Large eddy simulation of n-dodecane spray flame: Effects of injection pressure on spray combustion characteristics at low ambient temperature

Abstract

The present study uses large-eddy simulations (LES) to identify the underlying mechanism that governs the ignition and stabilization mechanisms of ECN Spray A flame for different injection pressures (Pinj) and ambient temperatures (Tam). Two Pinj of 50 MPa and 150 MPa, as well as two Tam of 750 K and 900 K are considered. The numerical model is validated against the experimental fuel penetration, radial mixture fraction distribution, ignition delay time, and lift-off length. The combustion characteristics of all four spray flames are well predicted, with a maximum relative difference of 15% to the measurements. At 900 K, high-temperature ignition (HTI) occurs in the fuel-rich mixture at the spray head of the high Pinj spray flame, but at the spray periphery of the low Pinj spray flame. This is due to the low Pinj case having fuel-richer mixture in the inner spray region. Nonetheless, the spray flames at both Pinj exhibit double-flame structure. At 750 K, HTI occurs at the fuel-rich and fuel-lean regions for spray flames with Pinj=50 MPa and 150 MPa, respectively. Reducing the Pinj leads to a lower injection velocity, less turbulent fluctuation, slower mixing, and hence the occurrence of HTI at the fuel-rich mixtures. The spray flame in the low Pinj case at 750 K exhibits a triple-flame structure at the lift-off position, while the high Pinj case exhibits a lean premixed reaction zone. This difference is attributed to the distribution of fuel-rich mixtures. Despite differences in the flame structures, auto-ignition process plays a key role to stabilize the lift-off position for all four spray flames. The auto-ignition process is also found to be dependent on the cool-flame products upstream of the lift-off position. In particular for the low Tam cases, the heat transfer effect from the main flame to the fuel-rich regions is suggested to also contribute to the flame stabilization mechanisms.