Soot formation in premixed C2H4-air flames for pressures up to 100 bar

Abstract

The influence of pressure on flat C₂H₄-air flames has been further investigated for pressures up to 70, resp. 100 bar by optical methods and by chemical analysis. In addition concentration profile computations have been performed for pressures up to 100 bar. For pressures above 10 bar the final soot volume fraction rises proportional with pressure f_v∞ ∼ P (T>1650 K). The same holds for the concentration of CH₄ and the much smaller concentration of C₂H₄. The final carbon density present as C₂H₂ decreases above 10 bar with increasing pressure. There is a change in mechanism of soot formation towards high pressures: The mass growth process ends because of a lack of substance which can be added readily. The final particle number density increases above the value obtained at normal pressures and becomes proportional to f_v∞ towards high pressure. Profile calculations show that the maximum C₂H₂ concentration in the reaction zone grows as p^1.3 and the higher PAH grow even more strongly with pressure. This is an indication that the formation of soot precursors can lead to high particle number densities and correspondingly a very high reactivity of the young soot which then causes this change in mechanism.