Time analysis of composite beams with partial interaction using available modelling techniques: A comparative study

This paper presents a comparison of available numerical structural formulations for the short- and long-term analysis of composite beams with partial shear interaction. The finite difference method, the finite element method, the direct stiffness method and the exact analytical model have been considered, and both the instantaneous analysis and the time analysis based on the age-adjusted effective modulus method (AEMM) have been carried out. For modelling based on the first two of these formulations, a spatial discretisation and a discretisation in the time domain are required, while only the time discretisation needs to be specified for the direct stiffness method. The results obtained using these formulations are compared qualitatively and their accuracy is estimated, adopting the exact analytical model as a benchmark reference with the objective of establishing the minimum spatial discretisations required to keep the error within an acceptable tolerance. These comparisons are carried out for simply supported beams, propped cantilevers and fixed ended beams, from which the qualitative behaviour of these formulations in the modelling of continuous beams can also be deduced.     

A predictive multi-step kinetic model of coal devolatilization

Devolatilization is the first step in coal combustion and gasification, thus an accurate kinetic modeling is relevant for the optimal design of these processes. In this work a relatively simple but flexible kinetic model is used to predict the thermal degradation of different coals in a wide range of operating conditions. The main feature of the model lies in its predictive capability: the elemental composition of the starting coal and the operating conditions are the only information required. Three reference coals are used to characterize the devolatilization process. The pyrolysis of each reference coal is described with a multi-step kinetic mechanism effective both at high and low heating rates. The devolatilization of the actual coal is simply obtained as a linear combination of the thermal degradation of the reference coals. The complete kinetic model refers to ∼30 reactions and lumped species, which makes this scheme suitable for being adopted in fluidynamic computations. A wide collection of comparisons between model prediction and experimental data validates this model both in terms of residual char and in terms of detailed gas and tar composition. The importance of secondary gas-phase reactions, mainly at high pressure, is also discussed and verified on the basis of an existing detailed kinetic scheme of pyrolysis and oxidation of hydrocarbon fuels.     

Solve separation units by combining sure and fast models

Problems met in countercurrent separation units have been solved by means of a proper combination of two different models, referred to as sure and fast, to describe the whole set of theoretical stages.The “sure” model is employed for trays showing particular features, or disturbances, i.e. feed trays, partial condensers, reboilers, side cuts, intercoolers, etc. as well as for trays adjacent to the previous ones. This model is written in terms of single theoretical tray equations. The “fast” model is applied to multitray sections, provided that no “disturbance” exists, with an assumption that molar flowrates and temperatures vary according to prefixed laws. The adoption of effective K-values allows one to use formally the same operating equations for the multitray sections as for single theroretical trays.A Newton—Raphson procedure, combined with a B. P. method, is employed to converge the set of equations describing the problem.     

In Saccharomyces cerevisiae,protein secretion into the growth medium depends on environmental factors

In the budding yeast Saccharomyces cerevisiae the cell wall, mainly composed of mannoproteins and glucans, constitutes a barrier to protein excretion in the growth medium. In this paper we have studied the effects of different environmental parameters on excretion of Escherichia coli β-galactosidase obtained by exploiting the glucoamylase II signal sequence. Excretion of the unglycosylated β-galactosidase was detectable only in cells grown in rich medium, was affected by temperature (36°C > 30°C >> 24°C) and slightly stimulated by reducing agents. On the contrary, glycosylated proteins, such as α-galactosidase and glucoamylase II, were excreted to a good extent under all tested conditions of medium composition, growth temperature and pH. These data indicate that optimization of environmental parameters may help the excretion of heterologous proteins, offering advantages for protein purification.     

The ignition,combustion and flame structure of carbon monoxide/hydrogen mixtures. Note 2: Fluid dynamics and kinetic aspects of syngas combustion

The kinetic characterization of the H₂/CO system in presence of nitrogen components was systematically revised in the first note of this work [Frassoldati A, Faravelli T, Ranzi E. The ignition, combustion and flame structure of carbon monoxide/hydrogen mixtures. Note 1: detailed kinetic modeling of syngas combustion also in presence of nitrogen compounds. Int J Hydrogen Energy; 2007, in press]. This second note analyses three different turbulent non-premixed syngas flames by using different approaches such as the Eddy dissipation (ED) the Eddy dissipation concept (EDC) and steady laminar flamelets (SLF) model.     

Generalised Beam Theory (GBT) for stiffened sections

This paper presents an extension to the Generalised Beam Theory (GBT) approach to describe the response of prismatic thin-walled members stiffened by means of generic plate arrangements at different cross-sections along their length. The conventional deformation modes to be included in the GBT formulation are obtained as the dynamic modes of a planar frame, which represents the cross-section. Two numerical procedures are implemented to account for the presence of the stiffeners. One approach identifies different sets of deformation modes for the unstiffened and stiffened sections, which are then combined for the member analysis. The second procedure relies on the use of constraint equations at the stiffened locations to be included in the member analysis. For the cross-sectional analysis, a new mixed finite element is presented which incorporates the inextensibility condition usually adopted in the framework of the classical GBT, therefore simplifying the steps required for the evaluation of the conventional deformation modes. The proposed technique is applicable to open, closed and partially-closed stiffened sections. Two numerical examples are provided to highlight the ease of use of the method of analysis considering open and partially-closed sections, and their results are validated against those obtained with the commercial finite element software Abaqus.     

Kinetic modeling study of ethanol and dimethyl ether addition to premixed low-pressure propene–oxygen–argon flames

The chemical composition of flames of mixed hydrocarbon–oxygenate fuels was examined systematically for a series of laminar, premixed low-pressure propene–oxygen–argon flames blended with ethanol or dimethyl ether (DME). All flames were established at a carbon-to-oxygen ratio of C/O = 0.5 at 40 mbar. Propene was replaced incrementally by either additive, so that the entire range from pure propene to pure ethanol or pure DME was accessible. Experimental results have been reported previously (J. Wang et al., J. Chem. Phys. A 112 (2008) 9255–9265), including temperature profiles measured with laser-induced fluorescence (LIF) and quantitative mole fraction profiles for a large number of species obtained from molecular-beam mass spectrometry (MBMS), using electron ionization (EI) and vacuum-ultraviolet (VUV) photoionization (PI). The effects of oxygenate addition to the propene base flame were seen to result in interesting differences, especially regarding trends to form aldehydes. The entire flame series is now analyzed with a comprehensive kinetic model that combines the chemistries of propene, ethanol, and DME combustion. The flames of pure fuels are also compared with the predictions of different detailed mechanisms taken from the literature. Quantitative comparison of C₁- to C₆-species from this model with the measurements is provided. Major trends of propene replacement by the oxygenates are reproduced in quantitative agreement with the experiments, enabling a more detailed understanding of the combined reaction sequences in such fuel blends.     

Formation of soot and nitrogen oxides in unsteady counterflow diffusion flames

The formation of pollutant species in turbulent diffusion flames is strongly affected by turbulence/chemistry interactions. Unsteady counterflow diffusion flames can be conveniently used to address the unsteady effects of hydrodynamics on the pollutant chemistry, because they exhibit a larger range of combustion conditions than those observed in steady flames.In this paper, unsteady effects on the formation of soot (and its main precursors) and nitrogen oxides (NO_x) are investigated by imposing harmonic oscillations on the strain rate of several counterflow diffusion flames fed with propane. Numerical results confirm that the dynamic response of each species is strongly affected by the strain rate oscillations and the characteristic time governing its chemistry. At low frequencies of imposed oscillations the soot and NO_x profiles show strong deviations from the steady-state profile. At large frequencies a decoupling between the concentration and the velocity field is evident. In particular, the formation of soot and NO_x is found less sensitive to velocity fluctuations for flames with large initial strain rate. The significant increase of soot and NO_x concentrations in unsteady conditions appears to be a function of both forcing frequency and flame global strain rate. Moreover, the cut-off frequency, defined as the minimum frequency above which the strain rate oscillations have negligible effects on the formation of each species, was found to be strongly dependent on the chemical characteristic time and the flame global strain rate, but only marginally affected by the amplitude of imposed oscillations.