| Soot processes within flames fueled with hydrocarbons are important because they affect the durability and performance of propulsion systems, the danger associated with unwanted fires, the pollutant and particulate emissions of combustion processes, local and global environments and the potential for developing computational combustion. The objective of the current work was to improve understanding of the processes controlling soot formation in combustion systems over a broad range of pressure conditions. The flame and soot structure, including primary soot particle nucleation, soot particle surface growth and oxidation properties, of round laminar diffusion flames were studied experimentally at pressures of 0.1-8.0 atm. Acetylene-nitrogen mixtures were used at pressures from 0.1 to 1.0 atm (in air coflow). Ethylene-helium mixtures were used at pressures from 1.0 to 8.0 atm (in oxygen/helium coflow). Soot concentrations, soot temperatures, soot structure, concentrations of major stable gas species, concentrations of radical species (H, OH, O), and flow velocities were measured along the axis of each flame studied.; The data were analyzed to determine local soot surface growth, oxidation and nucleation rates, as well as local flame properties that are thought to affect these rates. The measurements of soot surface growth rates were consistent with earlier measurements in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure. In addition, the growth rates from all the available flames were in good agreement with each other and with existing hydrogen-abstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature.; Measurements of soot surface oxidation rates were consistent with earlier measurements, and the oxidation rates from all available flame data could be explained by reaction with OH; supplemented to only a minor degree by direct soot surface oxidation by O2.; A simplified method to estimate the soot nucleation rate was developed. The results indicated the nucleation rate was independent of pressure. In general, pressure effects on soot formation, growth and oxidation processes were well captured by existing theory over the broad range of conditions considered. |
