Showing papers by "National Technical University of Athens published in 1991"

Modelling of the pyrolysis of biomass particles. Studies on kinetics, thermal and heat transfer effects

Abstract: The present work provides a rationally-based model to describe the pyrolysis of a single solid particle of biomass. As the phenomena governing the pyrolysis of a biomass particle are both chemical (primary and secondary reactions) and physical (mainly heat transfer phenomena), the presented model couples heat transport with chemical kinetics. The thermal properties included in the model are considered to be linear functions of temperature and conversion, and have been estimated from literature data or by fitting the model with experimental data. The heat of reaction has been found to be represented by two values: one endothermic, which prevails at low conversions and the other exothermic, which prevails at high conversions. Pyrolysis phenomena have been simulated by a scheme consisting of two parallel reactions and a third reaction for the secondary interactions between charcoal and volatiles. The model predictions are in agreement with experimental data regarding temperature and mass-loss histories of biomass particles over a wide range of pyrolysis conditions. On presente dans cet article un modele s'appuyant sur des bases rationnelles pour decrire la pyrolyse d'une particule solide simple de biomasse. Considerant que les phenomenes gouvernant la pyrolyse de la particule de biomasse sont a la fois chimiques (reactions primaires et secondaires) et physiques (principalement des phenomenes de transfert de matiere), le modele presente couple le transport de chaleur avec la cinetique chimique. Les proprietes thermiques incluses dans le modele sont considerees comme etant des fonctions lineaires de la temperature et de la conversion et ont ete estimees a partir de donnees pubiees dans la litterature ou en adaptant le modele aux donnees experimentales. On a trouve que la chaleur de la reaction etait representee par deux valeurs: une valeur endothermique qui predomine a de faibles conversions et une valeur exothermique qui predomine a des conversions elevees. Les phenomenes de pyrolyse ont ete simules par un schema comportant deux reactions paralleles et une troisieme reaction pour les interactions secondaires charbon-volatils. Les predictions du modele montrent un bon accord avec les donnees experimentales sur la temperature et l'histoire de la perte massique des particules de biomasse pour une vaste gamme de conditions de pyrolyse.

Antioxidant activity of some plant extracts of the family labiatae

Abstract: The antioxidant activities of methanol extracts of oregano, dittany, thyme, marjoram, spearmint, lavender and basil were tested in lard stored at 75°C. The concentration of extracts in lard varied from 0.01 to 0.20%. Oregano extract was found to be the most effective in stabilizing lard, followed by thyme, dittany, marjoram and lavender extracts, in a decreasing order. The induction period of lard increased with antioxidant concentration. After the induction period, peroxide formation proceeded rapidly, following pseudo-zero order reaction kinetics. The rate of the reaction decreased slightly with increasing plant extract concentration. Combined addition of plant extracts in lard showed a low synergistic action between thyme extract and spearmint extract.

A gradient flow theory of plasticity for granular materials

Abstract: A flow theory of plasticity for pressure-sensitive, dilatant materials incorporating second order gradients into the flow-rule and yield condition is suggested. The appropriate extra boundary conditions are obtained with the aid of the principle of virtual work. The implications of the theory into shear-band analysis are examined. The determination of the shear-band thickness and the persistence of ellipticity in the governing equations are discussed.

A four-step phase-fitted method for the numerical integration of second order initial-value problems

Abstract: A four-step method with phase-lag of infinite order is developed for the numerical integration of second order initial-value problems. Extensive numerical testing indicates that this new method can be generally more accurate than other four-step methods.

Dielectric permittivity and loss of an aluminum-filled epoxy resin

Abstract: Dielectric behaviour of an epoxy resin filled with 0–30 wt% aluminum powder is reported. Permittivity, loss index and dissipation factor are characterized as a function of temperature in the range 20–150°C and frequency in the range 20 Hz–20 kHz. The filled resin shows a higher permittivity and higher dielectric loss. An interfacial or Maxwell-Wagner-Sillars polarization effect is clearly evident and glass transition temperature is unaffected by the filler.

Energy dissipation and associated failure modes when axially loading polygonal thin-walled cylinders

Abstract: The crumpling mechanisms of thin-walled octagonal steel tubes subjected to axial static compression and their efficiency as energy absorbing components under impact conditions are investigated and compared with the equivalent circular and square tubes of same material. Both inextensional and extensible collapse mechanisms are theoretically analysed. Theoretically predicted values based on the deformation modes encountered, as well as experimentally obtained ones, are compared and found to be in good agreement.

Dielectric effects of water in water-containing systems

Abstract: The dielectric properties of several water-containing systems (synthetic polymers, saccharides, proteins, plant tissues) were systematically studied by means of the thermally stimulated depolarization currents (TSDC) method, following recently developed concepts for using the TSDC method to investigate the structures of water in different systems. The TSDC spectra recorded between 77 and 300 K exhibit several distinct relaxations which can be classified into three groups: relaxations due to the reorientation of water molecules themselves; dipolar relaxations, which are also present in the dry samples and which are influenced by water; and conductivity relaxations related with the water-dependent dc conductivity due to ions, mostly protons. In most cases, the results are compatible with a three-phase model: molecularly dispersed water tightly (non-rotationally) bound at primary hydration sites, loosely bound water in layers or clusters around primary hydration sites and free water at higher water contents.

Geotechnical properties of the Corinth Canal marls

Abstract: The Corinth Canal crosses the isthmus of Corinth and is of great importance for Mediterranean navigation as well as the railroad and highway connections between southern and central Greece. In the century-long history of the Canal, the slopes have shown only minor stability problems despite their significant length, very steep inclination and, more importantly, the strong earthquakes that have frequently shaken the Corinth area. This type of unsual behaviour has motivated research into the mechanical behaviour of the bluish grey marl which is the main geological formation in the Canal area. Geotechnical investigation of the Corinth Canal marl was performed with an extensive laboratory testing programme on high quality undisturbed samples of the intact marl. The material was shown to exhibit brittle behaviour, high stiffness and significant apparent cohesion at low and moderate stress levels. These characteristics indicate that the material possesses significant structural bonding which is believed to be due to cementation between individual particles, caused by the deposition of carbonates at the time of material genesis. With stressing, the material yields due to a gradual bond degradation. The locus of the initial yield points (yield surface) seems to be an ellipse oriented along the isotropic axis. The testing programme was supplemented with a series of tests on the de-structured marl obtained by thorough remoulding. These tests showed a significant difference in the pre-yield stiffness and the peak strength at low stress levels, but comparable post-rupture shear strengths.

Fuzzy reasoning in the development of geographical information systems FRSIS: a prototype soil information system with fuzzy retrieval capabilities

Abstract: The characteristics of a soil information system based on the fuzzy relational database model as defined by Zemankova-Leech and Kandel are presented. The proposed system maintains all the advantages of the more conventional relational implementations but enhances them in two ways: (1) the system can cope with incomplete or even imprecise data; and (2) the users are allowed to express their subjective view of the stored data. The retrieval and processing of data approximately resemble the way that humans think and reason. The INGRES relational database management system was used for the implementation of the system.

Jet Engine Fault Detection with Discrete Operating Points Gas Path Analysis

Abstract: A common feature of all Differential Gas Path Analysis methods is the necessity of measuring a number of performance variables greater or at least equal to the number of diagnostic parameters which have to be estimated. Discrete Operating Conditions Gas Path Analysis (DOCGPA) is an extended version of the conventional GPA algorithms, providing-among other things-the capability to overcome this problem. In the present paper, we describe how this method can be coupled with an engine computer model, in order to perform component directed fault diagnosis. Application to a commercial turbofan engine demonstrates the effectiveness of the proposed method. Nomenclature [C] = influence coefficient matrix EPR = engine pressure ratio / = scalar defined in Eq. (8) k = number of different operating points [M] = information matrix m = number of unknown component parameters n = number of measured variables [P] = covariance matrix of unknown components parameters m = number of unknown component parameters PEUI = performance estimation uncertainty index (equal to J) [R] = covariance matrix of measures variables tr{ } =' trace of a matrix u = operating condition vector x = component parameter vector y = measurement vector A( ) = percentage change from an initial value 2 = summation Superscripts T = transpose matrix -1 = inverse matrix

A two-step method with phase-lag of order infinity for the numerical integration of second order periodic initial-value problem

Abstract: A two-step method with phase-lag of order infinity is developed for the numerical integration of second order periodic initial-value problem. The method has algebraic order six. Extensive numerical testing indicates that the new method is generally more accurate than other two-step methods.

Charged particle multiplicity distributions in restricted rapidity intervals in Z0 hadronic decays.

Abstract: The multiplicity distributions of charged particles in restricted rapidity intervals inZ0 hadronic decays measured by the DELPHI detector are presented. The data reveal a shoulder structure, best visible for intervals of intermediate size, i.e. for rapidity limits around ±1.5. The whole set of distributions including the shoulder structure is reproduced by the Lund Parton Shower model. The structure is found to be due to important contributions from 3-and 4-jet events with a hard gluon jet. A different model, based on the concept of independently produced groups of particles, “clans”, fluctuating both in number per event and particle content per clan, has also been used to analyse the present data. The results show that for each interval of rapidity the average number of clans per event is approximately the same as at lower energies.

Simultaneous production of glucose oxidase and catalase by alternaria alternata

Abstract: A number of factors affecting simultaneous production of cell-bound glucose oxidase and catalase by the fungus Alternaria alternata have been investigated. Consecutive optimization of the type and concentration of nitrogen and carbon source, the initial pH and growth temperature resulted in a simultaneous increase in glucose oxidase and catalase by 780% and 68% respectively. Two second-order equations, describing the combined effect of pH and temperature on the activity of each enzyme, revealed that glucose oxidase had its optima at pH 7.9 and 32.3°C and catalase at pH 8.5 and 18.1°C. Under certain growth conditions, yields as high as 23.5 and 18,100 units/g carbon source for glucose oxidase and catalase, respectively, were simultaneously obtained.

Plasticized PVC films/petroleum oils: The effect of ultraviolet irradiation on plasticizer migration

Abstract: Plasticizer migration from flexible poly(vinyl chloride) comprises an important aspect, especially when packaging foodstuffs and pharmaceuticals. Much of the published work has been intended either to correlate migration into simulants with that into foods or to study migration into simpler extractants, enabling the various parameters involved in migration to be studied in isolation. According to the latter approach, the migration of dioctyl phthalate into petroleum oils has been studied already in our laboratory and in this paper results are presented in an attempt to reduce or prevent migration by u.v. irradiation. The effect of irradiation time on short and long-term migration behavior was examined together with the influence of the immersion temperature. The nature of the liquid environment seemed to be a predominant aspect: high viscosity oils presented a satisfactory behavior in contrast with those of lower viscosity in which the prevention effect was rather negligible. On the other hand, primary kinetics studies yielded similar results with those already established for untreated material (i.e., good conformity to the short time Fickian approximation).

Influence of pulse reversed current technique on the crystalline orientation and surface morphology of nickel electrodeposits

Abstract: The reversed current (RC) technique perturbs the nickel electrocrystallization process by modification of the adsorption-desorption phenomena occuring on the cathodic area and causes a radical change of the structural characteristics and properties of the deposits. The purpose of this work is to study the modifications occurring in the crystalline orientation and surface morphology of nickel deposits, prepared from an organic-free Watts bath, by application of the RC technique. The texture analysis of the deposits was realized by X-ray diffractometry and the surface morphology was studied by scanning electron microscopy techniques. It was observed that variation of the RC parameters results in a total modification of the texture and surface morphology of the deposited metal. The roughness of the metallic surface was also examined and conditions for the preparation of bright and smooth deposits were determined.

Localization of Rational B-Spline Surfaces

Abstract: Localization of curved surfaces is an important problem in the manufacturing and inspection of mechanical elements and in the simulation of manufacturing processes. The objective of this paper is to provide an efficient method for accurate localization of a single free-form surface of the rational B-spline form. Our approach is based on the following components—first, representation of position tolerances in terms of a ball offset tolerance region around the ideal rational spline surface; to facilitate interrogation and data exchange, this is followed by approximation of the bounding surfaces of the tolerance region with rational B-splines; next, optimal positioning of the target surface with respect to the ideal surface by minimizing a distance-based norm; finally, verification of compliance with position tolerance constraints. Examples illustrate our technique.