Development and application of a transported probability density function model for advanced compression-ignition engines.

摘要

A transported probability density function (PDF) method is coupled with a deforming/ moving grid with periodic removal/addition of layers of cells to accommodate piston motion in engine modeling. The coupled model is used to simulate incylinder combustion processes for heavy-duty compression-ignition engines. First, the influences of unresolved turbulent fluctuations in composition and temperature (turbulence-chemistry interactions -- TCI) on heat release, flame structure, and emissions are explored at four operating conditions in a conventional diesel engine. TCI are isolated and quantified by comparing results from the transported PDF model 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). 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. While the peak cylinder pressure values predicted by the PDF model are within 3% of the measured data, those predicted by the WSR model differ up to 10.5% from experimental data. 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. These results highlight the importance of TCI in compression-ignition engines. Second, one of the advanced combustion modes -- partially premixed combustion -- is studied using gasoline as fuel. It is observed that at least four components are required to form a gasoline surrogate to predict the ignition characteristics, flame structure and emissions accurately. A good surrogate chemical mechanism needs to be validated for two-component primary reference fuel (PRF) mixtures (mixtures of n-heptane and iso-octane) and three-component toluene reference fuel mixtures (mixtures of n-heptane, iso-octane and toluene) under heavy-duty engine conditions before using it to predict gasoline combustion characteristics. Several PRF chemical mechanisms are tested to model the combustion of two-component PRF mixtures, and none of them satisfactorily match the experimental data. Those mechanisms that have been primarily developed to study leaner combustion conditions predict a longer ignition delay compared to experiments. Finally, a new combustion concept based on advanced combustion strategies has been explored. A preliminary study of this concept shows tremendous potential to increase efficiency.