Turbulence-chemistry interactions in a heavy-duty compression-ignition engine

The influences of unresolved turbulent fluctuations in composition and temperature (turbulence-chemistry interactions - TCI) on heat release, flame structure, and emissions are explored in unsteady Reynolds-averaged simulations for a heavy-duty diesel engine at four operating conditions. TCI are isolated and quantified by comparing results from a transported probability density function (PDF) method with those from a model that neglects the influence of fluctuations on local mean reaction rates (a well-stirred-reactor - WSR-model), with all other aspects of the modeling being the same (e.g., spray model, gas-phase chemical mechanism, and soot model). The simulations feature standard fuel-spray and turbulence models, skeletal-level gas-phase chemistry, and a semi-empirical two-equation soot model. Computed pressure and heat-release traces, turbulent flame structure, and emissions from the WSR and PDF models show marked differences, with the PDF-model results being in closer agreement with experiment in most cases. The soot results are especially striking. Computed soot levels from the PDF model are within a factor of five of the measured engine-out particulate matter, and computed soot levels from the WSR and PDF models differ by up to several orders of magnitude, with the PDF-model results being in much closer agreement with experiment. The results show that TCI are important in compression-ignition engines, and that accurate accounting for turbulent fluctuations is at least as important as accurate kinetic rate parameters in the gas-phase chemistry and soot models.