Numerical simulation aided relative optical density analysis of TEM images for soot morphology determination

The relative optical density (ROD) method provides a means to measure three-dimensional information about soot aggregates from two-dimensional transmission electron microscopy (TEM) micrographs of soot. The method is dependent on accurate calibration of the relationship between the measured soot ROD in TEM images and the actual soot thickness perpendicular to the imaging plane. A novel calibration method based on the comparison between probability distributions of measured soot ROD in TEM images and that of virtual soot thickness of numerically simulated soot is introduced. Soot aggregates of various prescribed fractal structure parameters were numerically generated using a tunable cluster-cluster aggregation model. The probability histograms of the local soot thickness for the simulated soot aggregates and ROD of the TEM images of flame generated soot aggregates were found to be quite similar and were used as a basis to establish a quantitative relationship between ROD and the local soot thickness. The calibration constant obtained from the analysis of the simulated soot was found to be insensitive to the fractal structure parameters over a wide range. The calibrated ROD method is successfully applied to the morphology analysis of soot aggregates generated in an atmospheric laminar co-flow ethylene-air diffusion flame based on thermophoretic sampling (TS) and TEM analysis techniques. With the ROD method, an overlap coefficient is introduced to identify and eliminate non-soot-aggregate structures and the selection of a cut-off overlap coefficient was found to have little influence on the final results over a relatively wide range. ROD is independent of empirical constants and human judgments and has been found to be an accurate and reliable TEM image analysis method for studying the morphology of soot aggregates.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

Three-dimensional mid-infrared photonic circuits in chalcogenide glass

We report the fabrication of single-mode buried channel waveguides for the whole mid-IR transparency range of chalcogenide sulphide glasses (λ ≤ 11 μm), by means of direct laser writing. We have explored the potential of this technology by fabricating a prototype three-dimensional three-beam combiner for future application in stellar interferometry that delivers a monochromatic interference visibility of 99.89% at 10.6 μm and an ultrahigh bandwidth (3-11 μm) interference visibility of 21.3%. These results demonstrate that it is possible to harness the whole transparency range offered by chalcogenide glasses on a single on-chip instrument by means of direct laser writing, a finding that may be of key significance in future technologies such as astrophotonics and biochemical sensing.