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Simulating an Ignition Process

R. Sankaran, H. G. Im, E. R. Hawkes, and J. H. Chen, “The effects of nonuniform temperature distribution on the ignition of a lean homogeneous hydrogen-air mixture,” Proc. of the Combustion Institute 30 (in press). BES, SciDAC

To characterize the ignition process in homogeneous charge compression ignition (HCCI) engines, Sankaran et al. performed high fidelity simulations to study the effects of different temperature distributions on the autoignition of a turbulent homogenous mixture. The results showed that the temperature distribution and mixing rate have a major influence on the location of the first ignition sites and the subsequent combustion and heat release (Figure 6). It was found that the presence of a hot core gas leads to an increase in burn duration, while a cold core gas may lead to an undesirable slow combustion of the end gas. A predictive criterion to determine the two ignition regimes, the spontaneous propagation and the deflagration, was defined based on the ratio of their propagation speeds, suggesting a potential modeling strategy for large-scale simulations of HCCI engines.

Figure 6. Application of the ignition regime criterion on the solution fields at 1% heat release. The color background indicates the temperature field, and line contours of ß = 1 based on C_ß = 0.5 denote the regions where a flame front will be formed.

Controlling the Primary Event of Vision

S. C. Flores and V. S. Batista, “Model study of coherent-control of the femtosecond primary event of vision,” J. Phys. Chem. B 108, 6745 (2004). BES, RC, ACS, FWH, Yale

The cis/trans photoisomerization reaction in rhodopsin is the first step in vision, and as an ultrafast phototransduction mechanism, it has raised significant interest in the field of bioelectronics. Flores and Batista have computationally demonstrated the feasibility of coherently controlling this femtosecond reaction as modeled by an empirical Hamiltonian. Their approach involves selective photoexcitation of multiple vibrationally coherent wave packets by using two chirped femtosecond pulses. Control over product yields is achieved by externally changing the relative phases of the photoexcitation pulses and consequently affecting the interference phenomena between individual wave packet components.

Symmetric and Buckled Dimers on Silicon

Y. Jung, Y. Shao, M. S. Gordon, D. J. Doren, and M. Head-Gordon, “Are both symmetric and buckled dimers on Si(100) minima? Density functional and multireference perturbation theory calculations,” J. Chem. Phys. 119, 10917 (2003). BES, SciDAC

The silicon (100) surface undergoes reconstruction when cleaved — surface atoms dimerizing to form energetically more favorable bonds. However, the energy levels of these bonds, and whether they are symmetric or buckled (asymmetric), remain open questions. Head-Gordon et al. used density functional theory and multireference perturbation theory calculations to determine whether symmetric or buckled dimers are on the minimum (the energy “valley”) of the Si(100) surface. Results varied as to which type of structure was higher in energy; however, overall results indicate that both symmetric and buckled dimers are located on Si(100) minima.

Enhanced Reaction Rates in Fuel Cells

Y. Xu, A. Ruban, and M. Mavrikakis, “Adsorption and dissociation of O₂ on Pt–Co and Pt–Fe alloys,” J. Am. Chem. Soc. 126, 4717 (2004). BES, NSF, 3M

Recently, several platinum–base metal (Pt–M) alloys have been found to possess greater activity than pure Pt for catalyzing the oxygen reduction reaction (ORR) in acidic electrolytes, a primary electrochemical reaction in low-temperature fuel cells. Mavrikakis et al. studied the adsorption of O and O₂ and the dissociation of O₂ on the Pt–M alloys and the Pt “skins” covering these alloys. They found that thin Pt skins, though less reactive than pure Pt for O₂ dissociation, are nonetheless more active toward the ORR because they are less poisoned by O and because they facilitate the activation of O and O-containing intermediates in bond-making elementary reaction steps in the ORR mechanism. The Pt skins also bind CO less strongly than pure Pt does, indicating that they may be more resistant to CO poisoning, a problem relevant to direct methanol fuel cells.

Theorists Challenge Experimental Model

L. D. Speakman, B. N. Papas, H. L. Woodcock, and H. F. Schaefer, “The microwave and infrared spectroscopy of benzaldehyde: Conflict between theory and experimental deductions,” J. Chem. Phys. 120, 4247 (2004). BES, SciDAC, UGA

Recently, it has been proposed that ab initio calculations cannot accurately treat molecules comprised of a benzene ring with a π-conjugated substituent, for example, benzaldehyde. Theoretical predictions of the benzaldehyde barrier to internal rotation are typically a factor of 2 too high in comparison to the experimental values of 4.67 (infrared) and 4.90 (microwave) kcal mol^–1. However, both experiments use Pitzer’s 1946 model to compute the reduced moment of inertia and employ the experimentally observed torsional frequency to deduce benzaldehyde’s rotational barrier. When Pitzer’s model is applied to a system with a nonconjugated functional group, such as phenol, the model and theoretical values are in close agreement. Therefore, Speakman et al. concluded that (1) the model may not account for conjugation between the substituent and the π-system of benzene; (2) the experimental values of the benzaldehyde rotational barrier are misleading; and (3) the true rotational barrier should be closer to the theoretically extrapolated limit of 7.7 kcal mol^–1, based on coupled cluster theory.

Acoustic Speed Reduction Method

Y. Wang and A. Trouvé, “Artificial acoustic stiffness reduction in fully compressible, direct numerical simulation of combustion,” Combustion Theory and Modelling 8, 633 (2004). BES, SciDAC

Direct numerical simulation (DNS) studies of laminar or turbulent flames play a central role in our understanding of basic combustion phenomena, and different mathematical formulations are used for the DNS description of flow and combustion processes. Wang and Trouvé have developed a pseudo-compressibility method, called the Acoustic Speed Reduction (ASR) method, to allow for more efficient computations of slow flow problems using an explicit compressible flow solver. They tested the performance of this method in a series of problems ranging from isothermal sound propagation to laminar premixed flame problems. In all tested cases, the ASR method proved successful at improving the computational efficiency while maintaining solution accuracy.