Effect of ethanol addition on soot formation in laminar methane diffusion flames at pressures above atmospheric

Influence of ethanol addition on soot production in laminar diffusion flames of methane under elevated pressures was investigated experimentally. A high pressure vessel, equipped with a co-flow laminar diffusion flame burner having a 3 mm fuel nozzle diameter, was used for the soot experiments. The amount of ethanol in methane was 10% based on the total carbon of the fuel stream. Pressure range was from atmospheric to 6 bar. To have measurements to be comparable for the purpose of assessing the pressure dependence, the carbon mass flow rate of the ethanol and methane mixture was kept constant at 0.941 mg/s. Luminescent heights of the flames studied did not vary much with pressure excluding the heights of those at atmospheric pressure. Line-of-sight measurements of soot spectral emission were inverted by an Abel type algorithm, assuming axisymmetric flames, to evaluate variations of radial profiles of temperatures, soot concentrations, and soot yields of neat methane and ethanol-doped methane with pressure. Ethanol-doped methane flames displayed higher soot concentrations than those of neat methane flames at all pressures considered in the study; however, pressure dependence of maximum soot volume fraction is almost the same for both neat methane and ethanol-doped methane. The results showed that the maximum soot volume fractions scale with pressure as Pn, where n decreases from about 2.5 to 1.8, from atmospheric to 6 bar. These exponents are similar to measurements reported previously in the literature for gaseous paraffinic hydrocarbons. Maximum soot yields, on the other hand, are almost the same for both flames at 2 bar, but at 4 and 6 bar pressures ethanol-doped methane flames give higher maximum soot yields than neat methane flames.