Large Eddy Simulation Of Spray Combustion In IC Engines Based On The Representative Interactive Flamelet (Rif) Model

The In-cylinder flow of internal combustion engines is always in a highly unsteady turbulent movement, and coupled with complex fuel injection and air-fuel mixing as well as combustion processes. It is generally agreed that Large eddy simulation (LES) will be the next generation CFD tool for studying Internal combustion engines (ICE) as an alternative of RANS. In the last decade, a considerable effort has been put into advancing the LES researches of ICEs. Unfortunately, most of them have focused on non-reacting flow, such as intake, fuel injection, atomization, evaporation, and mixing. The researches on reacting flow in ICEs are relatively less. There is a lack of well-established turbulent combustion models for LES which are highly economic and engineering feasible. In this paper, based on the Representative Interactive Flamelet (RIF) model which had made reasonable successes on RANS based simulations, a new RIF-LES model was developed. The spray combustion in a diesel engine and a constant-volume vessel were simulated and validated. High resolution grids and a reduced chemical mechanism were employed in the calculations. The mechanisms of air-fuel mixing and combustion were discussed and the effects of various factors were analysed based on the predicted results. The conclusions made by this work are listed as the following.1.The domestic reports on the RIF model are extremely scarce, so a new KIVA-RIF code was developed by incorporating the RIF model with KIVA-3V2, and RANS based simulations of a heavy diesel engine were carried out by using the reduced chemical mechanism for the validation. The predicted ignition delay, in-cylinider pressure, heat release and pollutant emissions show good agreements with the experimental data. The RIF model can account for turbulence-chemistry effects through an interactive mode between the3D flow field and1D flamelets. The predicted results reveal the formation mechanisms of the main species. NOx appear in the high temperature region surrounding the exterior of the diffusion flame. Soot and CO appear in the relatively low temperature and fuel-rich region in the interior of the diffusion flame and soot was oxidized with the enrichment of oxygen. OH appeared in the high temperature region. The results were in accordance with Dec's conceptual combustion model. Further, the predicted results show that the growth of EGR rate resulted in an increase in the ignition delay, decrease in the in-cylinder temperature and pressure, decrease in NOx emission, and increase in soot. The advance of SOI results in an increase of ignition delay which enhances the air-fuel mixing, so higher in-cylinder temperature and pressure are formed. The NOx emission increases and the soot emission decreases. With increasing of the intake temperature, the ignition delay increases, and the average pressure increases.; the average temperature decreases; the NOx emission decreases and the soot emission increases. With increasing of the intake pressure, the ignition delay decreases, and the average pressure increases; the average temperature decreases; the NOx emission decreases and the soot emission increases.2. A new version of the CFD code KIVALES was developed, named as KIVALES-RIF, by incorporating a modified RIF (LES-RIF) combustion model with large eddy simulation (LES) turbulence model. The same diesel engine is chosen for the validation of the simulations by employing the refined grids and a reduce chemical mechanism. The predicted results indicate that the LES-RIF model is a very suitable combustion model for ICEs which can describe the turbulence-chemistry interactions and consider complex chemical mechanisms. The predicted ignition delay, in-cylinider pressure, heat release and pollutant emissions show good agreements with the experimental data. And the predicted results show that LES can provide better predictive capability than RANS and demonstrate more transient stochastic eddy flow structures.3. A CIP convective scheme of three-order precision is incorporated into KIVALES-RIF to improve computational accuracy."Baseline n-heptane", the Sandia constant-volume spray combustion experiments fueled by n-heptane are numerically investigated with KIVALES-RIF. The high resolution grids and the reduced chemical mechanism are employed in the simulation. The results indicate that the predicted liquid and vapor penetrations agree well with the measurement for the non-reacting case, and the temporal development of the vapor distribution contours was also well reproduced by the simulation. With the help of the Q-criterion, the instantaneous coherent vortical structures induced by the fuel jet are successfully identified and illustrated. For the reacting cases, the predicted ignition delays agree well with the measurement, although the auto-ignition positions from the nozzle are over-predicted than the measured values. The three stages during the spray combustion (auto-ignition, the distribution of flame development and quasi-steady combustion) are all well reproduced by the model. The simulated unsteady irregular twisted flame structures are consistent with the experiments. The predicted flame lift-off lengths and time-averaging soot distributions qualitatively agree with the experiments. The predicted and measured results show that with the growth of EGR rate, the flame temperature becomes lower and more homogenous, and the flame lift-off length increases. The NOx and soot emissions decrease. The predicted results show that an increase in the injection velocity enhances the air-fuel mixing, and it results in more high-temperature combustion regions. It accelerates the oxidation of soot, and hinders the formation of soot, but the NOx emission increases. A decrease in the initial drop diameter can accelerate atomization and enhance mixing. But a change in the initial drop diameter has limited effect on the spray combustion. As the influence of reduction of nozzle diameter was discussed, it was indicated that the variation in droplet speed plays a more critical role on fuel mixing and combustion processes compared to the variation of droplet size. With an increase in the initial ambient pressure, the ignition delay is shortened, leading to reduced penetration of air-fuel mixture. Consequently, more fuel-rich regions accumulate in the cylinder, which results in more soot emissions, while the NOx emissions are not significantly affected by the change in the ambient pressure.