Modeling of Turbulent Nonpremixed Combustion Using a Combined Probability Density Function/Moment Method

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A combined probability density function (PDF)/moment method has been developed to improve the accuracy of the Monte-Carlo method. The accuracy of Monte-Carlo method depends strongly on the number of Monte-Carlo particles and declines with the decrease in the PDF transport probability. The present method solves simultaneously a modeled PDF transport equation and moment equations of mean and variance. Both the mean and variance values obtained from the moment equations are just used as the reference values to match to the corresponding values of PDFs. The usefulness of the present method is evaluated in the configuration of a turbulent jet diffusion flame, with a comparison with other methods; the PDF method and the flamelet model method. In order to obtain the benchmark data of the combined method, we investigated the emission characteristics of jet nonpremixed flames confined in cylindrical furnaces and the statistics of the flame base structure of lifted jet nonpremixed flames. The re … More sults obtained are as follows.1) The comparison between the present combined PDF/moment method and the flamelet model method have verified the improvement from the PDF method. The combined PDF/moment method can reproduce the extinction and the re-ignition in jet nonpremixed flames.2) The flame characteristics of confined flames can be strongly dominated by the combination of the burner and furnace geometries. In the present study, the effects of the inner diameter of the cylindrical furnace, the turbulence at the flame boundary, and the global equivalence ratio are examined in terms of the emission of NOx. It was found that the emission index of NOx decreases roughly with these parameters.3) Nonpremixed flamelet statistics at the flame base of lifted turbulent nonpremixed flames were investigated experimentally using a planer temperature Rayleigh scattering method. The scalar dissipation rate has log-normal statistics and the quenching scalar dissipation rate is lower than the critical value predicted using the uniformly strained counter nonpremixed flame. Less