Ethanol supplement increases soot yields in nitrogen-diluted laminar ethylene diffusion flames at pressures from 3 to 5 bar

In spite of widely use of ethanol, mostly as a fuel extender in ground transportation engines, its sooting propensity with pressure and with ethanol content in the base fuel has not been clarified. Although, information on ethanol’s sooting and other combustion characteristics at atmospheric conditions is extensive, scaling this information to elevated pressures is problematic. Information that could be gathered from tractable laminar diffusion flames on the soot formation of ethanol’s response to pressure would be beneficial in assessing the soot emissions from engines fuelled with hydrocarbons supplemented with ethanol. To address this lack of information at pressures above atmospheric, high pressure soot formation in laminar co-flow diffusion flames fuelled by ethanol was investigated using nitrogen-diluted ethylene as the base fuel on a burner of 3 mm diameter fuel nozzle. Base fuel nitrogen to ethylene mass ratio was kept fixed at 6 for all experimental cases. In terms of total carbon mass flow, which was kept constant at 0.41 mg/s, ethanol’s contribution was varied as 0%, 10%, 30% and 40%, and the experiments were conducted at 3, 4, and 5 bar pressures. The main aim of the investigation was to evaluate the soot production at elevated pressures with increasing ethanol fraction in the fuel stream treating nitrogen-diluted ethylene as the other fuel component. Flame heights defined by the luminous visible tips did not display any significant changes as the ethanol fraction or pressure was changed. Spectrally-resolved line-of-sight soot radiation collected along the radial distance from the flame centreline at various heights above the burner exit with 1 mm increments was converted to radial soot temperature and volume fraction distributions through an Abel-type inversion algorithm. Within the bounds of pressure considered, soot production increased with the amount of ethanol added to the fuel stream. Largest incremental increase in soot production over ethylene diluted with nitrogen was observed at the condition with 10% carbon from ethanol in the fuel blend; further increases in ethanol fractions kept increasing the soot but at relatively lower rates. This non-linear effect with ethanol addition was discussed and it was argued that one of the reasons for this behavior is the influence of methyl radical produced by radical-induced decomposition of ethanol enhancing soot production for 10% ethanol case. With further increases in ethanol amount, this effect seems to slow down because of reduction in ethylene fraction and the increase in hydrogen to carbon ratio in the fuel stream at fixed carbon flow rate.