Search Results

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.

Large-eddy simulations (LES) of a motoring single-cylinder engine with transparent combustion chamber (TCC-II) are carried out using a commercially available computer code, CONVERGE. Numerical predictions are compared with high-speed particle image velocimetry (PIV) measurements. Predictions of two spatial discretization schemes, namely, numerically stabilized central difference scheme (CDS) and fully upwind scheme are compared. Four different subgrid scale (SGS) models; a non-eddy viscosity dynamic structure turbulence (DST) model of Pomraning and Rutland, one-equation eddy-viscosity (1-Eqn) model of Menon et al., a zeroequation eddy-viscosity model of Vreman, and the zeroequation standard Smagorinsky model are employed on two different grid configurations. Additionally, simulations are also performed by deactivating the LES SGS models. It is found that the predictions when using the numerically stabilized CDS are significantly better than using the fully upwind scheme.

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.