Improvements in multi-dimensional modeling of unsteady turbulent diesel sprays and engine combustion

Abani, N. Improvements in Multi-Dimensional Modeling of Unsteady Turbulent Diesel Sprays and Engine Combustion. University of Wisconsin-Madison, 2009.

This research is focused on an investigation of spray models used in multidimensional numerical simulations. Spray model predictions from CFD codes such as KIVA-3V are found to be sensitive to the computational mesh size and numerical time steps that are modeled using the Lagrangian-Drop and Eulerian-Fluid. To reduce the mesh size sensitivity, the Gas Jet model that is based on the assumption that the component of the gas velocity in the spray direction can be imposed using gas jet theory was formulated and tested with the standard KIVA methodology. The Gas Jet model predicts more accurately the relative velocity between the droplets and gas and also gives improved predictions of spray tip-penetration, drop size, number of spray drop parcels and spray structure at a wide range of mesh sizes (1 to 16 times the nozzle diameter).

To improve the prediction of vaporizing sprays, a new Vapor Particle Method (VPM) was formulated using Lagrangian approach. Vapor is transported as particles until a distance from the nozzle where the spray is resolved by the local mesh scale. At this distance the vapor in the particle is released to the computational cell to be solved following the standard Eulerian approach.

Steady turbulent round jet theory was also extended to unsteady turbulent jet theory based on the Helmholtz vortex theory. This theory accounts for the fact that any change in injection conditions takes time to affect a fluid particle in the jet. A Duhamel superposition integral is used to determine the effective injection velocity for time-varying injection velocity cases.

A Schlieren system was also setup to experimentally investigate unsteady turbulent jets (hole diameter 0.5 mm and 1 mm, respectively) using helium as the injection fluid. The jet tip penetration predicted from the model compares very well with the Schlieren experiments.

Finally, the above improved spray models were applied to investigate four engine cases for a medium and a heavy duty diesel engine at different operating conditions. It was found that both the Gas Jet and VPM spray models can be used to improve the accuracy of spray, jet and engine combustion predictions with coarse computational mesh sizes.