Effects of the Gas Preheat Temperature and Nitrogen Dilution on Soot Formation in Co-flow Methane, Ethane, and Propane Diffusion Flames

This work investigates the effects of gas preheating and N-2 dilution on soot formation in a series of laminar methane (CH4), ethane (C2H6), and propane (C3H8) co-flow diffusion flames. A modified Santoro-type burner was used to produce the flames with a series of preheat temperatures of both co-flow air and inlet fuels between 293 and 723 K. Planar laser-induced incandescence and line-of-sight beam extinction were employed to quantitatively measure the planar distributions of soot volume fractions, and two-color pyrometry was used to measure the soot temperature. The results show that the soot volume fraction significantly increases with the gas preheat temperature for all flames. A rise in the preheat temperature accelerates soot evolution and leads to the higher percentage of carbon conversion to soot in the flames. This enhancement is more remarkable in the flames of fuel with a lower sooting propensity than those with a high sooting propensity. Both axial and radial soot temperatures in the CH4 and C2H6 flames increase with the gas preheat temperature significantly. However, a reverse changing trend is observed in the central region of C3H8 flames, owing to the greater heat losses resulting from the incomplete combustion and strong soot radiation. Adding N-2 to the fuel flow suppresses soot formation via the direct dilution effect on the fuel concentration and the indirect effect of changing the flame temperature. As the gas preheat temperature increases, an increasing proportion of the contribution of the direct dilution effect is observed in the N-2-diluted C2H6 flames. In addition, the global activation energy of soot formation for the three fuels is estimated on the basis of the measured soot temperature and soot formation rate in the present work.