Hexagonal Tubes Research Articles

PLATE HEAT EXCHANGERS FOR OCEAN THERMAL ENERGY CONVERSION

Ocean Thermal Energy Conversion (OTEC) utilizes the temperature difference between the warm surface seawater and cold deep ocean water to generate electricity. The heat exchanger in an OTEC power plant is significant as it exchanges a large quantity (nearly 40-50 times of the gross power) of heat energy due to the low thermal efficiency of the power cycle. Nearly 30-40% of the total cost of the plant should be allotted to this single component. For the present, 1-MW OTEC plant plate heat exchangers are used both as an evaporator and a condenser. This paper reports a Computational Fluid Dynamics (CFD) investigation for a dimple plate heat exchanger. The analysis uses the Log Mean Temperature Difference Method (LMTD) in all its calculations. Whilst the shell side flow highly resembles the flow over a rough or wavy plate, the tube side passage in these represents the flow over short hexagonal tube banks with the flow across the sectional areas between the hexagons having the shape of a benzene ring. The CFD code employed is Fluent 5 using the Re-Normalization Group (RNG) turbulence model and the two-layer zonal model for near wall areas on the shell side. For tube side flow, the normal k-ε turbulence model is used with a turbulent Prandtl number of 0.63 suggested for flow in tube banks by the literature.

Micrometer‐Sized Hexagonal Tubes Self‐Assembled by a Cyclic Peptide in a Liquid Crystal

Gelöst in einer nematischen flüssigkristallinen Phase können starre scheibenförmige Makrolactame hexagonale Röhren bilden. Computermodelle (siehe Bild, O rot, N blau, C grau, H weiß), Raman- und IR-Mikrospektroskopie bringen eine hierarchische Selbstorganisation des Peptids in der flüssigkristallinen Phase zum Vorschein.

Micrometer-sized hexagonal tubes self-assembled by a cyclic peptide in a liquid crystal.

Dissolved in nematic liquid crystals, rigid disk-shaped macrolactams may form large hexagonal hollow tubes. Computer modeling (see picture, O red, N blue, C gray, H white), Raman and infrared microspectroscopy reveal a hierarchical and self-similar-structured self-assembly of the cyclic peptide in the liquid-crystal medium.

Synergy between spin crossover and metallophilicity in triple interpenetrated 3D nets with the NbO structure type.

Self-assembly of [Ag(CN)2]-, 3-cyanopyridine, and iron(II) gives triple interpenetration of three-dimensional spin crossover (SCO) networks with the NbO topology, which interact each other via argentophilic interactions. Superposition of the networks along the c-axis defines an infinite mosaic of trigonal and hexagonal cross-sectional tubes where water molecules are installed. The structural changes associated with the SCO tune the homoatomic interactions and induce an uncommon change of the lattice volume.

Experimental and numerical study in axial crushing of thin walled sections made of high-strength steels

The interest in dynamic plasticity has greatly increased in recent years driven by advances in computational power and by industrial needs. The knowledge of material properties is required as well as adequate constitutive equations capable of describing material behaviour. Results are presented in this study for the tensile testing of Dual-Phase and TRIP (Transformation Induced Plasticity) steels at different strain rates. The test results were used to obtain Cowper-Symonds coefficients with the intent of describing strain rate sensitivity of material properties. The crashworthiness of thin walled sections made of Dual-Phase and TRIP steels was subsequently investigated. Crush tests were performed at different speeds on top-hat and hexagonal tubes. The experimental results were compared with numerical simulation obtained with LS-DYNA software. The influence of the material models in the accuracy of simulations was examined.

Carbon nanotube bundles under high pressure: Transformation to low-symmetry structures

Structure transformations for crystalline bundles of single walled carbon nanotubes (10,10), (8,8), and (6,6), in response to external pressure are modeled by first-principles calculations. Upon pressure, the circular tube section is first transformed into an elliptical shape. Further pressure then leads to a flattened shape, similar to a 400-m track, with two flat sections connected by two cap sections. While the stress is taken up at the cap sections by bond buckling, the conjugate \ensuremath{\pi} bonding on the two flat sections becomes more effective and provides some stabilization for the structure. Such a transformation effectively squeezes the empty space inside a tube and thus reduces the intertube van der Waals repulsion. Collapse of the tube structures or linking between tubes via ${\mathrm{sp}}^{3}$ bonding is not observed up to a stress level of 20 GPa. Hexagonal tube sections are also observed, which is a metastable state, due to the the symmetry constraint of the triangular lattice during structure optimization. Such a structure is not favored as it is too rigid to adapt to external pressures.

Ab initio simulation of the ice II structure

We have carried out ab initio simulations on the high-pressure polymorph of solid water, ice II, a phase for which there is a surprising lack of experimental data. We report our calculated third-order Birch–Murnaghan equation of state for ice II: the zero pressure and temperature density, ρ0=1240.27±0.62 kg m−3, bulk modulus, K0=16.18±0.12 GPa, with the first pressure derivative of the bulk modulus, K0′, fixed equal to 6.0. These parameters, the unit cell dimensions, and the atomic positions are in good agreement with experimental values. We also describe the way in which the change in unit cell volume is accommodated within the structure, primarily by contraction of the distance between neighboring hexagonal tubes—the principal structural element of ice II. This is in agreement with existing experimental data.

Application of X-Ray Computer Tomography for Observing the Deflection and Displacement of Fuel Pins in an Assembly Irradiated in FBR

A nondestructive method making use of X-ray computer tomography (X-ray CT) has been applied to post irradiation examination of fast breeder reactor (FBR) fuel assemblies. In the study, an examination is made of the deflection and displacement of fuel pin in a fuel assembly irradiated to 74.2GWd/t peak burnup in the fast reactor “JOYO.” In the examination, X-ray CT images of transverse cross sections of fuel pin were obtained at different heights of fuel pin along its axis. Analysis of the resulting images indicated that: 1. The hexagonal wrapper tube had its lateral wall faces slightly bulged outward; 2. The fuel pins loaded in the outermost array were markedly displaced in the direction of wrapper tube, particularly in portions of fuel pin intermediate between positions constrained by wrapping wire. The latter behavior of fuel pins was substantiated by the contours of fuel pin along its axis, which were derived from cross section images obtained at different levels along axis. Such fuel pin displacement is surmised to have been caused by thermal stressing of the affected fuel assembly cladding.

Application of X-Ray Computer Tomography for Observing the Deflection and Displacement of Fuel Pins in an Assembly Irradiated in FBR

A nondestructive method making use of X-ray computer tomography (X-ray CT) has been applied to post irradiation examination of fast breeder reactor (FBR) fuel assemblies. In the study, an examination is made of the deflection and displacement of fuel pin in a fuel assembly irradiated to 74.2GWd/t peak burnup in the fast reactor “JOYO.” In the examination, X-ray CT images of transverse cross sections of fuel pin were obtained at different heights of fuel pin along its axis. Analysis of the resulting images indicated that: 1. The hexagonal wrapper tube had its lateral wall faces slightly bulged outward;2. The fuel pins loaded in the outermost array were markedly displaced in the direction of wrapper tube, particularly in portions of fuel pin intermediate between positions constrained by wrapping wire. The latter behavior of fuel pins was substantiated by the contours of fuel pin along its axis, which were derived from cross section images obtained at different levels along axis. Such fuel pin displacement is surmised to have been caused by thermal stressing of the affected fuel assembly cladding.

Improving the energy absorption of closed cell aluminum foams

Closed cell aluminum foams have received much recent attention as energy absorbing materials on account of their ability to undergo extensive deformation at a relatively low stress called the plateau stress. Several studies describe the improvements in energy absorption to be obtained, relative to their empty counterparts, when foam filled tubes are crushed either quasi-statically or dynamically [1–4]. Al foams are also of possible interest for ballistic applications because they present a very large acoustic impedance mismatch with common armor materials, offering the possibility of being able to modify the way in which elastic waves travel through multi-component armor [5]. Two cases of energy absorption are of particular practical interest. The first concerns blast protection, in which loading occurs over a broad area, and the second concerns resistance to penetration by small projectiles, in which severe point loading is experienced. The potential advantages that foam can offer in the first case are most clear if the foam remains in contact with a face sheet during deformation because the face sheet serves to spread the deformation throughout the foam and prevent local perforation. However, in the second case, the tendency is for the foam to perforate locally and little energy is absorbed when impacted by a small object. A key factor in this latter behavior is that metal foam has an approximately zero Poisson’s ratio. Hence, no lateral deformation occurs and the only contribution to energy absorpotion is from material directly ahead of the impacting object. The objective of the present study, therefore, was to investigate whether it is feasible to increase the energy absorption by causing the deformation to spread sideways, e.g., by inserting tubes into the foam (rather than vice versa). Experiments have been performed with novel foam/tube architectures and it has been shown that it is possible to increase the energy absorbed when foam is subject to loading by a small blunt object. While this preliminary series of experiments is not exhaustive, the tests definitely confirm that foam-based architectures can be tailored to increase the energy absorption beyond that available from foams alone. Aluminum foam blocks approximately 45 × 45 × 50 mm were cut from larger foam blocks and visually examined to ensure a relatively consistent pore size and freedom from any major, visible defects. Gentle core drilling was used to prepare holes 7.15 mm in diameter in either a square or hexagonal array, Fig. 1. Al-3003 aluminum tubes, 0.4 mm wall thickness, were then inserted into the holes. Samples were weighed at each stage of preparation so that energy absorption can still be compared on a “per kg” or density basis. Aluminum tubes were chosen so as not to increase the density of the assembly much beyond that of the foam; ideally the weight of tube inserted should equal the weight of foam removed if the density is to be maintained constant. Mechanical testing was carried out by subjecting the assemblies to indentation with a 12.7 mm diameter stainless steel indenter, at a penetration speed of 0.2 mm · s−1. Data acquisition software was used to record load and penetration depth measurements during testing. Force/indentation depth curves for typical samples are shown in Fig. 2 where it is clear that, for the same displacement, the hexagonal tube arrangement requires the greatest force. Although an attempt has been made to “standardize” the tests somewhat, it is clear that there are many factors related to the geometry of the samples, size, number of tubes, spacing of tubes, tube wall thickness, tube diameter, etc., that make direct comparison problematical. The data were also converted to display specific energy absorption as a function of displacement using the equation

New Indides EuAuIn2, EuPdIn4, GdRhIn2, YbRhIn4, and YbPdIn4

New intermetallic indium compounds EuAuIn2, EuPdIn4, GdRhIn2, YbRhln4, and YbPdIn4 were obtained by reaction of the elements. GdRhIn2 was synthesized in an arc-melting furnace, while EuAuIn2, EuPdln4, YbRhIn4, and YbPdIn4 were prepared in sealed tantalum tubes in a high-frequency furnace. The five compounds were investigated by X-ray diffraction both on powders and single crystals. EuAuIn2 and GdRhIn2 adopt the MgCuAl2 type structure with space group Cmcm. Single crystal X-ray data yielded a = 468.1(2), b = 1105.5(4), c = 753.5(4) pm, wR2 = 0.096, 343 F2 values for EuAuIn2 and a = 435.0(1), b = 1013.3(3), c = 783.6(2) pm, wR2 = 0.042, 608 F2 values for GdRhIn2 with 16 variables for each refinement. The two structures may be described as gold or rhodium filled versions of the host lattices Euln2 and GdIn2 . The three-dimensional indium networks of EuAuIn2 and GdRhIn2 resemble the lonsdaleite structure. Both structures are built up from three-dimensional [Auln2] and [Rhln2] poly anions in which the europium and gadolinium atoms occupy distorted hexagonal tubes. The modulations of the In-In distances within the indium networks are compared with other MgCuAl2 type indides. EuPdIn4 and YbPdIn4 crystallize with the YNiAl4 type, space group Cmcm: a = 454.8(2), b = 1703.2(8), c = 738.0(3) pm, wR2 = 0.044, 501 F2 values for EuPdIn4 and a = 445.8(2), b = 1666.0(4), c = 747.3(2) pm, wR2 = 0.050, 711 F2 values for YbPdIn4 with 24 variables for each refinement. In contrast, YbRhln4 adopts the LaCoAl4 type, space group Pmma: a = 863.7(2), b = 422.5(1), c = 743.1(1) pm, wR2 = 0.051, 467 F2 values and 24 variables. EuPdIn4, YbPdlIn4, and YbRhIn4 too consist of three-dimensional [Pdln4] and [Rhln4] polyanions in which the europium and ytterbium atoms are located in distorted hexagonal and pentagonal channels. Common structural motifs of these indides are distorted bcc-like indium cubes which are compared with the structures of Y2CoIn8, YCoIn5, EuRh2ln8, and elemental indium. Chemical bonding in these indides is briefly discussed

Structure and Properties of Sr2Au3In4 and Eu2Au3In4

Sr2Au3In4 was synthesized by reacting the elements in glassy carbon crucibles under an argon atmosphere in a high-frequency furnace. Eu2Au3In4 was obtained in the same way but starting with EuAuIn as a precursor and admixing indium and gold. Both compounds were investigated by X-ray diffraction on powders and single crystals: anti-Hf2Co4P3 type, P62m, a=1501.6(9) pm, c=457.9(2) pm, wR2=0.0734, 1030 F2 values, 45 variables for Sr2Au2.85In4.15 and a=1489.7(3) pm, c= 454.90(7) pm, wR2=0.1222, 1051 F2 values, 45 variables for Eu2Au3In4. One gold site in the strontium compound shows a mixed occupancy with indium atoms indicating a small homogeneity range according to Sr2Au3−xIn4+x. Striking structural motifs of Sr2Au3In4 and Eu2Au3In4 are gold-centered trigonal prisms formed by the strontium (europium) and indium atoms. All gold atoms have coordination number 9 in the form of tricapped trigonal prisms. The indium atoms in Sr2Au3In4 and Eu2Au3In4 occupy the cobalt positions of the Hf2Co4P3 type and the more electronegative gold atoms take the phosphorus positions, indicating that Sr2Au3In4 and Eu2Au3In4 may be considered as ternary aurides. The gold and indium atoms form three-dimensional [Au3In4]4− polyanions in which the divalent strontium and europium atoms occupy distorted hexagonal tubes. Magnetic susceptibility measurements of Eu2Au3In4 show paramagnetic behavior above 50 K with an experimental magnetic moment of 7.88(5) μB/Eu indicating Eu2+. Antiferromagnetic ordering is detected at 9.5(5) K at low external fields. A metamagnetic transition is observed at 2 K at a critical field of 0.5(2) T. The experimental saturation moment at 2 K and 5.5 T is 5.95(5) μB/Eu. Electrical conductivity investigations exhibit metallic behavior with a room temperature value of 75±10 μΩcm for the specific resistivity. 151Eu Mössbauer spectroscopic measurements show a broadened line with an isomer shift of −10.5(5) mm/s at 78 K in accordance with several sites of divalent europium. Below 10 K magnetic hyperfine field splitting is observed.

The CHORUS honeycomb tracker and its bitstream electronics

The CHORUS experiment searches for νμ↔ντ oscillation. To aid in the momentum reconstruction of charged hadrons, a honeycomb tracker was built with three orientations of six planes each. The planes are manufactured by point-welding together two precision folded conductive polycarbonate foils, forming hexagonal tubes with 30μm thick anode wires in the center.The honeycomb tracker in CHORUS is read out using a bitstream principle. The amplified signal of each wire is binary sampled every 5ns and stored in a 256 bit circular buffer, implemented in dual-port memories. This technique allows a full reconstruction of a 1.28μs history of each wire. Eighteen cards, each handling 72 wires, are read out over a single flat cable using a card-to-card pipeline.

Fiber Waviness and Other Mesostructures in Filament Wound Materials

Glass-epoxy filament windings were made with circular and hexagonal sections. They were sectioned and examined for fiber angular deviations and packing anomalies. Results were compared with those from a carbon-epoxy commercially wound circular section tube. In addition, flat regions from the hexagonal section tubes were tested using the short beam shear test. It was found that in-plane waviness was usually much more serious than out-of-plane, especially with the commercial tube. Winding pressure appears to be very important when tubes are not consolidated after winding. Thus the hexagonal sections had very high void contents (here the winding pressure is negligible) and the unsupported hexagonal section windings had relatively low fiber volume fractions. The apparent shear strength was reduced by the voids, but did not correlate with any other mesostructural parameter, nor with fiber volume fraction.

Incorporation of Vanadium in Mesoporous MCM-41 and Microporous AFI Zeolites

Vanadium-containing silicate MCM-41 (V-MCM-41) zeolite and aluminophosphate AFI (VAPO-5) zeolite were synthesized and characterized by spectroscopic techniques. In as-synthesized form, the vanadyl ions (VIVO)2+ were found to be the major vanadium species in the form of atomic dispersion on AFI by EPR and to exist simultaneously with tetrahedral (Td) V5+ in MCM-41 by UV−vis. 29Si MAS NMR investigations suggested that the vanadium ions might attach to MCM-41 through interaction with the silanol groups on the internal wall of hexagonal tubes. The V5+ (in Td) ions are incorporated into the lattice of MCM-41 during synthesis, while the VO2+ (in Td) is the loosely bound V species. The results of Raman spectroscopy indicated that the rodlike aggregation of cationic surfactant (cetyltrimethylammonium bromide, CTAB) was encapsulated in the intrachannel space of synthetic MCM-41 as in an aqueous solution. After calcination and hydration, the V4+ species in as-synthesized V-MCM-41 was totally oxidized to +5 as shown by UV−vis and EPR spectroscopies, and they further aggregated as two-dimensional vanadate chain species that were nonuniformly deposited on the wall of MCM-41 channels as verified by Raman and HREM with EDS spectroscopies, while the V5+ species of synthetic V-MCM-41 remains stable in a tetrahedral coordination. Comparatively, two types of VO2+ ions were observed in as-synthesized VAPO-5 by EPR and they could be oxidized by calcination treatment. The presence of water vapor facilitates the oxidation of (VIVO)2+ and the formation of V2O5 cluster instead of isolated (VVO)3+ species.

Comparison of an ordered mesoporous aluminosilicate, silica, alumina, titania and zirconia in normal-phase high-performance liquid chromatography

This paper investigates the behaviour of silica, alumina, titania, zirconia and the novel mesoporous aluminosilicate MCM-41 in normal-phase high-performance liquid chromatography under comparable conditions. The physicochemical properties of the oxides and MCM-41 are described. MCM-41 is an ordered mesoporous material with a regular pore structure composed of an assembly of hexagonal tubes with a pore diameter of 4 nm. In chromatography MCM-41 exhibits acid and basic properties and proves to be suitable for the separation of acid, neutral and basic compounds.

Tetraoleoylpyrophosphatidic acid: a four acyl-chain lipid which forms a hexagonal II phase with high curvature

Tetraoleoylpyrophosphatidic acid (bis phosphatidic acid), when hydrated in aqueous buffer, was shown to form an inverted hexagonal phase using 31P NMR. Low-angle x-ray diffraction provided verification of the formation of this phase in dilute aqueous buffer and in 2 M NaCl and permitted comparison of the tube diameter with that of cardiolipin in 2 M NaCl. By using the water cylinder diameters for tetraoleoylpyrophosphatidic acid, bovine cardiolipin, chloroplast monogalactosyl-diglyceride, and dioleoyl phosphatidylethanolamine as a means of estimating the spontaneous curvatures, tetraoleoylpyro-phosphatidic acid was shown to exhibit the greatest curvature of any of the above lipids, equaled only by the calcium salt of cardiolipin. Inverted micelles of hydrated tetraoleoylpyrophosphatidic acid and of cardiolipin in tetradecane were approximately the diameter of the inverted hexagonal tubes. A rationale is given for the differences.

A straightforward implantation method of radioactive sources by means of the Leksell frame.

Stereotactic brachytherapy of cerebral gliomas often involves insertion of catheters through the tumor bed. We adopted the hexagonal tube configuration around a central tube. Such a configuration is applicable to most commonly occurring brain tumors. In order to perform a low-cost, accurate and straightforward implantation, a robust mathematical derivation of all relevant formulas is used to transform CT coordinates to Leksell frame coordinates, including derivations for the two stereotactic angles. The coordinates of the seven tubes (which are essential if no template is available) and the length of the radioactive source are also derived, and an example is presented. The calculations have been implemented on a personal computer (Intel processor 386 or higher). Additional options, such as choosing other stereotactic angles than the calculated ones and rotating the tube configuration around the central tube, are integrated into the program.

Molecular dynamics simulations of phospholipid bilayers.

Molecular dynamics (MD) simulations at 37 degrees C have been performed on three phospholipid bilayer systems composed of the lipids DLPE, DOPE, and DOPC. The model used included 24 explicit lipid molecules and explicit waters of solvation in the polar head group regions, together with constant-pressure periodic boundary conditions in three dimensions. Using this model, a MD simulation samples part of an infinite planar lipid bilayer. The lipid dynamics and packing behavior were characterized. Furthermore, using the results of the simulations, a number of diverse properties including bilayer structural parameters, hydrocarbon chain order parameters, dihedral conformations, electron density profile, hydration per lipid, and water distribution along the bilayer normal were calculated. Many of these properties are available for the three lipid systems chosen, making them well suited for evaluating the model and protocols used in these simulations by direct comparisons with experimental data. The calculated MD behavior, chain disorder, and lipid packing parameter, i.e. the ratio of the effective areas of hydrocarbon tails and head group per lipid (a(t)/ah), correctly predict the aggregation preferences of the three lipids observed experimentally at 37 degrees C, namely: a gel bilayer for DLPE, a hexagonal tube for DOPE, and a liquid crystalline bilayer for DOPC. In addition, the model and conditions used in the MD simulations led to good agreement of the calculated properties of the bilayers with available experimental results, demonstrating the reliability of the simulations. The effects of the cis unsaturation in the hydrocarbon chains of DOPE and DOPC, compared to the fully saturated one in DLPE, as well as the effects of the different polar head groups of PC and PE with the same unsaturated chains on the lipid packing and bilayer structure have been investigated. The results of these studies indicate the ability of MD methods to provide molecular-level insights into the structure and dynamics of lipid assemblies.

Analytic formulae for the effective conductivity of a square or hexagonal array of parallel tubes

A set of analytic formulae are presented for the effective thermal conductivity of either a square or a hexagonal array of parallel tubes which have distinct core, tube and fill conductivities. The formulae are based on a generalization of Rayleigh's [ Phil. Mag. 34(5), 481–502 (1892)] method to include hexagonal (as well as square) arrays, tubes (as well as solid rods), and higher-order terms in the analytic series. The accuracy of the analytic formulae is determined by comparison with essentially exact numerical calculations. The formulae are applied to the problem of an array of dry, spent, nuclear fuel rods.