Three-dimensional CFD modeling in-cylinder ignition, combustion and pollutant formation processes with detailed chemistry and mixing effects.

A series of combustion sub-models, which represent the physical mechanisms of ignition, turbulent combustion, transition, and soot formation were developed and investigated towards the goal of creating a robust and accurate computational engine design and performance evaluation tool. All sub-models were validated via either comparison with appropriate experimental measurements and/or alternative modeling results. In addition, all sub-models incorporated detailed chemistry. The sub-models were implemented in a modified version of KIVA-3V and used to predict the effects of the combustion processes on in-cylinder combustion performance of a Direct Injection Natural Gas (DING) engine. The most significant contributions of the study are the modification of Eddy Dissipation Concept (EDC) model for engine combustions the development of a chemistry and mixing model to represent the transition from ignition to turbulent combustion, and the development of soot model that includes soot transport effects. The EDC model originally proposed by Magnussen was modified for engine combustion processes and used to predict ignition delay. Using the modified EDC model, reaction rates during the ignition/turbulent combustion phases were predicted via interactions between the mixing rate (i.e. physical mixing of the fuel and oxidizer) and the chemical rate (i.e. the detailed chemical kinetics of the reaction mechanism). The results for ignition delay agreed well with experimental data, and it was found that mixing effects play an important role in predicting ignition delay, particularly at high temperatures. The transition phenomenon was well captured using the transition model. The soot model initially developed by Frenklach and co-workers was modified for use in predicting soot production in a DI engine. The model was validated by comparison with experimental data for an ethylene flame/perfectly stirred reactor system. When the soot model was implemented in engine studies, transport effects were found to play an important role in predicting soot formation in a DING engine. The soot model represents a considerable improvement in evaluating and predicting soot emissions in engine studies due to the basis on relevant physical phenomena and due to the inclusion of particle transport effects. The effects of detailed chemistry on the in-cylinder engine combustion processes were also investigated. Detailed chemistry produces similar cylinder pressures and engine power compared to modeling results using a global reaction. However, lower cylinder temperatures and changes in species profiles were obtained when detailed chemistry was used. The changes in temperature and species concentrations significantly affect the pollutant emissions and substantiate the need to include detailed chemistry. Engine performance studies were also conducted using the newly developed and validated sub-models. Retarding injection timing leads to incomplete combustion, and eventually produces low IMEP and high soot concentrations. Increasing exhaust gas recirculation (EGR) reduces NO significantly without notable changing the IMEP.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125489/2/3016866.pd