Morphology of soot sampled from N2-diluted methane/air counterflow flames at elevated pressures via TEM imaging

An experimental work is carried out to investigate the influence of pressure on morphological parameters of soot in counterflow flames of N-2-diluted methane and air. Flames are stabilized at 3, 5, 7 and 10 atm in a pressure vessel and a global strain rate of 30 s(-1) is maintained at all pressures by adjusting the inlet mass flux. The mole fraction of methane is maintained at 0.7. The entire soot zone of the counterflow flames are sampled using a thermophoretic sampling device attached to the pressure vessel. Our sampling method minimizes the flow disturbances to a level that they are visually negligible during the sampling process. Collected samples are analyzed under a transmission electron microscope and information about mean primary particle diameter, fractal dimension (D-f), fractal prefactor (k(f)) and aggregate size distribution is inferred at different pressures. To investigate the effects of carbon flux on primary particle size, fuel mole fraction is decreased to 0.5 and primary particle size is investigated at 5, 7 and 10 atm. Mean primary particle size increases by 70% when pressure is changed from 5 to 10 atm and remains independent of fuel mole fractions. Geometric mean and geometric width of aggregate size distributions also increase by increasing the pressure. Fractal properties of soot aggregates are found to be insensitive to the pressure. Fractal dimension varies between 1.56 and 1.65 while fractal prefactor values range between 1.96 and 2.1. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.