Analysis of high-pressure Diesel fuel injection processes using LES with real-fluid thermodynamics and transport

Imaging has long shown that under some high-pressure conditions, the presence of discrete two-phase flow processes becomes diminished. Instead, liquid injection processes transition from classical sprays to dense-fluid jets with no drops present. When and how this transition occurs, however, was not well understood until recently. In this paper, we summarized a new theoretical description that quantifies the effects of real fluid thermodynamics on liquid fuel injection processes as a function of pressure at typical Diesel engine operating conditions. We then apply the Large Eddy Simulation (LES) technique coupled with real-fluid thermodynamics and transport to analyze the flow at conditions when cylinder pressures exceed the thermodynamic critical pressure of the injected fuel. To facilitate the analysis, we use the experimental data posted as part of the Engine Combustion Network (see www.sandia.gov/ECN); namely the "Spray-A" case. Calculations are performed by rigorously treating the experimental operating conditions. Numerical results are in good agreement with available experimental measurements. The high-fidelity simulation is then used to analyze the details of transient mixing and understand the processes leading to auto-ignition. The analysis reveals the profound effect of supercritical fluid phenomena on the instantaneous three-dimensional mixing processes. The large density ratio between the supercritical fuel and the ambient gas leads to significant penetration of the jet with enhanced turbulent mixing at the tip and strong entrainment effects. Using detailed chemistry, a map of the auto-ignition delay time was calculated in simulation results. This map shows that a large flammable region with low velocity and mixture gradients is generated 250 diameters downstream of the injector. In the experiment, the first ignition site is observed at this location. This correspondence seems to indicate that the ignition location is piloted by the efficient mixing operating at the extremity of the jet coupled with long residence times, low strain rates and low scalar gradients. Published by Elsevier Inc. on behalf of The Combustion Institute.