Search Results

A method for automatically generating hex-dominant meshes for Computational Fluid Dynamics (CFD) applications is presented in this article. Two important regions of the mesh for any CFD simulation are the interior mesh and the boundary layer mesh. The interior mesh needs to be fine in the critical flow regions to ensure accurate solutions. The proposed method uses Bubble Mesh algorithm which packs bubbles inside the geometry to generate the mesh nodes. Algorithm was tested for sample flow problems and improvements were made to interior and boundary layer mesh generation methods. The interior mesh is generated using directionality and sizing control functions specified on the points of a 3D grid generated over the entire geometry. This offers a flexible control over mesh sizing and local mesh refinement. Boundary layer mesh is important to accurately model the physics of boundary layer near the geometry walls.

In previous publications, Singh et al. [1, 2] have shown that direct integration of CFD with a detailed chemistry auto-ignition model (KIVA-CHEMKIN) performs reasonably well for predicting combustion, emissions, and flame structure for stratified diesel engine operation. In this publication, it is shown that the same model fails to predict combustion for partially premixed dual-fuel engines. In general, models that account for chemistry alone, greatly under-predict cylinder pressure. This is shown to be due to the inability of such models to simulate a propagating flame, which is the major source of heat release in partially premixed dual-fuel engines, under certain operating conditions. To extend the range of the existing model, a level-set-based, hybrid, auto-ignition/flame-propagation (KIVA-CHEMKIN-G) model is proposed, validated and applied for both stratified diesel engine and partially premixed dual-fuel engine operation.

In the Premixed-Charge Compression Ignition (PCCI) combustion mode, fuel is injected fairly early before top-dead-center (TDC) of compression compared to the conventional near-TDC injection combustion mode. Early fuel injection into a low temperature in-cylinder environment results in long ignition delay and high peak heat release rate. Since the onset of ignition occurs after the end of injection, importance of spray and bowl induced flow field and mixing is not so obvious. In the present work, computational analysis is used to investigate the effects of spray-bowl interactions on PCCI combustion and emissions at a light-load (4Bar BMEP) operation of a medium-duty, direct injection diesel engine. Multidimensional CFD code KIVA-3V coupled with detailed chemical kinetics is used to perform combustion simulations.

Present paper discusses the advantages and disadvantages of fuel injection rate shaping for a medium-duty diesel engine using computational analysis. The analysis was performed using three-dimensional (3D) Computational Fluid Dynamics (CFD) code KIVA-3V. Fuel injection rate-shape is parameterized and a Design of Experiments (DoE) is constructed. CFD simulations are performed at discrete DoE points to construct a statistical model, which is then used to predict the engine response for variation in injection shape parameters. Trends of NOx, soot, noise and Indicated Mean Effective Pressure (IMEP) are investigated to understand the impact and potential of injection rate-shaping on engine performance. It is found that slow increase in injection rate leads to reduction in NOx and IMEP, but has almost no effect on soot emissions. Effect of rate shaping during fall of injection rate does not show strong influence on emissions and performance.

The nitrogen dioxide (NO₂) emissions of compression ignition diesel engines are usually relatively small, especially when operated at medium and high loads. Recent experimental investigations have suggested that adding hydrogen (H₂) into the intake air of a diesel engine leads to a substantial increase in NO₂ emissions. The increase in NO₂ fraction in the total NOx is more pronounced at lower engine load than at medium- and high-load operation, especially when a small amount of H₂ is added. However, the chemistry causing the increased NO₂ formation in H₂-diesel dual-fuel engines has not been fully explored. In the present work, kinetics of NO and NO₂ formation in a H₂-diesel dual-fuel engine are investigated using a CFD model integrated with a reduced hydrocarbon oxidation chemistry and an oxides of nitrogen (NOx) formation mechanism. A low-load and a medium-load operating condition are selected for numerical simulations.

This paper presents the results from experiments and Computational Fluid Dynamics (CFD) simulations performed to understand the impact of piston geometry on ignition delay for Dimethyl Ether (DME)/air mixtures inside a Rapid Compression Machine (RCM). Three piston shapes and two dilution ratios are studied using CFD simulations validated by experiments. The three piston geometries under consideration are: a flat piston, a piston with an enlarged crevice, and a bowl piston. Key phenomena analyzed in the study include fluid flow patterns, heat transfer, temperature homogeneity of the mixture, and ignition delay. The CFD model provides reasonable predictions of ignition delay when compared with experimental data. Simulations indicate that flat and bowl pistons show similar heat transfer, ignition delay, and combustion characteristics, while the enlarged creviced piston shows lower peak temperatures and a cooler mixture core due to higher wall heat transfer.