Sooting propensity dependence on pressure of ethylbenzene, p-xylene, o-xylene and n-octane in laminar diffusion flames

Abstract

Alkylbenzenes constitute a significant portion of middle-distillate transportation fuels, and they are judged as the key contributors to the extent of soot production and consequent exhaust particulate emissions in combustion engines. Because of the uncertainties involved in engine experiments due to boundary condition variation among test platforms and the difficulties involved in controlling engine variables independently, benchmark information on pressure dependence of sooting propensities of alkylbenzenes is essential to understand their influence on particulate emissions and provide a more consistent interpretation of engine data. One of the approaches to acquire such benchmark data is to investigate laminar diffusion flames doped with alkylbenzenes under tractable conditions at elevated pressures. Here, we study the influence of doping a methane laminar diffusion flame with alkylbenzenes having eight carbon atoms, namely ethylbenzene, p-xylene, and o-xylene and compare their sooting characteristics to that of n-octane at pressures up to 10 bar. Liquid hydrocarbons added to methane replaced 3% of the carbon in the base fuel keeping the carbon mass flow in the fuel stream fixed, at all pressures considered, to have tractable measurements. The flames of base fuel methane and those doped with liquid C8 hydrocarbons were stabilized on a laminar co-flow diffusion burner positioned inside a combustion chamber capable of elevated pressures and has suitable optical access. Radially resolved temperatures and soot volume fractions were evaluated from spectral emission of soot radiation from the flames. Soot yields, inferred from the radial distributions of soot volume fractions, of ethylbenzene, p-xylene, and o-xylene doped flames were found to be a factor of about 2.6–3.2 higher than that of n-octane at 2 bar; however, this difference gradually disappeared with increasing pressure and soot yields were almost the same above 6 bar. It appears that pressure dependence of sooting propensities of ethylbenzene, p-xylene, and o-xylene is weaker than that of n-octane within the pressure range of 2 to 10 bar.