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Technical Paper

An Investigation of the Dependence of NO and Soot Formation and Oxidation in Transient Combusting Jets on Injection and Chamber Conditions

NO and soot emissions from Diesel engines are dependent on several parameters related to the engine design and operating conditions. Multidimensional models are increasingly employed to study the effect of these parameters. In this paper, a multidimensional model for flows, sprays and combustion in engines is employed to study the dependence of NO and soot formation and oxidation on injection timing, injection pressure, chamber temperature, EGR and ignition delay, and compare the computed trends with those observed in experimental studies reported in the literature. Computations are carried out in a typical heavy-duty Diesel engine and additional computations in a constant volume chamber are used to clarify the engine results when appropriate. For several parametric changes, the experimentally observed trends are reproduced. However, several limitations are identified. The structure of the computed combusting jet has differences with those suggested from recent experiments.
Technical Paper

A Mixture Fraction Averaged Approach to Modeling NO and Soot in Diesel Engines

Multidimensional models are increasingly employed to predict NO and soot emissions from Diesel engines. In the traditional approach, the ensemble-averaged values of variables are employed in the expressions for NO and soot formation and oxidation. In the mixture fraction averaged approach, the values of state variables and species concentrations are obtained from the structure of laminar diffusion flames. The source terms for NO and soot are then obtained by averaging across the mixture fraction coordinate with a probability density function. The clipped-Gaussian probability density function and profiles obtained by employing the OPPDIF code (part of the CHEMKIN package) for the laminar flame structure are employed in this work. The Zeldovich mechanism for NO formation and the Moss et al. formation and Nagle-Strickland-Constable oxidation model for soot have been employed to study the qualitative trends of pollutants in transient combusting Diesel jets.