Large scale effects in weakly turbulent premixed flames

In this study we numerically investigate large scale premixed flames in weakly turbulent flow fields. A large scale flame is classified as such based on a reference hydrodynamic lengthscale being larger than a neutral (cutoff) lengthscale for which the hydrodynamic or Darrieus–Landau (DL) instability is balanced by stabilizing diffusive effects. As a result, DL instability can develop for large scale flames and is inhibited otherwise. Direct numerical simulations of both large scale and small scale three-dimensional, weakly turbulent flames are performed at constant Karlovitz and turbulent Reynolds number, using two paradigmatic configurations, namely a statistically planar flame and a slot Bunsen flame. As expected from linear stability analysis, DL instability induces its characteristic cusp-like corrugation only on large scale flames. We therefore observe significant morphological and topological differences as well as DL-enhanced turbulent flame speeds in large scale flames. Furthermore, we investigate issues related to reaction rate modeling in the context of flame surface density closure. Thicker flame brushes are observed for large scale flames resulting in smaller flame surface densities and overall larger wrinkling factors.     

Strain rates,flow patterns and flame surface densities in hydrodynamically unstable,weakly turbulent premixed flames

Recent numerical and experimental studies have unveiled a potentially marked difference between the laminar as well as turbulent propagation of premixed flames exhibiting Darrieus–Landau (DL) (or hydrodynamic) instabilities from flames for which instabilities are inhibited. In this study we utilize two-dimensional numerical simulations of slot burner flames as well as experimental Propane–Air Bunsen flames to analyse differences in turbulent propagation, strain rate and induced flow patterns of hydrodynamically stable and unstable flames. We also investigate the effects of hydrodynamic instability on quantities which are directly related to reaction rate closure models, such as flame surface density and stretch factor. A clear enhancement of turbulent flame speed can be observed for unstable flames, generally mitigated at higher turbulence intensity, which is attributed to a flame area increase induced by the characteristic cusp-like DL-induced corrugation, absent in stable flames, which occurs concurrently and in synergy with turbulent wrinkling. Unstable flames also exhibit, both numerically and experimentally, a different correlation between strain rate and flame curvature and are observed to give rise to a channeling of the induced flow in the fresh mixture. Conditionally averaged flame surface density is also observed to attain smaller values in unstable flames, as a result of the thicker turbulent flame brush, indicating that closure models should incorporate instability-related parameters in addition to turbulence-related parameters.     

Calorimetric study of the room-temperature transitions of polytetrafluoroethylene: The influence of thermal history

The trend of the room-temperature transitions of native polytetrafluoroethylene versus annealing temperature has been studied. A method of deconvolution for the calorimetric curves C_p(T), based on Pearson VII type functions and the Marquardt's nonlinear leastsquares method, allowed the evaluation of the temperatures and of the heat of each transition. Annealing at 330°C determines the loss of the crystalline fraction responsible of the lowest room-temperature transition. An interpretation of the lowest room-temperature transition is suggested.     

Photolysis and Thermolysis of N-Alkoxycarbonyl-α-diazoamide Acetals

In the photoysis and thermolysis of 1-phenyl-2,2-dimethoxy-2-(N-alkoxycarbonylamino)-diazoethanes the presence of the N-alkoxyearbonylamino substituent is of fundamental importance. In both cases the carbene intermediate, through intramolecular addition to carbonyl oxygen, gives, as main products, oxazole derivatives. In the thermolysis, especially at relatively low temperature (160°), the diazoamide acetals in part cyclize to form triazole derivatives.     

Reactions of ketenes XVII. Intermediates in the reactions between ketone acetals and ethyl azidoformate

The reaction of the ketene acetals with ethyl azidoformate proceeds via I-ethoxycarbonyl-5,5-dialkoxy-Δ² -1,2,3-triazolines. The latter decomposes easily at room temperature, the reaction pathway depending on the presence of the substituents at the 4-position. Thermolysis and photolysis of the 4-alkyl-substituted triazolines proceed via 1,3-diradicals. The photodecomposition of the 4-unsubstituted triazolines proceeds similarly. The thermal decomposition of the latter is considered either to involve homolytic cleavage of the N₁? N₂ bond to give a 1,5-diradical, or to proceed by a concerted mechanism.     

TRMC, XPS, and EPR characterizations of polycrystalline TiO2 porphyrin impregnated powders and their catalytic activity for 4-nitrophenol photodegradation in aqueous suspension

Characterization of polycrystalline TiO(2) bare or porphyrin impregnated powders, used as photocatalysts for the degradation of 4-nitrophenol (4-NP) in aqueous suspension, was performed by time-resolved microwave conductivity (TRMC) measurements and electronic paramagnetic resonance (EPR) and X-ray photoelectron (XPS) spectroscopies. The presence of porphyrin sensitizers, as the metal-free or Cu [5,10,15,20-tetra (4-tert-butylphenyl)] porphyrin, impregnated onto the TiO(2) surface improved the photocatalytic activity of the bare TiO(2). TRMC measurements indicate that the number and lifetime of the photoinduced excess charge carriers increase in the presence of the macrocycles, and EPR and XPS spectroscopies support the mechanistic hypotheses based on the photoreactivity experiments.     

Trennung und Nachweis vom Osmium und Ruthenium

Ohne Zusammenfassung     

Classification and technology of byzantine mosaic glass

Glass-making is a very sophisticated skill and the contribution given by the chemical analyses of glass materials is fundamental for the classification of glass types and for identifying compositional groups according to consistent characteristics that can be associated with chronological and geographical differentiations. The chemical composition of glasses is particularly complex: to a few basic constituents many components were added, either derived from impurities in the raw materials or intentionally incorporated into the glass mix. The field of study concerning the chemical composition and the technology of Byzantine mosaic production has not been dealt with in a systematic manner and certainly not exhaustively from the view point of classification according to the reconstruction of chronological and geographical development. Nevertheless, it is of great interest because it is probable that during the Byzantine period the production of mosaic glass was greater than for any other type of glass. We propose a methodology for classifying Byzantine mosaic glasses on the basis of simple statistical treatment of the chemical composition data. Compositional data relative to basic and accessory constituents together with colorants were analysed and elaborated through binary diagrams. Byzantine glasses are also compared to glasses of different epoch and provenance.     

The recovery of biophenols from wastewaters of olive oil production

In this article, the phenolic composition of wastewaters prepared from different cultivars of Olea europaea have been described. The main aim is the recovery of these compounds for technological utilization.     

Crystallization kinetics and morphology of the mesomorphic form of syndiotactic polypropylene

For the first time the kinetics of formation of syndiotactic polypropylene mesophase close to 0 °C through differential scanning calorimetry has been investigated. The calorimetric data have been analyzed through the well‐known Avrami analysis. We have obtained Avrami coefficients less or at most equal to 2, which are likely to indicate a nearly bidimensionality of the crystallites. The morphology of the mesophase, not known up to now, has been examined through atomic force microscopy (AFM): lamellar structures have been observed. The fractal dimension of the AFM images has been evaluated through the box counting method. It resulted about 2, in a very good agreement with the calculated Avrami coefficients. An infrared and diffractometer analysis on samples kept for increasing times at 0 °C has also been performed to confirm the calorimetric data. The experimental results allow one to better clarify the kinetics of mesophase‐formation. It has been found that a time of 3 h at 0 °C is actually necessary for the complete crystallization of the trans‐planar mesophase, a longer time is needed to stabilize the mesophase domains when the sample is extracted from 0 °C at room temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 936–944, 2007