The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN “Spray A” configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (fv), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered

The processes of ignition and formation of soot precursor and soot particles in a diesel spray flame achieved in a rapid compression machine (RCM) were imaged two-dimensionally using the laser sheet techniques.For the two-dimensional imaging of time and of location where ignition first occurs in a diesel spray, planar laser-induced fluorescence (PLIF) of formaldehyde was applied to a diesel spray in an RCM. Formaldehyde has been hypothesized to be one of the stable intermediate species marking the start of oxidation reactions in a transient spray under compression ignition conditions. In this study, the laser-induced fluorescence (LIF) images of the formaldehyde formed in a diesel fuel spray during the ignition process have been obtained by exciting formaldehyde with the third harmonic of a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. The LIF images of formaldehyde in a spray revealed that the time when the first fluorescence is detected is almost identical with the time when the total heat release due to low-temperature oxidation reactions equals the heat absorption by fuel vaporization in the spray. The formaldehyde level rose steadily until the high-temperature reaction phase of diesel spray ignition. At the start of this 'hot-ignition' phase, the formaldehyde concentration fell rapidly, thus signalling the end of the low-temperature ignition phase. Increases in the initial ambient gas temperatures advanced the hot-ignition starting time. The first hot ignition occurred in the periphery of spray head at initial ambient gas temperatures between 580 and 660 K. When the ambient gas temperature was increased to 790 K, the position of the first ignition moved to the central region of the spray head.For the investigation of soot formation processes in a diesel spray flame, simultaneous imaging of the soot precursor and soot particles in a transient spray flame in an RCM was conducted by PLIF and by planar laser-induced incandescence (PLII) techniques. The third harmonic (355 nm) and the fundamental (1064 nm) laser pulses from an Nd:YAG laser, between which a delay of 44 ns was imposed by 13.3 m of optical path difference, were used to excite LIF from the soot precursor and laser-induced incandescence (LII) from soot particles in the spray flame. The LIF and the LII were separately imaged by two image-intensified charge-coupled device cameras with identical detection wavelengths of 400 nm and bandwidths of 80 nm. The LIF from the soot precursor was mainly located in the central region of the spray flame between 40 and 55 mm (between 270 and 370 times the nozzle orifice diameter d o ) from the nozzle orifice. The LII from soot particles was observed to surround the soot precursor LIF region and to extend downstream. The first appearance of the LIF from the soot precursor in the spray flame preceded the appearance of the LII from soot particles. The intensity of the LIF from the soot precursor reached its maximum immediately after rich premixed combustion. In contrast, the intensity of the LII from soot par...

No abstract

The major challenge of the post-processing of soot aggregates in transmission electron microscope (TEM) images is the detection of soot primary particles that have no clear boundaries, vary in size within the fractal aggregates, and often overlap with each other. In this study, we propose an automated detection code for primary particles implementing the Canny Edge Detection (CED) and Circular Hough Transform (CHT) on pre-processed TEM images for particle edge enhancement using unsharp filtering as well as image inversion and self-subtraction. The particle detection code is tested for soot TEM images obtained at various ambient and injection conditions, and from five different combustion facilities including three constant-volume combustion chambers and two diesel engines. Through a comparison between automatically detected and manually selected primary particles from extensive datasets, five key image-processing parameters of the self-subtraction level, negative Laplacian shape parameter, maximum and minimum diameter of primary particles, and CHT sensitivity are optimised. From the analysis of the size distribution and mean diameter of primary particles, it is found that the automatic method is much more dependent upon the minimum primary particle diameter and CHT sensitivity than the other three parameters. With the optimised set values, the new particle detection code shows a good agreement with the results from the manual method.

For a better understanding of soot formation and oxidation processes in conventional diesel and biodiesel spray flames, the morphology, microstructure and sizes of soot particles directly sampled in spray flames fuelled with US#2 diesel and soy-methyl ester were investigated using transmission electron microscopy (TEM). The soot samples were taken at 50mm from the injector nozzle, which corresponds to the peak soot location in the spray flames. The spray flames were generated in a constant-volume combustion chamber under a diesel-like high pressure and high temperature condition (6.7MPa, 1000K). Direct sampling permits a more direct assessment of soot as it is formed and oxidized in the flame, as opposed to exhaust PM measurements. Density of sampled soot particles, diameter of primary particles, size (gyration radius) and compactness (fractal dimension) of soot aggregates were analyzed and compared. No analysis of the soot micro-structure was made. The overall morphology of the biodiesel soot bears similarity to that of #2 diesel, but the soot density, primary particle size, and fractal dimension are smaller for biodiesel.