Experimental Investigations on Ethylene Laminar Flames at Elevated Pressures

Hydrocarbon species concentrations and flame temperatures are measured in a laminar jet diffusion flame at elevated pressures, the objective is to better understand soot production and oxidation which will ultimately help reduce soot emissions from engines. Quantification of hydrocarbon species was done by extraction of samples using a quartz micro probe along the centerline of the flame and analyzing them using both a GC-MS/FID and a TCD to calculate the mole fraction of the various species in the sample volume. The flame temperature was measured by both thermocouples and two color pyrometry. The pressure effects on the measured quantities are investigated. The presence of benzene and acetylene suggests soot formation and growth, and two trends were observed with respect to hydrocarbon species concentrations. The first trend shows an increase in peak concentrations of stable hydrocarbon species, specifically benzene and acetylene, with increase in pressure. The second trend is the migration of the occurrence of the peak concentration towards flame base as the pressure increased. With respect to temperature an increase in peak flame temperature along with reduction in the reaction zone thickness was observed as the pressures increased. Physical and chemical effects of addition of diluents to the fuel stream (He, CO2, N2 and Ar) are also investigated. The measured hydrocarbon species concentrations are dramatically different among the diluted flames with the helium and carbon dioxide diluted flame yielding the largest and smallest amounts of soot precursors, respectively. The peak flame temperatures varied with diluents tested, with helium diluted flame being the hottest and carbon dioxide diluted flame the coolest. Soot surface temperatures and soot volume fractions were investigated using two color pyrometry. Addition of the diluents to the fuel stream had a pronounced effect on both the soot surface temperatures and soot volume fractions, with helium diluted flame and carbon dioxide flame yielding the maximum and minimum soot surface temperatures and soot volume fractions respectively. At low pressures peak soot volume fractions exist at the tip of the flame, with increase in pressure the location of peak soot volume fractions shift to the center of flame wings.