Dependence of sooting characteristics and temperature field of co-flow laminar pure and nitrogen-diluted ethylene–air diffusion flames on pressure

Pressure dependence of sooting characteristics and the flame temperature field of pure ethylene and ethylene diluted with nitrogen in co-flow laminar diffusion flames was investigated experimentally. The pressure range for ethylene was from atmospheric to 7 atm and to 20 atm for nitrogen-diluted ethylene flames. Spectrally-resolved line-of-sight soot radiation emission measurements were used to obtain radially resolved temperatures and soot volume fractions by using an Abel type inversion algorithm. A constant mass flow rate of ethylene was maintained at 0.48 mg/s at all pressures to match the carbon flow rates of gaseous alkane fuels experiments reported previously. Visible flame heights, as marked by the luminous soot radiation, initially increased with pressure, but changed little above 5 atm. Maximum local soot volume fraction of ethylene flames seems to scale with pressure raised to the third power (about 2.8). This is argued to be a relatively stronger pressure dependence of maximum soot volume fraction as compared to other gaseous fuels. A similarly higher pressure dependence was observed when the maximum soot yields of ethylene and other gaseous fuels were compared. It was shown that the soot yield dependence of ethylene flames does not conform to the unified dependence on pressure which was demonstrated for gaseous alkane fuels recently. The sooting propensity of nitrogen-diluted ethylene flames was shown to be less than that of n-heptane flames diluted with similar amount of nitrogen. Flame temperature profiles and averaged temperatures of ethylene flames showed similar characteristics as the other gaseous fuels, however radial temperature gradients in ethylene flames were much higher than those in gaseous alkane fuel flames.