Effects of turbulence-chemistry interactions in direct-injection compression-ignition engines.

摘要

Advanced combustion strategies are emphasized in modern compression-ignition engine systems, aiming at improving diesel engine efficiency and reducing pollutant emissions, especially soot and NOx, together with strategies to accommodate unconventional fuels. Recent studies have shown the importance of turbulence and turbulence-chemistry interactions on emissions from laboratory flames and compression-ignition engines.;Constant-volume, high-pressure spray combustion is an important intermediate step for model validation and scientific understanding of combustion in direct-injection compression-ignition engines. The Engine Combustion Network (ECN) provides a series of well-documented experimental data for spray combustion under typical diesel-engine conditions, and this serves as a good resource for simulation and validation purposes. Here simulations for the ECN constant-volume, n-heptane spray configuration have been performed using OpenFOAM, an object-oriented C++ based code. The effects of exhaust-gas recirculation (EGR), ambient temperature and density on combustion were investigated computationally. The simulations demonstrate that the CFD model is capable of predicting sprays, mixing, ignition and combustion, quantitatively, for engine-relevant conditions reasonably well. The numerical results show that the ignition delay and lift-off lengths are strongly influenced by EGR, ambient gas temperature and ambient gas density, in agreement with measurements. Results from a model using a transported probability density function (PDF) method that explicitly accounts for turbulence-chemistry interactions have been compared to those from a model that simplistically accounts for turbulence-chemistry interactions, including mixture fraction profiles, ignition delays, lift-off lengths and flame structures under various ambient conditions. Significant differences between these two models have been observed, which shows the importance of turbulence-chemistry interactions. The turbulent flame structure predicted by the PDF method is more realistic than that obtained from a simplistic model to account for turbulence-chemistry interactions. The choice of chemical mechanism also plays a strong role.;Next, the high-fidelity CFD-based models have been used to simulate fuel effects and complex interactions between turbulence and gas-phase chemistry on emissions for biodiesel combustion and hydrogen-assisted diesel combustion in common-rail diesel engines. The sensitivity of predicted NOx emissions to variations in the physical properties of the fuel (density and viscosity) has been explored to determine the origins of the so-called biodiesel-NOx effect: the increase in NOx emissions that has been observed when petroleum-based diesel fuel is replaced with biodiesel fuel. Interactions between turbulence and gas-phase chemistry have been found to be important in the fuel density effect on NOx emissions. CFD also has been used to explore the changes in NOx emissions with hydrogen substitution that have been observed experimentally in hydrogen-enriched diesel combustion over a range of operating conditions. In spite of the significant simplifications and approximations, the model is able to reproduce the experimentally observed trends for some operating conditions. A model using a transported PDF method that explicitly accounts for turbulence-chemistry interactions does somewhat better than a model using well-stirred reactor model which ignores turbulence-chemistry interactions, in low-speed conventional diesel combustion cases. The CFD results are consistent with the hypothesis that in-cylinder HO2 levels increase with increasing H2, which enhances the conversion of NO to NO2.;In close collaboration with engine experiments, this research shows that fuel physical properties and complex interactions between turbulence and chemistry have important effects on emissions. It has provided new physical insight into in-cylinder processes, which in turn allows better understanding for advanced engine development.