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Chemistry and Dynamics of Counterflow Cool Flames
Reuter, Christopher B Princeton University ProQuest Dissertations & Thes 2019 해외박사(DDOD)
Cool flame experiments offer an unexplored platform in addressing challenges relevant to the design of advanced engines. Advanced engine designs often focus on premixed or partially premixed strategies involving reduced flame temperatures and, consequently, near-limit combustion with heightened sensitivity to the fuel reactivity and ignition timing. Despite the emissions and efficiency advantages, however, these designs have not been widely implemented due to the limited knowledge of the combustion chemistry required to operate them. A complete understanding of the chemical reactivity of real transportation fuels has been particularly difficult to achieve when considering the complexity of low-temperature combustion phenomena. By investigating low-temperature cool flames in counterflow burners, this dissertation advances the fundamental understanding of the chemistry and dynamics of both nonpremixed and premixed cool flames.In the first section of this dissertation, the counterflow cool flame platform is presented as an important tool in the quantitative validation of chemical kinetic models at low temperatures. Measurements of the nonpremixed cool flame extinction limits are shown to magnify relatively small differences in low-temperature chemistry, giving the platform a potential use for screening the reactivity of different fuels. It is found that kinetic model predictions of the cool flame extinction limits cannot reproduce experimental measurements accurately due to their inability to capture low-temperature heat release rates in cool flames. An updated kinetic model for dimethyl ether/methane mixtures is developed and validated by targeting reactions disproportionally important to cool flame extinction.In the second part of the dissertation, the dynamics of both cool flames and hot flames are investigated. The structure and stability of lean premixed cool flames are measured over various conditions, and it is observed that cool flames can sometimes burn under conditions that hot flames cannot, resulting in an extension of the lower flammability limits. In some cases, hot flames can extinguish into cool flames via a transitional double flame structure, composed of a leading cool flame and a trailing hot flame. The interactions between double flames and vortices are also investigated, revealing new and interesting transient flame structures. These findings highlight the relevance of cool flames to near-limit combustion.
Coherent Control of Electric Current at the Nanoscale
Reuter, Matthew Gregory Northwestern University 2011 해외박사(DDOD)
This dissertation discusses several computational and theoretical aspects regarding the application of coherent control schemes for manipulating electric current through molecules. In the first part, we discuss challenges in modeling nanometer-scale systems over more conventional systems. One key problem is the persistence of surface effects deep into nanoscale systems---possibly on the order of hundreds of nanometers---making accurate computations expensive. Besides showing that the system's (effective) dimensionality is largely responsible for this behavior, we discuss and develop methods to accurately and efficiently compute surface effects. In the second part, we use model systems to explore the fundamentals of electron transport through electrode-molecule-electrode junctions. Semi-analytical results from the first part facilitate these investigations by providing more accurate and easier-to-use open-system boundary conditions for the electrodes. In particular, we first examine the role of the molecule-electrode adsorption chemistry on the electron transport properties. Our results expose a loose set of guidelines (a chemical intuition) for choosing molecular "alligator clip" binding motifs when linking the molecule to an electrode. Second, we study the ramifications of cooperative effects between multiple molecules on electron transport. In systems with either two wires or infinite wires (i.e., adlayers), we show that molecules sometimes behave like conventional electronic wires (where cooperative effects are undesirable), but that conduction through multiple molecules is usually enhanced by cooperative effects. Moreover, we attribute this observation to quantum interference effects between the molecules' conduction channels. Lastly, we consider the effects of using semiconducting, as opposed to metallic, electrodes in these junctions. Both the band gap and the possibility of surface states lead to markedly different electron transport properties. Finally, the third part considers the application of coherent control schemes to surface-adsorbed molecules. In addition to developing more efficient computational techniques for these simulations, we show that the coherence properties of a laser field can be used to manipulate the (librational) motion and orientation of surface-adsorbed molecules. Ultimately, since the molecular geometry is a key parameter for electron transport though molecules, this leads to the "Coherent Control of Electric Current at the Nanoscale.".