Exploring 3-D morphology and mechanical properties for the soot particles produced within a transient diesel reacting jet spray flame under diesel engine-like operating conditions by using atomic force microscopy (AFM)

Abstract

In this work, soot particles were directly sampled within a transient reacting jet spray flame under conditions close to practical diesel engine combustion based on the principle of the thermophoretic probe sampling. The experiments were performed in a constant volume combustion chamber (CVCC) with five ambient oxygen concentrations (O₂: 21 %, 18 %, 15 %, 12 %, 9 %), accompanied by the subsequent 3-D atomic force microscopy (AFM) imaging to examine not only to what extent the 3-D morphology of the soot particles samples but also their mechanical properties were affected by the variations in the oxygen concentration in the CVCC.

The results showed relatively larger particles were observed in the 3-D AFM images, indicating soot coagulation occurred at the very beginning of combustion process. Three different types of morphology were found for isolated particles samples. The equivalent diameter (ED) of the samples exhibited a broad distribution of about 2–100 nm. When the ambient oxygen concentration was reduced from 12 % to 9 %, similar distribution patterns of ED were found, especially over the range ED < 10 nm. The population-averaged ED decreased firstly and then increased, and was found to decrease again as the oxygen concentration was gradually lowered. The distribution of sphericity ratio for the particles samples fell within the range of 0–0.35, and very low sphericity ratio values (<0.1) were found for most of the isolated particles samples.

Three types of force curves were found for the particles samples. The attractive force fell within the range of 1.4–4.8 nN for all the cases studied. As the ambient oxygen concentration was lowered, the population-averaged attractive force decreased from 2.60 nN to 2.23 nN. The Van der Waals force accounted for over 65 % of the attractive force, and thus played a dominant role in the attractive force. The adhesive force mainly fell within the range of 10–24 nN. As the oxygen concentration was gradually lowered, the population-averaged adhesive force increased from 14.19 to 14.46 nN; the adhesive energy fell within the range of 0–5.1 × 10⁻¹⁶ J, and the population-averaged adhesive energy decreased initially from 1.92 × 10⁻¹⁶ J to 1.79 × 10⁻¹⁶ J, and then increased to 1.89 × 10⁻¹⁶ J. Especially, it was found that the population-averaged adhesive energy was four orders of magnitude higher than the thermal kinetic energy. The Young's modulus fell within the range of 15–520 MPa, while the population-averaged Young's modulus fell within the range of 205–235 MPa, and the population-averaged Young's modulus showed a completely opposite trend to that of the fringe separation distance as the oxygen concentration was lowered.