Verification and Improvement of Flamelet Approach for Non-Premixed Flames

Studies in the mathematical modeling of the high-speed turbulent combustion has received renewal attention in the recent years. The review of fundamentals, approaches and extensive bibliography was presented by Bray, Libbi and Williams. In order to obtain accurate predictions for turbulent combustible flows, the effects of turbulent fluctuations on the chemical source terms should be taken into account. The averaging of chemical source terms requires to utilize probability density function (PDF) model. There are two main approaches which are dominant in high-speed combustion modeling now. In the first approach, PDF form is assumed based on intuitia of modelliers (see, for example, Spiegler et.al.; Girimaji; Baurle et.al.). The second way is much more elaborate and it is based on the solution of evolution equation for PDF. This approach was proposed by S.Pope for incompressible flames. Recently, it was modified for modeling of compressible flames in studies of Farschi; Hsu; Hsu, Raji, Norris; Eifer, Kollman. But its realization in CFD is extremely expensive in computations due to large multidimensionality of PDF evolution equation (Baurle, Hsu, Hassan)

Flamelet Model Application for Non-Premixed Turbulent Combustion

The current Final Report contains results of the study which was performed in Scientific Research Center 'ECOLEN' (Moscow, Russia). The study concerns the development and verification of non-expensive approach for modeling of supersonic turbulent diffusion flames based on flamelet consideration of the chemistry/turbulence interaction (FL approach). Research work included: development of the approach and CFD tests of the flamelet model for supersonic jet flames; development of the simplified procedure for solution of the flamelet equations based on partial equilibrium chemistry assumption; study of the flame ignition/extinction predictions provided by flamelet model. The performed investigation demonstrated that FL approach allowed to describe satisfactory main features of supersonic H 2/air jet flames. Model demonstrated also high capabilities for reduction of the computational expenses in CFD modeling of the supersonic flames taking into account detailed oxidation chemistry. However, some disadvantages and restrictions of the existing version of approach were found in this study. They were: (1) inaccuracy in predictions of the passive scalar statistics by our turbulence model for one of the considered test cases; and (2) applicability of the available version of the flamelet model to flames without large ignition delay distance only. Based on the results of the performed investigation, we formulated and submitted to the National Aeronautics and Space Administration our Project Proposal for the next step research directed toward further improvement of the FL approach

Experimental characterization of aircraft combustor soot: Microstructure, surface area, porosity and water adsorption

International audienceThe laboratory combustion technique operating on a typical combustor of a gas turbine engine is used for soot sampling. Soot particles are derived by combustion of a hydrocarbon C3H8–n-C4H10 mixture at typical cruise conditions. Size, morphology, microstructure, surface area, porosity, and the chemical nature of the soot surface particles are studied by transmission electron microscopy (TEM), Raman and Auger electron spectroscopies (AES), volumetry and gravimetry. Structural irregularities such as micropores determine the specific adsorbability of non-polar gases such as Kr, CH4 and C6H6. With respect to water adsorption, aircraft combustor soot is far from being hydrophobic. Initial water adsorption on polar heterogeneities leads to pore filling at increasing pressures. The microstructure of soot particles is easily transformed under the influence of adsorbates, giving rise to swelling effects. Due to its specific physico-chemical properties aircraft combustor soot may act as contrail condensation nuclei at low sulfur content in the jet fuel

Abstracts of the 6th FECS Conference 1998 Lectures

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