CFD Simulation of Turbulent Combustion for Description of Non-Uniform Process by a Stochastic Approach

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A stochastic approach was applied to computational fluid dynamic (CFD) simulation to establish efficient program valid for predicting turbulent combustion. The program placed a special emphasis on description of concentration and density fluctuations due to local heat release which are essential to the pollutant formation in highly turbulent combustion, such as an engine combustion. Obtained results in the present study are summarized as follows.(1)The probability density function (pdf) of the mass fraction of fuel was taken into consideration in each computational cell of a finite difference CFD program to allow description of the microscopic heterogeneity of mixture concentration. Changes of pdf due to convection and diffusion were solved using hybrid scheme in an implicit method. Furthermore, the process of turbulent mixing was expressed by a collision-redispersion model. The collision rate was estimated from the turbulence characteristics based on the k-epsilon model.(2)The accurac … More y of the above prediction program was investigated for a simple case of one- and two-dimensional turbulent flows. It was shown that the time evolution of the mean concentration was consistent with theoretical one as long as twenty or more fluid elements in each cell are chosen. In addition, the simulation program for a more practical in-cylinder flow was performed for a further test.(3)To investigate the effect of mixture heterogeneity on the formation of oxides of nitrogen in the engine combustion, the histories of temperature distribution and NO concentration were calculated using a thermodynamic model based on the measured rates of heat release for the mixture condition having different degrees of heterogeneity. It was shown that a higher NO concentration originates from the hot burnt gas initially ignited. Thus, a lean burning of spark-ignition engines with a more homogeneous mixture should be used for NOx reduction. It was also suggested that much reduction of NO may be achieved once the burnt gas is made stirred. Less