Current combustion models have been developed and validated with low-pressure experimental data, and they fail at the high pressures of real devices. The goal of my research is to explore the fundamental effects of high pressure on the chemical kinetics of combustion relevant reactions. This knowledge will be used in the development of accurate models for combustion at the high pressures of current and future engines and novel alternate fuels. Some recent experimental studies of the recombination reactions have demonstrated a second-rise in the rate constant for loss of reactants at very high pressures. It has been speculated that this second-rise is due to the effect of the formation of complexes between the radical and the bath gas, however, no theoretical rate calculations have every been made. I will present a two transition state model; one at long-range, correlated with centrifugal barriers, and one at short range, correlated with the loss of entropy as a chemical bond is formed. The pressure-induced stabilization of an intermediate metastable complex (between the two transition states) could explain this increase. The onset of the radical-complex mechanism is governed by the equilibrium constant of the radical-complex species and the recombination rate coefficient is expected to rise above the traditional high-pressure limit, thus explaining the ``second-rise". An outlook to new class of reactions will be presented.
Beyond the gas phase, radical chemistry is very important in the atomic/molecular scale dynamics of materials. Plasma-wall interaction in the fusion reactor results in the formation of codeposits and hydrocarbon flakes. Adsorption isotherms and transmission electron microscopy measurements reveal a multiscale structure made of meso- and macro-pores separated by semi-crystalline graphite and amorphous hydrocarbons. I will present a dynamical atomistic model of hydrocarbon radical interaction with amorphous hydrocarbon flake. A multiscale model for hydrogen isotope diffusion in codeposits will be presented. Similarities in the structural properties between energy storage materials, elementary transport processes in electrolyte membrane and the reactive-diffusion process of H atom in codeposits is seen. An outlook to develop multiscale modeling capabilities for materials modeling in energy storage devices will be presented.
Physical Sciences and Mathematics | Physics
Sharma , A. R. (2013). Dynamics and Kinetics of Radicals: Combustion to Multiscale Materials Modeling. .