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978-3-8439-0403-2, Reihe Energietechnik
Numerical Modelling and Simulation of Hydrogen Enriched Premixed Turbulent Flames with RANS and LES Approaches
191 Seiten, Dissertation Universität Hannover (2012), Softcover, A5
The existing technique to reduce emission levels of pollutants released from the power generation sector is the ultra lean premixed turbulent combustion method. However, here the flame is susceptible to instability, high stretch rate, local extinction and lean blow out. Adding hydrogen to hydrocarbon fuels increases burning velocity, flame stability, resistance to stretch and extinction limit. The main objective of this work is to model and simulate such hydrogen blended methane flames.
Modelling of turbulent premixed hydrocarbon flames is not trivial, due to the complex interaction between molecular transport, turbulent flow and chemical reaction. This work addresses two important topics. Firstly, reaction subclosures are investigated to allow predictive calculations of such hydrogen/methane flames. This is done in the frame of Reynolds Averaged Navier-Stokes (RANS) simulations. Secondly, the reaction sub-closure is extended to the Large Eddy Simulation (LES) technique.
Concerning to the first task, three group of subclosures are considered and tested by incorporating them to the Algebraic Flame Surface Wrinkling (AFSW) turbulent premixed reaction model, which has been developed before by Muppala and Dinkelacker for high pressure flames. One subclosure is based on an effective Lewis number approach, assuming an importance of the local diffusion effects. The second approach is based on stretched laminar burning velocities to include local curvature and stretch effects, while a third approach includes a critical chemical time scale to the leading edge concept. Three sets of experimental data are used for comparisons, including high pressure cases. The studies show that together with a suitable definition of an effective Lewis number this approach leads to a good agreement with experimental data. The stretched burning velocity subclosures fail to follow the experimental trend at higher pressure conditions. Similary, the critical chemical time scale approach leads to under-predicted reaction rates.
As the second task, the AFSW model is extended into the LES framework. Simulation results indicate that even the subgrid scale reaction models have to be accommodated with pressure terms for accurate prediction. In an additional study, the grid sensitivity is analysed using three grids. Two quality assessment techniques are applied partly for the first time to combustion LES, a two-grid estimator by Celik and a three solution method by Klein.