To obtain information about the morphology of soot, thermophoretic sampling of soot particles is conducted followed by the analysis of the images of the samples captured by a transmission electron microscope (TEM). The sampling technique used in the present work is based on earlier works by Köylü and coworkers. A copper electron microscopy grid, 3 mmin diameter, with standard square mesh and a mesh size of 200 acts as the cold surface. The grid is held by two 0.3 mm-thick stainless steel sheets. Figure shows a sketch of this grid holder. The holder is attached to a double acting pneumatic air cylinder with 1–1/16 in. bore diameter and one inch stroke. The cylinder is actuated by two high flow rate 3/2-way and normally closed solenoid valves, from Festo. The operating pressure is 80 psi. To synchronize the motion of the piston, the timer function of an Omron ZEN V2 programmable logic controller (PLC) is used. A sketch of the sampling setup is presented in Figure above. The ‘exposure time’, the time during which the TEM grid is exposed to the desired soot containing position in the flame, is chosen to keep the grid coverage below 15% to prevent overlapping of the soot aggregates. The grid travels through other soot containing regions and the high temperature soot oxidation region in the flame to reach the intended position. To minimize the unavoidable contamination and oxidation in those areas, the ‘travel time’ should be small. In this work, our goal is to keep the travel time smaller than 10% of the exposure time. The timing is monitored
by the laser-based arrangement shown in Figure above. Samples are characterized using a Hitachi H-700 TEM operating at an accelerating voltage of 100 kV. The images are analyzed using the ImageJ software, based on the method detailed in by koylu et al.
Thermophoretic Sampling, High Speed filming, (Video is provided by Dr. Greg Smallwood, NRC)
An illustration of the evolution of soot morphology in two flames. A Jet A1 surrogate on the left and a surrogate for GtL on the right. the effects of the presence of aromatics in the Jet A1 surrogate is clearly observed.