Numerical Modeling of Soot and NOx Formation in Non-Stationary Diesel Flames with Complex Chemistry

Doctoral thesis, 2003

A complex chemistry model of reduced size (65 species and 268 reactions) derived on the basis of n-heptane auto-ignition kinetics, small hydrocarbon oxidation chemistry, polyaromatic hydrocarbon (PAH) and NOx formation kinetics together with a phenomenological soot model has been implemented in the KIVA code for multidimensional Diesel spray combustion simulations. An EDC (Eddy Dissipation Concept) based partially stirred reactor model is used to handle the turbulence-chemistry interaction. The results obtained from numerical simulations for direct-injection (DI) Diesel sprays, injected into a high-pressure combustion vessel at engine-like conditions or a real engine geometry, show that the approach is able to reproduce the transient Diesel auto-ignition and combustion processes as observed in optical imaging studies. The simulated results (for the cases tested) indicate that the auto-ignition of DI Diesel spray occurs at a site close to the mean stoichiometric surface. The ignition spot grows on the lean side, crosses over the mean stoichiometric surface, enters into the rich zone and develops further in a very short time. The prediction demonstrates that the post-ignition, fully developed combustion process occurs in a lifted diffusion flame stabilized at a large distance from nozzle exit. The spatial distributions of soot and NOx in the predicted lifted flame are similar to those described in Dec's conceptual Diesel combustion model. Further numerical investigations performed show that, the lower the ambient gas pressure and temperature, the longer the auto-ignition delay times of the sprays and vice versa. Increase in ambient gas pressure or temperature causes a reduction in the flame liftoff length. The results demonstrate also that the flame liftoff length is more sensitive to the change in the ambient temperature. The liftoff has a strong influence on the soot and NOx formation. The farther the flame is stabilized, the lower the emissions. Studies of air dilution effects were also performed to investigate the EGR effects on ignition delay, soot and NOx emissions of Diesel flames. The simulations suggest that Diesel auto-ignition delays are controlled by the oxygen concentrations not by the nature of diluents. The soot and NOx formation is suppressed by the dilution. Moreover, studies demonstrate that the initial temperature has a strong effect on soot formation whereas the pressure effect is much weaker. For the same amount of fuel injected, the longer the fuel injection duration time, the higher the maximum value of the averaged soot mass concentration produced. It is expected that the present numerical study combined with experimental studies may provide a better insight into Diesel spray combustion and pollutant formation in Diesel flames.