Determining fractal properties of soot aggregates and primary particle size distribution in counterflow flames up to 10 atm

Experimental investigations of soot morphology are performed in counterflow flames of N-diluted ethylene and air, up to 10 atm. A thermophoretic sampling device is attached to a pressure vessel containing a counterflow burner where flames with an ethylene mole fraction of 0.3 are stabilized at 3, 5, and 6 atm. To allow measurements at higher pressures, the fuel mole fraction is lowered to 0.2 to reduce the soot loading and flames are studied at 5, 7, and 10 atm. Thermophoretic sampling of the soot zone is performed using TEM grids. The sampling process causes minimal flame disturbances. Soot collected on TEM grids is analyzed under transmission electron microscope (TEM). Primary particle size distributions are inferred at each pressure by manually analyzing the primary particles from TEM images. Fractal properties of soot at each pressure are also obtained by analyzing the TEM images at comparatively low magnifications. Mean primary particle diameter increases from 17.5 to 47.1 nm as the pressure is increased from 3 to 10 atm, whereas the fractal dimension and prefactor do not change with pressure up to 10 atm. For the flames studied here, fractal dimension lies between 1.61 and 1.67 whereas fractal prefactor varies between 1.68 and 1.86 without following any apparent trend with pressure.