Experimental and numerical study of soot formation in hydrocarbon sprays under high-pressure fuel pyrolysis conditions

Abstract

This study combined high-speed optical diagnostics and numerical simulation to investigate soot formation in n-dodecane sprays under conditions characterized by fuel pyrolysis and low oxygen concentrations. Numerical models were employed to predict the evolution of polycyclic aromatic hydrocarbons (PAHs), while the experiments focused on soot formation. A 186-µm single-hole orifice nominal diameter injector was employed to inject well-controlled fuel sprays into a constant-volume chamber operating at 76 bar. We use a short injection duration of approximately 100 µs to maximize the residence time of the fuel, with variations in the ambient gas temperature within the range of 1,400 to 1,700 K, and the oxygen concentration was ranged from 0 to 5 %. Additionally, we conducted closed-homogeneous-reactor and two-stage Lagrangian simulations with various kinetic mechanisms to predict PAH formation and compared the results with experimental data. The experimental results revealed that variations in the ambient temperature and oxygen percentage significantly influenced the pyrolysis and oxidation processes. Soot onset occurred at 1,450 K for oxygen levels of 0, 1, and 3 %, whereas at 5 % oxygen, soot formed at temperatures below 1,400 K. Interestingly, higher oxygen concentrations increased the rates of soot formation at all temperatures tested. By contrast, elevated temperatures reduce the total soot mass owing to enhanced oxidation. The present study also evaluates the influence of fuel composition on soot formation and observes that a higher aromatics content in the fuel leads to a lower soot onset temperature and increased soot mass. Notably, similar trends for both ethanol and n-dodecane fuels are identified in this study. Furthermore, the numerical calculations revealed distinct trends in PAH formation. Although the different mechanisms reasonably captured the trends in benzene formation, they differed in their predictions of the formation rate of pyrene, resulting in potential differences in soot processes. This disparity highlights the need for a comprehensive review and potential modification of the current soot modeling approach.