Effects of Hydrogen Addition on NOx Emissions in Hydrogen-Assisted Diesel Combustion

Computational fluid dynamics (CFD) 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 the experiments, it is found that engine-out NO tends to decrease while NO2 increases with increasing levels of H2 substitution. In spite of the significant simplifications and approximations that are made in the CFD model, the model is able to reproduce the experimentally observed trends for some operating conditions. A model that explicitly accounts for turbulence-chemistry interactions using a transported probability density function (PDF) method does somewhat better than a model that ignores the influence of turbulent fluctuations on mean chemical production rates, although the importance of the fluctuations is not as strong as has been reported in some other recent modeling studies. The CFD results confirm that temperature changes alone are not sufficient to explain the observed reduction in NO and increase in NO2 with increasing H2. The CFD results are consistent with the hypothesis that in-cylinder HO2 levels increase with increasing H2, and that the increase in HO2 enhances the conversion of NO to NO2.