Experimental and numerical study of soot formation in laminar ethylene diffusion flames at elevated pressures from 10 to 35 atm

The effects of pressure on soot formation and flame structure were studied experimentally and numerically in coflow ethylene–air laminar diffusion flames between 10 and 35 atm. Reliable measurements up to 35 atm were permitted by diluting the fuel with nitrogen and suppressing smoke formation. These measurements were compared with numerical predictions to assess the accuracy of current soot models applied to high pressure diffusion flames. The numerical framework used in the present work represents the current state of the art in computational modelling, making use of a block-based parallel implicit finite-volume scheme and detailed radiation heat transfer. In the current implementation, a semi-empirical acetylene-based model is used to predict the nucleation, growth, and oxidation of soot particles. Although the soot model is based on experimental data at atmospheric pressure, it correctly predicted many of the observed trends with pressure. A narrowing flame with constant height was observed as pressure was increased in both the experiments and numerical results. The model also captured the observed relationship between the maximum amount of carbon converted to soot and pressure, although soot volume fractions were generally over-predicted everywhere in the flames. In both the experiments and predictions, soot volume fractions increased with pressure while the tendency of the fuel to produce more soot declined as pressure was increased. Interestingly, the calculations predicted soot inside the fuel tube at 20 atm whose concentrations increased with pressure. An analysis of the numerical results concluded that this early appearance of soot was attributed to fuel pyrolysis inside the tube which accelerated as pressure was increased from 10 to 35 atm.