Ab initio study of [001] GaN nanowires

We present the results of a study of structural, electronic, and optical properties of the unpassivated and H-passivated GaN nanowires having diameters in the range of 3.29 to 18.33 Å grown along [001] direction by employing the first-principles pseudopotential method within density functional theory in the local density approximation. Two types of nanowires having hexagonal and triangular cross-sections have been investigated. The binding energy increases with the diameter of the nanowire because of a decrease in the relative number of the unsaturated surface bonds. The binding energies of the triangular cross-sectional nanowires are somewhat smaller than those of the hexagonal cross-sectional nanowires in accordance with the Wulff’s rule except the smallest diameter triangular cross-sectional nanowire, where the binding energy is comparable with the corresponding hexagonal cross-sectional nanowires. The band gap varies rapidly with the diameter of the nanowire in the case of the smaller diameter nanowires, and quite slowly for the larger diameter nanowires. After atomic relaxation, appreciable distortion occurs in the nanowires, where the chains of Ga- and N-atoms are curved in different directions. These distortions are reduced with the diameters of the nanowires. The optical absorption in the GaN nanowires is quite strong in the ultra-violet region but an appreciable absorption is also present in the visible region for the larger diameter nanowires. The present results indicate the possibility of engineering the properties of nanowires by manipulating their diameter and surface structure. The presently predicted smaller diameter GaN nanowire possessing the triangular cross-section should be observable in the experiments.     

A surrogate fuel for kerosene

Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene.A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.     

Benzene formation in premixed fuel-rich 1,3-butadiene flames

Detailed kinetic modeling and flame-sampling molecular-beam time-of-flight mass spectrometry are combined to unravel important pathways leading to the formation of benzene in a premixed laminar low-pressure 1,3-butadiene flame. The chemical kinetic model developed is compared with new experimental results obtained for a rich (? = 1.8) 1,3-butadiene/O₂/Ar flame at 30 Torr and with flame data for a similar but richer (? = 2.4) flame reported by Cole et al. [Combust. Flame 56 (1) (1984) 51-70]. The newer experiment utilizes photoionization by tunable vacuum-ultraviolet synchrotron radiation, which allows for the identification and separation of combustion species by their characteristic ionization energies. Predictions of mole fractions as a function of distance from the burner of major combustion intermediates and products are in overall satisfactory agreement with experimentally observed profiles. The accurate predictions of the propargyl radical and benzene mole fractions permit an assessment of potential benzene formation pathways. The results indicate that C₆H₆ is formed mainly by the C₃H₃ + C₃H₃ and i-C₄H₅ + C₂H₂ reactions, which are roughly of equal importance. Smaller contributions arise from C₃H₃ + C₃H₅. However, given the experimental and modeling uncertainties, other pathways cannot be ruled out.     

Solubility of pyrene in simple and mixed solvent systems

The solubility of pyrene was experimentally determined in simple and complex solvent systems (single, binary, ternary, quaternary and pentinary solvent systems) composed of benzene, ethylbenzene, hexane, hexanol and methylcyclohexane over a temperature range from 293 to 318 K. In addition, six models were used in this study to represent pyrene solubility in the different solvent systems. The interaction parameters for modified Wilson, NIBS/Redlich-Kister, UNIQUAC and NRTL models were estimated using the solubility data generated for pyrene in single, binary and ternary solvent systems. By re-adjusting the interaction parameters reported for Dortmund UNIFAC and ASOG models, a better representation of the solubility of pyrene was obtained compared to using reported values. Furthermore, a correction term is introduced for the ASOG model in this study to better improve pyrene solubility prediction in simple and mixed solvent systems. These estimated or re-adjusted interaction parameters for the different models, along with the reported parameters for Dortmund UNIFAC and ASOG models, were tested on complex solvent systems (quaternary and pentinary solvent mixtures), in order to check their validity and accuracy for such predictions.     

Service network design with management and coordination of multiple fleets

We present a new optimization model for the tactical design of scheduled service networks for transportation systems where several entities provide service and internal exchanges and coordination with neighboring systems is critical. Internal exchanges represent border crossings necessitating changes of vehicles, while the coordination with neighboring systems represents intermodal operations. For a given demand, the model determines departure times of the services such that throughput time of the demand in the system is minimized. The model is an extension of the design-balanced capacitated multicommodity network design model that we denote service network design with asset management and multiple fleet coordination to emphasize the explicit modeling of different vehicle fleets. Data from a real-world problem addressing the planning of new rail freight services across borders serves to illustrate the capabilities of the formulation. We analyze how synchronization with collaborating services and removal of border-crossing operations impact the throughput time for the freight. We identify a significant potential for system performance enhancement from synchronization among collaborating services for the problem studied.     

Bidimensional shallow water model with polynomial dependence on depth through vorticity

In this paper, we obtain a bidimensional shallow water model with polynomial dependence on depth. With this aim, we introduce a small non-dimensional parameter ε and we study three-dimensional Euler equations in a domain depending on ε (in such a way that, when ε becomes small, the domain has small depth). Then, we use asymptotic analysis to study what happens when ε approaches to zero. Asymptotic analysis allows us to obtain a new bidimensional shallow water model that not only computes the average velocity (as the classical model does) but also provides the horizontal velocity at different depths. This represents a significant improvement over the classical model. We must also remark that we obtain the model without making assumptions about velocity or pressure behavior (only the usual ansatz in asymptotic analysis). Finally, we present some numerical results showing that the new model is able to approximate the non-constant in depth solutions to Euler equations, whereas the classical model can only obtain the average velocity.     

Multiobjective optimization of an ultrasonic transducer using NIMBUS

The optimal design of an ultrasonic transducer is a multiobjective optimization problem since the final outcome needs to satisfy several conflicting criteria. Simulation tools are often used to avoid expensive and time-consuming experiments, but even simulations may be inefficient and lead to inadequate results if they are based only on trial and error. In this work, the interactive multiobjective optimization method NIMBUS is applied in designing a high-power ultrasonic transducer. The performance of the transducer is simulated with a finite element model, and three design goals are formulated as objective functions to be minimized. To find an appropriate compromise solution, additional preference information is needed from a decision maker, who in our case is an expert in transducer design. A realistic design problem is formulated, and an interactive solution process is described. Our findings demonstrate that interactive multiobjective optimization methods, combined with numerical simulation models, can efficiently help in finding new solution approaches and possibilities as well as new understanding of real-life problems as entirenesses. In this case, the decision maker found a solution that was better with respect to all three objectives than the conventional unoptimized design.