Reduced-order numerical model for transient reacting diesel sprays with detailed kinetics

Abstract

A reduced-order model of transient diesel spray combustion is presented that utilizes simplified fluid mechanics and detailed chemical kinetics, premised on the similarity between dense turbulent gaseous jets and diesel sprays at engine conditions. The presented model offers a new capability for detailed chemistry predictions in transient diesel sprays since the use of large chemical mechanisms is prohibitively expensive in more detailed modeling approaches such as multidimensional computational fluid dynamics. The numerical model is validated against Engine Combustion Network spray-H experimental data. Predictions of vapor penetration, axial mixture fraction distribution, ignition delay, axial location of cool-flame reaction, and end-of-injection combustion recession show excellent agreement with experimental measurements. The model is applied to study modern diesel injection strategies that involve significant transient mixing and combustion behavior, including fuel injection rate shaping and close-coupled split-injection strategies. In general, the model is shown to enable a detailed examination of modern diesel injection strategies and the expected impact of these strategies on emissions. A slow ramp down of fueling rate at the end of injection is found to limit over-mixing in the near field of the injector, enabling recession of second-stage ignition toward the injector after end of injection. This is advantageous for consumption of unburned hydrocarbons and improved combustion efficiency. Compared to slow ramp-down injection strategies, close-coupled split injections are less effective for unburned hydrocarbon reduction due to a strong end-of-injection entrainment wave that accompanies both injections, causing rapid over-leaning and no recession of second-stage ignition.