Numerical Investigation Of Diesel Combustion

Energy and environmental problems are worldwide challenge for humanity. It is essential to improve traditional energy application, such as diesel. Numerical simulation technology,which benefits from the development of computer science, plays a very important role in the design process of modern engines. To improve diesel combustion, higher fuel injection pressure, higher exhaust gas recirculation (EGR) rate, higher boost pressure are widely used in the development of new diesel combustion. These technologies accelerate the atomization and evaporation of fuel droplets, reduce the chemical reaction rate and retard the ignition in diesel engine, so emissions can be effectively reduced. Meanwhile, the new diffusion flame in diesel engine shows some similarity of premixing flame. In order to reduce the calculation cost, detailed chemical reactions have to be ignored or simplified in traditional diesel combustion simulation, which usually focuses on the mixing progress. Unfortunately these models cannot describe the modern diesel combustion.To develop new simulation method for modern diesel combustion, this thesis firstly studied the injection process of liquid fuel, focusing on the initialization of injection simulation. A1D spray model and a droplet diameter profile have been established based on theoretical analysis and hypothesis, and validated with experimental data in literatures. Diesel spray in constant volume vessel has also been investigated with3D CFD simulation and validated by high speed photography. Calculated fuel penetration and spray shape fits the experimental results reasonably well besides the initial stage.Based on the low temperature oxidation mechanism of alkane and the assumption of alkyl and aromatic species thermal cracks, a reduced mechanism of n-heptane and toluene oxidation containing26species and28reactions is achieved. The calculated ignition delays fit the experimental data reasonably well. Diesel diffusion flame in a constant volume vessel is investigated with CFD simulation using this reduced mechanism, and validated by high speed photography. The calculated results show very good agreement with the experiment, both chemiluminescent flame and high temperature flame could be evaluated accurately.Finally, a single cylinder diesel engine test bench was built, and three cases were chosen for test, two were new combustion model with high boost, high EGR and high injection pressure, the other one was operated in the traditional model. Numerical simulation was performed with the method developed in this thesis to investigate the combustion process of these cases. The pressure curves of case1and case3fit the experimental data very well, but case2show some error because of unknown injection rate. The calculated emissions are several times higher than the test results, which cannot be refined since the stoichiometric ratio of reference species is difference with diesel. Generally speaking, this thesis built an appropriate simulation method for numerical investigation of new type diesel combustion.