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Project ID: G4RD-CT-2002-00644
Finanziato nell'ambito di: FP5-GROWTH
Paese: Germany

2.4 The influence of turbulent mixing and reaction on Lean Blowout characteristics

Models that have been developed the in the past to explain important mechanisms that control extinction and stability concentrated on simple 1-D flame configurations. Additionally effects of turbulence usually were of minor interest. Since most practical combustion devices burn liquid fuels like kerosene and exhibit complex turbulent flow patterns, the results of those idealised systems cannot be extrapolated to turbulent reacting two-phase flow systems. On the other hand, due to computational limitations, it is not possible to include all mechanisms describing accurately extinction phenomena into a turbulent combustion model. Therefore, it is important to identify the key parameters that are responsible for the extinction of turbulent aerodynamically stabilised flames.

The stability performance of practical systems is strongly influenced by several hardware specific parameters like swirl number, air distribution, fuel placement and spray characteristics. In a first step the investigation will concentrate on gaseous flames in order to reduce the complexity of the system.

The presumably most important parameters are the turbulent length scales and the fluctuation intensities of both velocity and species mass fractions as well as an appropriate chemical time scale. Turbulent combustion models, that incorporate these parameters mentioned above while not making excessive computational demands are models based on a presumed PDF approach (PDF = Probability Density Function). Such approaches that are based on a mixture fraction and a single reaction progress variable have proven to be suitable for describing important characteristics of swirling flames such as the lean blow off limit.

In any case the success of a PDF-based model is crucially dependent on the ability to describe marginal PDFs and a reliable reaction model that is able to capture the main characteristics of the combustion chemistry near extinction. The aim of this task is the extension and validation of an existing JPDF model, already used and verified within the scope of the CFD4C project.

The knowledge of the JPDFs of mixture fraction and reaction progress near extinction is crucial in order to assess the applicability of the turbulent reaction model. Existing detailed experimental data for turbulent flow and mixing will serve as a database for the validation and evaluation of the presumed PDF model.

For the selected test cases, numerical simulations will be performed and the calculated PDFs will be compared with the experimental data. Based on this evaluation it will be possible to assess the necessity for the more sophisticated but computationally more expensive transported PDF models in order to describe the physics of flames near extinction. If the key parameter is instead the proper representation of the chemical kinetics, the reaction model will be altered in order to describe lean blowout with satisfactory accuracy. Finally the model will be applied to the same combustor but for different operating conditions and its applicability will be shown.

The results of the project will deepen the understanding of physical sub processes that lead to extinction of turbulent aerodynamically stabilised diffusion flames and will shed a light on the necessity to utilise more complex turbulent combustion models. The resulting model will improve the capability of predicting lean stability limits of aero engine combustors and might guide the design and development procedure for combustors that meet stringent demands on increasing combustion performance.

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