Computational Studies of Soot Production for Varying Fuels, Pressures, and Flow Conditions

Whether desired or not, combustion processes will continue to be the largest energy source in the world for the foreseeable future as the slow transition to a low-carbon energy matrix occurs. During the combustion of hydrocarbons, a series of complex processes and mechanisms transforms heavy gaseous hydrocarbons into larger solid particles known as soot, which are linked to environmental and health threats. Three studies are presented to understand soot formation better, each tackling different combustion aspects. Firstly, forced laminar flames are used to study soot formation in a time-varying environment. A numerical procedure to solve the Navier-Stokes equations in a transient scheme was developed and implemented successfully, allowing matching the flow field results with experimental measurements on steady-state and forced laminar sooting ethylene flames. Comparing the simulated results with the experimental measurements on both steady and time-dependent cases showed good agreement in peak soot concentration, and the location and distribution of soot within the flame. Secondly, Dimethyl ether, a non-toxic and renewable fuel known for its soot emissions reduction tendencies, is studied to unravel the mechanisms behind a sudden reversal in soot concentration observed when a 2% increase in the oxygen content in the fuel side reduces soot by 60%. Numerical simulations were performed, showing that the chemical effects of adding oxygen to the fuel stream are exceedingly more important than the thermal and dilution effects. It was found that the reversal occurred when nearly all DME disassociated before exiting the fuel tube. Finally, flames formed in ammonia/ethylene blends are studied as a promising pathway to expedite its use in different industries, as it might offer a good compromise between energy output and minimizing the emissions of both NOx and carbonaceous combustion products, including soot. A high-pressure chamber was used to study the flames at pressures between 1 to 7 atm, and ammonia addition mole fractions between 0 and 50%. Numerical simulations were used to further characterize the ammonia soot suppression characteristics when increasing pressure. The results showcase the potential of NH3 in high-pressure applications when mixed with hydrocarbon fuels to reduce carbon emissions.