engleski

Numerical Simulation of Jet Diffusion Flames with Radiative Heat Transfer Modeling

Beside appropriate turbulence and combustion modeling, the problem of accurate predictions of turbulent diffusion flames, especially in industrial applications, usually requires accurate radiative heat transfer predictions as well. However, it seems at the moment that a full account of accomplished radiation modeling is too much of a burden for efficient overall modeling of diffusion flames, and simplified radiation models are usually retained in the industrial applications. In most of the early works concerned with the modeling of the relatively simple jet diffusion flames, like these simulated in this work, the radiation heat transfer has been neglected, presuming the radiant heat transfer fraction having negligible impact onto overall energy balance. When used, then usually the optically thin radiation model has been applied, mostly due to its simplicity and ease of implementation. On the other hand, in recent publications the importance of radiation inclusion into simulations of simple jet flames has been recognised, where even subtle radiation details, like spectral effects, showed to play an important role. Two different jet diffusion flame configurations are simulated in this work by using a computational fluid dynamics code FIRE – a hydrogen jet flame and a methane piloted jet diffusion flame. Predictions are compared with the experimental measurements. Based on the stationary laminar flamelet methodology, chemistry related calculations were obtained in pre-processing step by using the CSC solver together with detailed chemical mechanisms used in both cases. Statistical behaviour of the reactive scalars is presumed according to the beta probability density function of the mixture fraction moments. Turbulence modeling is based on a hybrid closure and comparisons against the classical k-eps model are performed. The aim of such a hybrid turbulence modeling was to partly overcome the limitations of the classical k-eps model by using the resolved Reynolds stresses to calculate the turbulent kinetic energy only, but still retaining the eddy-viscosity based closure in the momentum equations. This way a more accurate, but still a numerically robust turbulence closure is obtained. A conservative form of the discrete transfer radiation method (DTRM) is applied in the radiative heat transfer calculations, with the radiative properties being modeled as a weighted sum of grey gases. Conservative DTRM considerably contributed to improved temperature profiles of the jet flames simulated in this work, proving to be a sufficiently accurate method as well. Number of rays used in DTRM calculations has been varied and its impact on the predictions has been investigated.

Discrete transfer radiation method; laminar flamelet model; hybrid turbulence closure; jet diffusion flames

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