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

Abstract

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.