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The calculation of the Nitrogen Oxide (NO) formation emitted from diesel engines usually involve direct integration of a set of nitrogen chemistry elementary reactions that involve formation and destruction of NO. The primary hydrocarbon chemistry is usually simplified as long as the main species and heat release are predicted correctly. The result of the integration is the net NO formation rate evaluated using the local concentrations and thermodynamic parameters. In the present work a method for calculating NO emissions from diesel engines is proposed that takes into consideration the effect of residence time as a measure of turbulence effects on chemistry. This is based on the assumption that for mixing-limited conditions the turbulent eddy turn-over time can be taken as a characteristic reaction residence time. The proposed procedure depends on a detailed investigation of the primary hydrocarbon combustion chemistry decoupled from the flow-field prediction.

The purpose of the present study is a to demonstrate and validate the use of the conserved scalar/assumed pdf approach to simulate turbulent mixing. To achieve this goal a numerical simulation of an experimental bench-mark problem is performed. A two-dimensional finite volume axisymmetric model numerically simulates the steady state passive scalar/air mixing. The computational study is conducted by solving the Reynolds Averaged Navier Stokes (RANS) flowfield governing transport equations. The turbulent fluctuations are computed by the two-equation k-e model. In addition to the flowfield variables, the model also solves for the mean and variance of the passive scalar concentration. To calculate the mean density, the probability density function (pdf) of the mixture fraction is assumed to follow the Beta-function distribution. The mean density is then computed by the convolution integral of the instantaneous density over the mixture fraction space.