Self-Ignition and Early Combustion Process of n-Heptane Sprays Under Diluted Air Conditions: Numerical Studies Based on Detailed Chemistry

This paper reports the numerical studies of self-ignition and early combustion process of n-heptane sprays under various diluted air conditions. The numerical simulations employ a detailed chemistry approach, coupled directly with the computational fluid dynamics (CFD). A “subgrid” Partially Stirred Reactor (PaSR) model has been developed to account for the turbulence-chemistry-interaction. This model has been implemented into the KIVA3V CFD code. A detailed chemical mechanism of reduced size (65 species and 273 elementary reactions) for the n-heptane fuel has been derived and applied to the simulations of spray combustion. The studies focus on sprays injected into a high-pressure constant-volume chamber. Firstly, the validation of the current numerical model has been carried out for the case in which the injection and initial conditions are similar to those used in the “classical” Aachen experiments (50bar and 800K). The computed self-ignition and combustion patterns are compared with the measured data. Secondly, a high-speed injection mode of sprays is applied to simulate the liftoff of spray combustion. Finally, a parametric study of self-ignition and early combustion of sprays is carried out for various diluted air conditions. The air in the high-pressure chamber is assumed to be initially diluted by homogeneously mixed with nitrogen (N₂), steam (H₂O), hydrogen peroxide (H₂O₂), carbon monoxide (CO), carbon dioxide (CO₂), or methane (CH₄). The simulations illustrate that the addition of N₂, H₂O, CO or CO₂ to the air increases the ignition delay times of n-heptane sprays but a small amount of H₂O₂ or CH₄ additives works in an opposite way. The numerical studies have shown that the current numerical approach can reproduce qualitatively many expected phenomena of spray combustion.