Combustion-generated soot particulates from land-based sources are now acknowledged to pose a significant health risk and have been the subject of stringent new EPA regulations. From the engine designer's standpoint, the concern about soot goes beyond these regulatory issues. High soot concentrations in combustor primary zones contribute to high thermal radiation loads on combustor liners. Soot coatings on liner surfaces drastically increase underlying metal temperatures. These issues are exacerbated by the high pressures at which new combustors are operated, because soot production is very sensitive to pressure. If soot and the problems associated with it are to be controlled, quantitative understanding of the soot growth and oxidation mechanisms is essential. In this collaborative research award, groups at Yale and George Washington University are studying molecular-growth chemistry in forced, time-varying flames. Major species concentrations, small radical concentrations, velocity, and temperature are determined using a suite of laser-based techniques. Both major and trace hydrocarbon species are sampled using a novel pulsed microprobe coupled to a mass spectrometer. They are also performing new optical diagnostics to quantify the concentration of acetylene in regions inaccessible to probe sampling and to follow PAH molecular growth and aggregation. The experimental work will be complemented by time-dependent computations of flame structure that include molecular growth chemistry. The proposed work will provide critical insight into the subtle interplay between molecular growth chemistry and the time-temperature history experienced by a packet of fluid. The goal of the work is not only to obtain experimental data for a rigorous test of competing soot growth models, but also to guide the development of our intuitive understanding of the process. Within the team, each PI leads specific project areas: Marshall Long coordinates the optical diagnostics activities, J. Houston Miller supervises the extractive sampling and line-of-sight optical diagnostic activities, and Mitchell Smooke leads the computational efforts. Data "mining" and interpretation proceed collaboratively. An essential component of the proposed work is education of undergraduate and graduate students in chemistry and mechanical engineering. Results of this work will feed into educational outreach activities in their communities, including (1) the six-week Frontiers of Science program, designed to expose high school juniors and seniors to leading-edge scientific research, (2) the Yale-New Haven Futures in Science Research Fellowship, which supports a summer research collaboration between a New Haven high school student and a Yale graduate student or post-doc supervised by a Yale faculty member, and (3) the Piedmont Environment and Education Foundation, a newly-chartered nonprofit incorporated in the State of Maryland, established to promote environmental science educational programs for middle and secondary schools in the District of Columbia and suburban Maryland.
M.D. Smooke and M. B. Long Yale University Practical combustion devices are often both time-dependent and sooting. A step toward the modeling of such devices is the simulation of systems that incorporate both particulate formation and time dependence. To this end, a number of investigators have studied forced, time-varying, laminar diffusion flames. These investigations have been either experimental or computational or they have combined both experimental and numerical techniques in their approach. The present research, funded in part by the NSF, was focused at first on a combined numerical/experimental simulation of a periodically-forced, sooting, laminar, coflow diffusion flame with ethylene as the fuel. The periodic forcing involved a sinusoidal perturbation of the fuel tube velocity. Key processes involved in soot formation (inception, surface growth, and oxidation) are heavily dependent on the residence time of a soot particle within the flame, specifically within regions of high temperature and high mixture fraction. Therefore, it is expected that an oscillating fluid field will significantly affect soot formation in the ethylene/air diffusion flame. Although forced flames represent an important intermediate between laminar and fully turbulent systems, practical combustion systems generally burn complex fuels that are not well understood. As the simplified systems are becoming better understood, research applications are moving towards more complex configurations by increasing the flow complexity and the details of the chemical model. The final portion of this study (which was part of a larger investigation funded in part by the NSF and the AFOSR) focused on adapting computational and experimental methodologies to study the larger molecules that can be found in realistic combustion systems. Existing chemical mechanisms had to be expanded to include a larger set of molecules so as to provide suitable agreement with experimental efforts and new experimental approaches had to be developed to quantify the larger molecules that were present in these systems.
