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Large Eddy simulation of turbulent combustion: numerical study and applications

dinsdag, 23 januari, 2007 - 18:00
Campus: Brussels Humanities, Sciences & Engineering campus
Tim Broeckhoven

Given the importance of fossil fuel combustion for energy supply, ground and air transport on one hand and problems like pollution and global warming due to CO2 production on the other hand, an improved understanding of combustion is strongly desired. Although combustion is known to mankind for millennia there are still many aspects which are not completely understood. The interaction of complex chemical reactions, transport phenomena, turbulence and radiation effects make combustion systems difficult and challenging to simulate. However because of the wide range of application, the reward of design modifications in combustion systems that can lead to higher efficiency and pollutant reduction is high.

In technical processes, combustion nearly always takes place within a turbulent rather than a laminar flow field. To improve a given combustion system, possible effects leading to reduced efficiency and mechanisms forming pollutants must be identified. This requires the knowledge of the flow and species fields which can be determined through experiments or numerical simulations. An experimental investigation of a combustor requires expensive hardware to extract all the needed quantities and in a design phase the system would have to be rebuilt after each modification. Numerical simulations on the other hand can provide more detailed information and the effects of modifications in geometry and operating conditions can be investigated much faster, thus reducing the cost and time for design and production.

This dissertation focuses on gaseous combustion and uses the Large Eddy Simulation technique (LES) to accurately describe the turbulent flow field at an acceptable cost. Before applying numerical models to practical combustion systems, they have to be validated on simplified laboratory flames, which are well investigated experimentally. Both flow field data and scalar data is required to adequately validate the models used for flow and combustion. In the present work such combustion systems are being simulated numerically and the quality of the models used is assessed by comparing with the available experimental data. Flames can be classified depending on the way the reactants are introduced in the combustion zone with two limiting cases: perfectly premixed flames and non-premixed flames. In the present work both types of combustion have been investigated.

In this work first a general introduction of the governing equations describing the fundamental properties of turbulent reacting flows and the simulation techniques to model these flows are discussed. Then the modeling of turbulent non-premixed and premixed combustion is discussed in depth, giving an overview of the state of the art models described in literature. The numerical methods used and some aspects of their implementation into the two different numerical codes used in the current work are described. For the simulation of turbulent non premixed flames a complete 3D LES code was developed at the department of Mechanical Engineering of the VUB as well as several tools to generate flame tables and inflow boundary conditions. For the simulation of premixed turbulent combustion the artificially thickened flame model was implemented in a research code developed at the TU Darmstadt. Results of the investigation of two non-premixed flames (the piloted flame Sandia D and the DLR A jet flame) defined in the TNF-workshop of the Combustion Institute are discussed as well as a premixed combustor named ORACLES, especially designed for validation of Large Eddy Simulation of turbulent premixed combustion.