Showing papers by "Istanbul Technical University published in 2001"

Biomass conversion to carbon adsorbents and gas

Abstract: A process is created for the utilization of biomass by producing carbon adsorbents and gas. Carbon adsorbents with alkaline character of the surface, tar and gas products are obtained by steam pyrolysis of biomass (almond shells, nut shells, apricot stones, cherry stones, grape seeds). Mixtures of tar obtained during this process and furfural are used for obtaining carbon adsorbents with low ash and sulphur contents and different chemical character of the surface. The chemical character of oxygen functional groups on their surface reveals that they are polyfunctional cationites. Carbon adsorbents obtained have a hydrophilic surface and are suitable for removing metal ions and other pollutants from water. The gas products can be used as energy source.

Obduction, subduction and collision as reflected in the Upper Cretaceous–Lower Eocene sedimentary record of western Turkey

Abstract: Late Cretaceous–Early Eocene Tethyan evolution of western Turkey is characterized by ophiolite obduction, high-pressure/low-temperature metamorphism, subduction, arc magmatism and continent–continent collision. The imprints of these events in the Upper Cretaceous–Lower Eocene sedimentary record of western Anatolia are studied in thirty-eight well-described stratigraphic sections. During the Late Cretaceous period, western Turkey consisted of two continents, the Pontides in the north and the Anatolide-Taurides in the south. These continental masses were separated by the Izmir-Ankara Neo-Tethyan ocean. During the convergence the Pontides formed the upper plate, the Anatolide-Taurides the lower plate. The arc magmatism in the Pontides along the Black Sea coast is biostratigraphically tightly constrained in time between the late Turonian and latest Campanian. Ophiolite obduction over the passive margin of the Anatolide-Tauride Block started in the Santonian soon after the inception of subduction in the Turonian. As a result, large areas of the Anatolide-Tauride Block subsided and became a region of pelagic carbonate sedimentation during the Campanian. The leading margin of the Anatolide-Tauride Block was buried deeply and was deformed and metamorphosed to blueschist facies during Campanian times. The Campanian arc volcanic rocks in the Pontides are conformably overlain by shaley limestone of Maastrichtian–Palaeocene age. However, Maastrichtian sedimentary sequences north of the Tethyan suture are of fore-arc type suggesting that although arc magmatism ceased by the end of the Campanian age, continent–continent collision was delayed until Palaeocene time, when there was a change from marine to continental sedimentation in the fore-arc basins. The interval between the end of the arc magmatism and continent–continent collision may have been related to a northward jump of the subduction zone at the end of Campanian time, or to continued obduction during the Maastrichtian.

The tectonics of the Strandja Massif: late-Variscan and mid-Mesozoic deformation and metamorphism in the northern Aegean

Abstract: The Strandja Massif is a mid-Mesozoic orogenic belt in the Balkans build on a late-Variscan basement of gneisses, migmatites and granites. New single-zircon evaporation ages from the gneisses and granites indicate that the high-grade metamorphism and plutonism is Early Permian in age (∼271 Ma). The late-Variscan basement was unconformably overlain by a continental to shallow marine sequence of Early Triassic–Mid-Jurassic age. During the Late Jurassic–Early Cretaceous (Oxfordian–Barremian) the lower Mesozoic cover and the basement were penetratively deformed and regionally metamorphosed in greenschist facies possibly due to a continental collision. An Rb–Sr biotite whole-rock age from a metagranite dates the regional metamorphism as Late Jurassic (155 Ma). Deformation involved north-vergent thrust imbrication of the basement and the emplacement of allochthonous deep marine Triassic series over the Jurassic metasediments. The metamorphic rocks of the Strandja Massif are unconformably overlain by the Cenomanian shallow marine sandstones. During the Senonian, the northern half of the Strandja Massif formed a basement to an intra-arc basin and to a magmatic arc generated above the northward-subducting Tethyan oceanic lithosphere. The Srednogorie arc closed during the early Tertiary through renewed northward thrusting of the Strandja Massif.

Characteristics of the reaction zone in a combustor operating at mild combustion

Abstract: The aerodynamic, chemical and thermal aspects of the mild combustion process have been studied with emphasis on mixing rates, flue gas recirculation and strong shear produced by reactants supplied from discrete jets. Time-averaged and instantaneous structures of turbulent flow were examined by visualization and local measurements within a 5400 W burner operating with methane with an overall equivalence ratio varying from 0.8 to 1.2 and at non-premixed and premixed modes. The results showed that the entrainment of the flue gases into the fresh mixture was very important for the initiation and progress of the reaction, and occurred in two successive mechanisms. Initially, the flue gases were driven with the reverse flow towards the annular exit where, by Biot–Savart induction, they acquired some momentum from the supply streams provided at the center. The resulting mixing process in the close vicinity of the burner was less intermittent and this was evident in relatively lower values of the second order moments of the residence time distribution. Slightly downstream, the second order moments were, however, increased by large-scale turbulent fluctuations and this led to the enhancement of the mixing process and introduced some further intermittency. The latter entrainment mechanism caused the flue gases to partially encapsulate the discrete jets, which resulted in islands of flammable mixture surrounded by the inert gases. Hence, as the instantaneous OH radical visualizations revealed, the reaction was only initiated away from the burner and in disconnected regions where the Rayleigh pictures showed strong temperature gradients. As the distance from the nozzle increased further, the reaction seemed to follow local flow patterns in that it progressed radially outwards with large structures, which resulted in an increased space-averaged temperature. Furthermore, the residence time decreased away from the burner and the flame came close to extinction due to the high stretching rates of the large structures. However, the flue gases entrained up to this point increased the inert content of the fresh mixture with chemical time scales comparable to the time scales of the flow. This allowed the reactants to attain temperatures near to those of the flue gases and to ignite with a small temperature rise, which led to a much lower thermal NO formation. The results also showed that when the equivalence ratio of the non- premixed mixtures was increased, the region where the combustion took place was shifted away from the burner and extended further downstream towards the roof. In the case of premixed combustion, however, the reaction started and terminated earlier and was confined to regions in close proximity to the axis. The emissions of OH radical occurred rather patchily and in relatively high concentrations.

Underplatform Dampers for Turbine Blades: Theoretical Modeling, Analysis, and Comparison With Experimental Data

Abstract: This paper describes a theoretical model for analyzing the dynamic characteristics of wedge-shaped underplatform dampers for turbine blades, with the objective that this model can be used to minimize the need for conducting expensive experiments for optimizing such dampers. The theoretical model presented in the paper has several distinct features to achieve this objective including: (i) it makes use of experimentally measured contact characteristics (hysteresis loops) for description of the basic contact behavior of a given material combination with representative surface finish, (ii) the damper motion between the blade platform locations is determined according to the motion of the platforms, (iii) three-dimensional damper motion is included in the model, and (iv) normal load variation across the contact surfaces during vibration is included, thereby accommodating contact opening and closing during vibration. A dedicated nonlinear vibration analysis program has been developed for this study and predictions have been verified against experimental data obtained from two test rigs. Two cantilever beams were used to simulate turbine blades with real underplatform dampers in the first experiment. The second experiment comprised real turbine blades with real underplatform damper. Correlation of the predictions and the experimental results revealed that the analysis can predict (i) the optimum damping condition, (ii) the amount of response reduction, and (iii) the natural frequency shift caused by friction dampers, all with acceptable accuracy. It has also been shown that the most commonly used underplatform dampers in practice are prone to rolling motion, an effect which reduces the damping in certain modes of vibration usually described as the lower nodal diameter bladed-disk modes.

Stratigraphic and metamorphic inversions in the central Menderes massif. A new structural model

Abstract: The Menderes Massif is a large area of dominantly Tertiary metamorphic rocks in western Turkey. It is bordered in the west by the Cycladic Metamorphic Complex with Eocene high-pressure/low-temperature (HP/LT) metamorphism. In the Central Menderes the Aydin mountains are made up of a thrust stack of Eocene age. At the base of the thrust stack, greenschist-facies Paleozoic metasediments of the Menderes Massif form an inverted stratigraphic sequence. The Barrovian-type metamorphism is also inverted with garnet-bearing metapelites lying over the lower-grade biotite-bearing metapelites. The P–T conditions in the garnet zone are estimated as 530°C and 8 kbar. This schist sequence of the central Menderes Massif is interpreted as the inverted lower limb of a major southward closing recumbent fold, with the southern Menderes Massif representing a section from the near hinge of this fold. The Paleozoic metamorphic rocks of the central Menderes Massif are tectonically overlain by gneiss klippen possibly originating from the sheared and southward translated core of the Menderes fold. Lying also tectonically over the Paleozoic metamorphic rocks is a major thrust sheet belonging to the Cycladic metamorphic complex. It consists of garnet micaschist, Mesozoic marble, serpentinite and amphibolitised eclogite. Although it has a highly sheared internal structure, it probably represents an initially coherent sequence that has undergone HP/LT metamorphism during the Eocene. The Aydin mountains are dominated by contractional structures with subordinate extensional structures.

Phase transformations in Ti-6Al-4V-xH alloys

Abstract: Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pct hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 °C for 24 hours. The phases present in the temperature range of 20 °C to 1000 °C were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pct lowered the beta-transus temperature of the alloy from 1005 °C to 815 °C, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pct H. A hydride phase was detected in specimens containing 20 and 30 at. pct hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 °C to 580 °C, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pct.

Investigation of groundwater contamination using electric and electromagnetic methods at an open waste-disposal site : a case study from Isparta, Turkey

Abstract: Geophysical methods, when integrated with soil chemical and hydrogeological methods, can be used to investigate groundwater contamination. Direct current (DC) resistivity geo-electrical sounding and very-low-frequency electromagnetic (VLF-EM) data were collected at an open waste site used by the municipality of the city of Isparta, Turkey. The groundwater at shallow depths in alluvium, which is composed of gravel, sand and clay, were expected to be hazardously contaminated under and around the open waste-disposal site, in which both household and industrial wastes are known to be disposed of improperly. In this study, we mapped the spread of groundwater contamination using a VLF-EM method, which allows fast and inexpensive data collection. The method complements the results of geo-electrical sounding. There is a good correlation between the results of the VLF-EM and the DC-resistivity methods employed for the investigation of subsurface structure of the site, where soil chemical and previous hydrogeological surveys have indicated high levels of chemical concentrations.

n-scroll chaos generators: a simple circuit model

Abstract: n-scroll attractors which are generated from a generalised and simple circuit model are presented. The nonlinear characteristics can be systematically designed by adding comparators. A 5-scroll attractor circuit realisation is shown, which has been experimentally verified using current feedback opamps.

Modification of crosslinked glycidyl methacrylate-based polymers for boron-specific column extraction

Abstract: Terpolymers of glycidyl methacrylate (0.4 mol) with methyl methacrylate (0.5 mol) and divinyl benzene (0.1 mol) in spherical beads form have been modified with N -methyl- d -glucamine in N -methyl pyrrolidone solvent. Polymers with (2.05 mmol g −1 ) functionality have moderate swelling ability (1.81 v / v 0 ) and show excellent boron uptake ability (2.15 mmol g −1 ) for extraction of trace quantities of H 3 BO 3 from aqueous solutions. The resinous polymer with 110–210 μm size has a reasonable breakthrough capacity (0.835 mmol g −1 ) and a sharp elution profile in the column extractions. No interferences come from Ca(II), Mg(II) and Fe(III) ions (0.1 M), though slightly in the presence of these ions due to coprecipitation of borates by the metal hydroxides formed on the resin particles. The capacity of the resin and flow rate do not change even after 20 recyclings in the column extractions. The method has been shown to be promising for large-scale boron extractions from aqueous solutions.

Removal of ammonia by natural clay minerals using fixed and fluidised bed column reactors

Abstract: A series of fixed and fluidised bed ion exchange column runs were conducted to identify the ability of natural clay minerals, sepiolite and clinoptilolite, to remove ammonia from a contaminated drinking water reservoir. Ion exchange column tests using both fixed and fluidised bed were initially carried out with synthetic water composed of NH 4 Cl. Breakthrough curves as a function of flow rate, particle size, and initial ammonia concentration reveal that sepiolite does not have as high ion exchange capacity as clinoptilolite but maintains a steady adsorption up to higher bed volumes. The adsorption capacity was modified by using regeneration cycles at both acidic and alkaline pH. Furthermore, fluidised bed runs with clinoptilolite utilising water and air as fluidiser resulted in inferior results compared to those of fixed bed runs. This was respectively ascribed to the presence of ammonia in the circulating water and competition of exchangeable ions released in water and the ability of air to adsorb nitrogen. Tests conducted with natural raw water contaminated with sewage indicated that clinoptilolite adsorbs ammonia the same as the synthetic water. Regenerated clinoptilolite is capable of removing ammonia from both synthetic and actual raw water at a much higher rate than the untreated clinoptilolite.

Cretaceous to Miocene Palaeogeographic Evolution of Turkey: Implications for Hydrocarbon Potential

Abstract: The Cretaceous to Miocene palaeogeographic development of Turkey in general reflects the evolution of the various oceanic branches of Neo-Tethys together with interactions between the Laurasian and Gondwanan margins. At the beginning of the Cretaceous, the first-order palaeotectonic units which make up present-day Turkey either formed parts of these supercontinents or were isolated continental blocks within the Neo-Tethys. In Aptian to Albian times, north-dipping subduction commenced along the southern margins of these units and resulted in the development of magmatic arcs and arc-related sedimentary basins. Coeval with the start of subduction, large-scale ophiolite obduction occurred on the units??? passive margins. Terminal closure of the oceans??? branches took place between the latest Cretaceous and the Miocene, forming the Anatolian orogenic collage whose outline defines present-day Turkey. Post-collisional intra-continental convergence continued until the late Miocene and resulted in north-south shortening of the collage. This shortening led to internal imbrication, thrusting and crustal thickening. It forced the Anatolian landmass westwards, away from east Anatolia where there has been continuous north-south compression since the Miocene between Laurasia and the Arabian Platform.