Experimental comparison of soot formation in turbulent flames of Diesel and surrogate Diesel fuels

Soot formation is compared in turbulent diffusion flames burning a commercial Diesel and two Diesel surrogates containing n-decane and α-methylnaphthalene. A burner equipped with a high-efficiency atomisation system has been specially designed and allows the stabilisation of liquid fuels flames with similar hydrodynamics conditions. The initial surrogate composition (70% n-decane, 30% α-methylnaphthalene) was previously used in the literature to simulate combustion in Diesel engines. In this work, a direct comparison of Diesel and surrogates soot tendencies is undertaken and relies on soot and fluorescent species mappings obtained respectively by Laser-Induced Incandescence (LII) at 1064 nm and Laser-Induced Fluorescence at 532 nm. LIF was assigned to soot precursors and mainly to high-number ring Polycyclic Aromatic Hydrocarbons (PAH). The initial surrogate was found to form 40% more soot than the tested Diesel. Consequently, a second surrogate containing a lower α-methylnaphthalene concentration (20%) has been formulated. That composition which presents a Threshold Soot Index (TSI) very close to Diesel one is also consistent with our Diesel composition that indicates a relatively low PAH content. The spatially resolved measurements of soot and fluorescent soot precursors are quite identical (in shape and intensity) in the Diesel and in the second surrogate flames. Furthermore the concordance of the LII temporal decays suggests that a similar growth of the primary soot particles has occurred for Diesel and surrogates. In addition, the comparison of the LII fluence curves indicates that physical/optical properties of soot contained in the different flames might be similar. The chemical composition present at the surface of soot particles collected in Diesel and surrogate flames has been obtained by laser-desorption ionisation time-of-flight mass spectrometry. An important difference is found between Diesel and surrogate samples indicating the influence of the fuel composition on soot content.