Soot inception in highly controlled counterflow flames at pressures up to 4MPa

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The emission of soot from combustion processes is a problem that has been plaguing the world for centuries, with repercussions on health and climate that have been fully appreciated only in recent decades. The formation of soot remains a thorny challenge in combustion research because of difficulties in following the critical growth process steps from fuel to soot nuclei. To understand the chemical pathway to soot, it is necessary to sample the gases in a flame in order to conduct chemical analyses. However, inserting a probe into the flame creates a perturbation, and its impact on the resulting analyses needs to be understood. The majority of combustion applications, such as in engines, occur at high pressures, with modern combustion turbines operating in the 2.5-4.0 MPa pressure range. The challenges of understanding soot formation in flames are compounded at high pressures, because the flames become invariably thinner and more sensitive to probe perturbations. This research project aims to understand the chemistry of soot formation in flames at high pressures, by creating controlled laboratory flame environments and using a unique technique to quantify the growth of soot nuclei (i.e., small molecule carbon compounds). These studies will impact our fundamental understanding of the soot process, with potential consequences on the design of practical engines, the reduction of the environmental footprint of combustion, and beneficial effects on air quality, public health and climate. In this research project, the flame structure and the evolution from parent molecule to large soot precursors, and eventually carbonaceous soot nanoparticles, will be determined as a function of changes in pressure and peak temperature. Gaseous species and temperature measurements will be complemented by soot measurements by two-color pyrometry, laser light-scattering and laser induced incandescence for the determination of soot volume fraction, size and dispersion index, the latter being indirectly related to soot age and carbon-hydrogen ratio. Both diffusion flames and rich partially premixed flames will be examined. Key novelties include: the determination of species profiles with good spatial resolution even in very thin high-pressure flames; the quantification, through a collaboration with MIT, of key building blocks of soot, namely, aromatic precursors up to 6-ring in size, well beyond the typical range of analytical chemistry instrumentation used in conjunction with microprobe sampling; and an unprecedented level of control of the examined flames, encompassing conditions of constant and/or accurately scaled time-temperature history. The experimental program is supplemented by modeling of the flames with detailed chemical kinetic mechanisms through collaborations with researchers at the University of Aachen (Germany) and the University of Milan (Italy) for the refinement of chemistry mechanisms and soot models on the basis of the experimental database produced from this research project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.