Collaborative Research: Molecular Growth in Forced, Time-varying Flames

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Summary This is a coordinated, comprehensive study of molecular growth chemistry in forced, time-varying flames involving groups at Yale and George Washington University. Major species concentrations, small radical concentrations, velocity, and temperature are determined using a suite of laser-based techniques. Larger species are sampled using a novel pulsed microprobe coupled to a mass spectrometer. The experimental work is complemented by time-dependent computations of flame structure that include molecular growth chemistry. Three time-varying, non-premixed flames are studied: a non-luminous, nitrogen-diluted methane flame, a40% ethylene-60% nitrogen flame that is slightly luminous, and a 60% ethylene-40% nitrogen flame that is more luminous. The first flame will provide a base case and demonstration environment for the approach. The steady "40/60" flame has been observed to have peak soot loadings along the centerline while the "60/40" flame shows highest soot levels near the edges, indicative of the subtle interplay between molecular growth chemistry and the time-temperature history experienced by a packet of fluid. The goal is to provide experimental data for a rigorous test of competing soot growth models, and also to guide the development of intuitive understanding of the process. Time resolved measurements of the concentrations are made for a host of intermediate hydrocarbons formed from fuel pyrolysis and molecular growth chemistry; these four-dimensional data are combined with high-fidelity temperature and velocity fields to map experimentally the entire particle-inception process; and molecular growth chemistry is simulated numerically using three of the most evolved soot-formation chemistry models available.Broader impactsCombustion-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 standpoint of an engine designer, 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 will 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 the pressure. If soot and the problems associated with it are to be controlled, quantitative understanding of the soot growth and oxidation mechanisms in the form of computational models supported by experimental measurements is essential. An essential component of the proposed work will be education of undergraduate and graduate students in chemistry and mechanical engineering. Also, on-going activities in K-12 education will continue and be expanded through the Research Experience for Teachers (RET) program.