Soot formation in counterflow non-premixed ethylene flames at elevated pressures

Quantitative soot volume fraction measurements were conducted in a counterflow non-premixed flame configuration using ethylene/nitrogen as the fuel stream, oxygen/nitrogen as the oxidizer stream, and a pressure range of 1-8 atm. The laser-induced incandescence technique, calibrated using the light extinction method, was used to measure the soot volume fraction distributions. The variations of soot formation along the centerline of the counterflow flame with pressure were compared by keeping the density-weighted strain rate constant. Maintaining a constant density-weighted strain rate allows the overall flame thickness, as well as the reactant mass fluxes entering the flame, to remain unchanged for all pressures. As such, the effect of pressure on soot chemistry can be isolated from the effect of convective-diffusive transport. Based on the measured soot volume profiles, the soot layer thickness variation with pressure was determined. It was found that when keeping the density-weighted strain rate constant, the soot layer thickness remains similar over the pressure range investigated. However, the soot layer thickness was seen to decrease with increasing pressure when holding the strain rate fixed. In addition, the effects of fuel mole fraction and oxygen mole fraction on soot formation were investigated. Furthermore, the pressure scaling factors of soot formation under varying mixture conditions were deduced from experimental measurements. A literature gas-phase reaction mechanism including polycyclic aromatic hydrocarbon (PAH) chemistry up to pyrene was also used to simulate the experimental counterflow flames. The pressure effect on PAH formation was presented and discussed.