in a k-epsilon framework has been developed and tested against experimental data. The flamelet library concept was proposed in the literature some decades ago and has been successfully applied to non-premixed combustion. The present work is however one of the first applications of the flamelet library concept to premixed turbulent combustion modelling. In the flamelet library approach, the mean turbulent flame is viewed as an ensemble of locally laminar flamelets fluctuating around a mean flame position. Each flamelet has its own local structure of temperature, species and reaction rates as a function of the flamelet coordinate. This structure may be simulated separately from the flow simulations and be stored in a table, called a flamelet library. Since the local structure is laminar and low-dimensional (often one-dimensional), it is affordable for numerical calculations, even using detailed chemical kinetics. This gives opportunity to model detailed species, such as CO, NOx and soot. A comparison with alternative models for premixed turbulent combustion, such as the eddy dissipation concept, shows that the present concept is promising in terms of accuracy, low computational demands and generality in the flamelet regime. The central components of the modelling have been identified as the following: the description of the mean flame position, the description of the variance of flame position, the probability density function for the flame position, the Lagrangian/Eulerian interpretation of the ensemble averaging procedure, the formulation of the level-set as a distance function and the construction of laminar flamelet libraries. Submodels for these components have been proposed. The performance of the present level-set flamelet library approach and the submodels has been tested and compared with experimental data. Measurements from the VAC Validation Rig 1, in which a lean premixed propane/air V-shaped flame is stabilized behind a triangular prismatic flame holder in a rectangular channel, are utilized. Comparisons of calculated distributions with measured data indicates that the current approach not only yields good simulations of temperature and major species such as O2 and CO2, but also gives reasonable predictions of the minor species CO and NO thanks to special modelling treatment. For the modelling of intermediate species, such as CO, it is necessary to account for flame wrinkling and flame stretch. For species with formation paths which are too slow to be contained in the flamelet, an approach based on flamelet libraries containing formation rates is developed. The sensitivity of the numerical results to different submodels is studied. It is found that there is need for further refinement of the submodels identified here.

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