Modeling of a two-color, two-stage, ultrafast Yb-doped fiber amplifier

We have modeled a two-stage, Yb-doped fiber amplifier system to amplify two-color ultrafast laser pulses in near-infrared. The first and second stages namely preamplifier and power amplifier are single-clad, single-mode and double-clad, single-mode Yb-doped fibers respectively. From numerical simulations of the modeled fiber amplifiers, the optimal lengths of the two fibers to have equal power at two colors centered at 1035 nm and 1105 nm are in agreement with our experimental results. Numerical simulations have also been performed to demonstrate the impact of using a flat-top, two-color seed spectrum on the system performance.     

Surface energies of metals in both liquid and solid states

Although during the last years one has seen a number of systematic studies of the surface energies of metals, the aim and the scientific meaning of this research is to establish a simple and a straightforward theoretical model to calculate accurately the mechanical and the thermodynamic properties of metal surfaces due to their important application in materials processes and in the understanding of a wide range of surface phenomena. Through extensive theoretical calculations of the surface tension of most of the liquid metals, we found that the fraction of broken bonds in liquid metals (f) is constant which is equal to 0.287. Using our estimated f value, the surface tension (γ_m), surface energy (γ_SV), surface excess entropy (−dγ/dT), surface excess enthalpy (H_s), coefficient of thermal expansion (α_m and α_b), sound velocity (c_m) and its temperature coefficient (−dc/dT) have been calculated for more than sixty metals. The results of the calculated quantities agree well with available experimental data.     

Availability optimization of a redundant system through dependency modeling

This paper presents a constructive approach to optimize the availability of a system through modeling the dependency of the components. Our goal is to minimize the system cost under the constraint that system availability must not be less than a given level. In particular, the components are dependent of each other. A function noted as the dependence function is introduced to model the dependency. It is demonstrated that, for a general form of the system cost, the dependence function guarantees a finite set of feasible solutions. An approach is then developed with the help of the dependence function to obtain the optimal solution. The resolution is illustrated by an interesting example, in which the system cost depends on the strength of the dependency. Our study reveals that the dependency is an essential and effective option to improve system reliability. Moreover, the modeling of dependency, i.e. the introduction of the dependence function is valuable for resolving the optimization problem.     

Instant controlled pressure drop extraction of lavandin essential oils: Fundamentals and experimental studies

Détente Instantanée contrôlée (DIC), French for Instant Controlled Pressure Drop, was performed on laboratory apparatus as well as on a pilot plant for proving its feasibility, and identifying the optimized processing conditions and recognizing the energy consumption and the quantity of water used for such an operation. GC–MS and SPME analysis of the extracts and residue material were carried out to assess the extracts and solid residues. The lavandin essential oils obtained by using the new DIC extraction process was studied, modeled and quantitatively and qualitatively compared to the conventional hydrodistillation method. The most important differences between the two essential oils were reflected in the yields, with 4.25 as against 2.3 g EO/100 g of raw matter, and in the extraction time, with 480 s as against some hours for respectively the DIC and the hydrodistillation operations. These differences have been previewed through the fundamental analysis. They can normally explain the great decreasing of energy consumption to be 662 kWh/t of raw material. The amount of water to be added was about 42 kg water/t of raw material.     

Influence of the position of the double bond on the autoignition of linear alkenes at low temperature

The influence of the position of the double bond on the autoignition of linear alkenes has been investigated by modeling the behavior of the three isomers of linear hexene and those of linear heptene. Low-temperature kinetic mechanisms for the oxidation of these six alkenes have been obtained after some improvements made to the system EXGAS, for the automatic generation of mechanisms, which had been previously adapted to model the oxidation of 1-pentene and 1-hexene. Quantum mechanical calculations have shown that cis-trans conformations should be taken into account and that isomerizations of alkenyl and alkenyl peroxy radicals involving a transition state including a double bond could be neglected. The new mechanisms have been validated using experimental data obtained in two rapid compression machines between 600 and 900 K with a good prediction of cool flame and ignition delay times. The model reproduces well the decreasing reactivity at low temperature when going from 1- to 3-alkene. While the profiles of products are well reproduced for 1-hexene in a jet-stirred reactor above 780 K, more problems are encountered for the prediction of products in a rapid compression machine at 707 K, showing persisting problems in the understanding of the chemistry of the low-temperature oxidation of alkenes. Reaction rates analysis have been used to explain the difference of reactivity between the isomers of hexene.     

Thermal oxidation of clay-nanoreinforced polypropylene

The thermo-oxidation process at low temperatures for a montmorillonite-nanoreinforced polypropylene (PP) was studied. Experimental aging kinetic data at 100, 80 and 60 °C have been obtained and compared with a computational simulation in which a kinetic model based on the closed loop approach was used. As a result, it has been found that the montmorillonite role is not limited to a role of inert filler in the polymer matrix but induces a slight catalytic effect leading to induction period reduction. This effect has been well simulated by increasing initial hydroperoxyde concentration. The consequences of kinetic control by oxygen diffusion have also been investigated by using micro ATR-FTIR mapping to assess concentration profiles of the oxidation products across the sample thickness. It has been found that the oxidized layer thickness is close to 17 μm for the pure polypropylene whereas it is around 10 μm for the nanocomposite at 100 °C. These profile variations have been attributed to differences in oxygen diffusion coefficient values. Simulations based on the kinetic model including diffusion-reaction coupling describe these profiles well.     

Thermogravimetric analysis and kinetics modeling of isothermal carbonization of olive wood in inert atmosphere

The kinetics of olive wood carbonization is investigated by means of isothermal thermogravimetric analysis method. Measurements were carried out in a thermobalance for different fixed temperatures between 498 and 648 K. A two-stage semi-global kinetic model consisting of four sequential steps was proposed to derive kinetic parameters. The olive wood is classified in three pseudo-components. For the first two, similar thermal degradation mechanisms take place in a single reaction step. For the third, the thermal degradation takes place in two consecutive steps. The isothermal conditions allow the kinetic constants (activation energy and pre-exponential factors) to be estimated by means of the analytical solution of the mass conservation equations. An overall good agreement was obtained with activation energy values available in the literature.     

Microscopic model of CO oxidation on Pt(1 1 1)

A new microscopic model, based on DFT/LDA modeling, is suggested for the Langmuir-Hinshelwood reaction of catalytic CO oxidation in coadsorbed O-CO layers on Pt(1 1 1). It has been found that only the oxygen atoms occupying threefold hollow sites of hcp type are chemically active. The potential barrier for the oxidation reaction significantly decreases due to changes in the adlayer oxygen states in the proximity to CO. The oxygen electronic density distribution is affected by approaching CO molecule which alters the oxygen position. Height of the barrier is estimated as 1.15 eV, which may be attributed to the upper limit of activation energy for the net reaction process.