Large Diameter Tubes Research Articles

Effects of inert gases on the vortex bursting in small diameter tubes

A flame propagation in a small diameter tube has been experimentally investigated with use of two meso-scale combustors. Methane was used as fuel, and four inert gases, Ar, N2, He and CO2 were used as diluents. Results show that, in the large diameter tube (Dg=10.0mm), the flame propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. In the small diameter tube (Dg=3.6mm), the propagation ranges of Ar and N2 are narrowed in fuel-rich region, where the Lewis number is greater than unity. For CO2 and He-dilution, however, the propagation has not been observed. Because the quenching diameters for CO2- and He-dilution are significantly large, the flames are strongly affected by the tube wall. Further experiments have shown that, under high oxygen concentrations, the flame can propagate in the small diameter tube for all diluents, and the propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. The flame is slender for CO2, whereas the flame is wide for He, as compared with Ar and N2. These differences in flame appearance clearly indicate that the Lewis number of deficient reactant affects strongly on the flame propagation in small diameter tube, and for CO2-dilution, as the Lewis number is sufficiently less than unity, the flame diameter can be small, and thus, the wall effects can be reduced. Whereas for He-dilution, as the Lewis number is sufficiently greater than unity, the flame diameter becomes large, and thus, the wall effects can be enhanced. The flame speed of He-dilution becomes considerably lower than the other three mixtures. This retardation seems to be resulted from an increase in the burned gas density due to the cooling by the enhanced wall effect.

Experimental investigation of condensation heat transfer and pressure drop of R22, R410A and R407C in mini-tubes

Condensation heat transfer and pressure drop of R22, R410A and R407C were investigated experimentally in two single round stainless steel tubes with inner diameter of 1.088mm and 1.289mm. Condensation heat transfer coefficients and two phase pressure drop were measured at the saturation temperatures of 30°C and 40°C. The mass flux varies from 300 to 600kg/m2s and the vapor quality 0.1–0.9. The effects of mass flux and vapor quality were investigated and the results indicate that condensation heat transfer coefficients increase with mass flux and vapor quality, increasing faster in the high vapor quality region. The experimental data was compared with the correlations based on experimental data from large diameter tubes (dh>3mm), such as the Shah and Akers correlations et al. Almost all the correlations overestimated the present experimental data, but Wang correlation and Yan and Lin correlation which were developed based on the experimental data from mini-tubes predicted present data reasonably well. Condensation heat transfer coefficients and two phase pressure drop of R22 and R407C are equivalent but both higher than those of R410A. As a substitute for R22, R410A has more advantages than R407C in view of the characteristics of condensation heat transfer and pressure drop.

Tunable deformation and electronic properties of single-walled ZnO nanotubes under a transverse electric field

The effects of the electric field on the radial deformation and electronic properties of armchair and zigzag single-walled ZnO nanotubes are investigated by the density functional theory. Under the transverse electric field, the circular cross-sections of ZnO nanotubes are deformed to elliptic. In addition, the bandgap of ZnO nanotube gradually reduced with the increasing electric field strength. Such field-induced modulation effect is enhanced in ZnO nanotubes with larger tube diameter, but is not sensitive to the chirality. The mechanisms of the radial deformation and the variations of bandgaps of the ZnO nanotubes are analyzed.

Influence of nanotube characteristics on electrical and thermal properties of MWCNT/polyamide 6,6 composites prepared by melt mixing

Differential scanning calorimetry (DSC), X-ray diffraction, dynamic mechanical analysis (DMA) and electrical conductivity measurements were performed on multiwalled carbon nanotube/polyamide 6,6 composites with four different types of well-characterized tubes manufactured using fixed-bed catalytic processes. The tubes differed in diameter, number of walls and surface chemistry. Except for one very poorly formed set of tubes, there were not large differences in behavior with respect to modulus, maximum electrical conductivity and percolation threshold; none of which differed by more than a factor of 2 for the three types of tubes. However, the shape of the percolation region was very different for the larger diameter tubes in that the percolation region occurred over a much wider concentration. Glass transition and crystallization/melting characteristics also showed very small differences between the three types of tubes. One unique observation made in this work was that the glass transition temperature (Tg) showed a qualitatively different behavior depending on what was used to measure the transition: DSC indicated an increase in Tg with added nanotubes while DMA showed a decrease in Tg. Other than the difference being related to how the Tg was measured, it is not clear what caused the qualitatively different behavior.

Air-side performance of herringbone wavy fin-and-tube heat exchangers under dehumidifying condition – Data with larger diameter tube

This study examines the airside performance of the herringbone wavy fin-and-tube heat exchangers in dehumidifying condition having a larger diameter tube (Dc=16.59mm) with the tube row ranging from 2 to 12. Test results are compared to that of dry conditions and plain fin geometry. Upon the influence of surface condition (dry or wet) on the heat transfer performance, the heat transfer performance in dehumidifying condition normally exceeds that in dry condition, and is more pronounced with the rise of tube row or reduction of fin pitch. By contrast, it is found that the heat transfer coefficient for plain fin geometry in dehumidifying condition is slightly lower than that in dry condition. The pressure drops in wet condition is much higher than that in dry condition. However, the difference in pressure drop amid dry and dehumidifying condition for wavy fin configuration is less profound as that of plain fin geometry.

Li2 Trapped inside Tubiform [n] Boron Nitride Clusters (n=4–8): Structures and First Hyperpolarizability

The geometries and electronic properties of tubiform [n] boron nitride clusters entrapping Li(2) (Li(2)@BN-cluster(n,0); n=4-8), obtained by doping BN-cluster(n,0) with Li(2) molecules, are investigated by means of DFT. The effects of tube diameter n on the dipole moment μ(0), static polarizability α(0), and first hyperpolarizability β(0) are elucidated. Both the dipole moment and polarizability increase with increasing tube diameter, whereas variation of the static first hyperpolarizability with tube diameter is not monotonic; β(0) follows the order 1612 (n=4)<3112 (n=5)<5534 (n=7)<8244 (n=6)<12,282 a.u. (n=8). In addition, the natural bond orbital (NBO) charges show that charge transfer takes place from the Li(2) molecule to the BN cluster, except for BN-cluster(8,0) with larger tube diameter. Since the large-diameter tubular BN-cluster(8,0) can trap the excess electrons of the Li(2) molecule, Li(2)@BN-cluster(8,0) can be considered to be a novel electride compound.

Penetration of charged particles through metallic tubes

Several theoretical studies have shown that charged particles increased the particle deposition efficiency in cylindrical tubes by image force. Some experimental investigations have also found that the deposition loss increased with increasing aerosol charge. However, the amount of charges on the aerosols or the electrical mobility was relatively low in these previous experimental studies. In order to extend to higher aerosol charge, a TSI vibrating orifice monodisperse aerosol generator was modified to generate 1-μm DEHS aerosols with charge up to 24,000 elementary units. Metallic tubes were employed to exclude the effect of Coulombic force. Tube diameter, tube length, and average velocity in tubes were among the major operating parameters. Aerosol charge was monitored using a TSI electrometer, while a TSI aerodynamic particle sizer was utilized to measure both aerosol concentrations and size distributions upstream and downstream of the tubes. Aerosol deposition loss in the sampling train of the TSI electrometer was measured to back-calculate the average aerosol charge. A closed-form theoretical model, showing the deposition efficiency as a function of particle charge, was validated by the experimental data produced.The aerosol deposition efficiency increased with increasing aerosol charge and tube length, due to stronger image force and longer retention time, respectively. The deposition efficiency decreased with increasing average velocity because of shorter retention time. Under the same average velocity, the deposition loss decreased with increasing tube diameter because the fraction of aerosols near the inner wall was higher for small-diameter tube than for large-diameter tube. The model developed from parabolic flow showed good agreement with most of the experimental data, except for data of highly charged particles. The discrepancy was probably due to the space charge effect.

Recognition of carbon nanotube chirality by phage display

We report a virus-based biological template, M13 bacteriophage, for selecting single-walled carbon nanotubes’ (SWCNTs’) chirality, based on the premise that binding affinity between carbon nanotubes (CNTs) and peptides are sensitive to amino acid sequences. Bacteriophage pIII capsid fusion peptides, which have high binding affinity for CNTs, were identified through an evolutionary screening process by a phage display technique. Most of the binding sequences are rich in aromatic amino acids, begin with histidine, and possess hydrophobic properties. Direct evidence of binding was provided by the attachment of SWCNTs to selected bacteriophage-coated microspheres via an anti-M13 monoclonal antibody and by the attachment of CdTe quantum dots coupled with the selected peptides to SWCNTs. Efficient dispersion of debundled SWCNTs was achieved by the selected peptides whose conformation is characterized by circular dichroism. Quantitative analysis of the binding affinity was carried out using molecular dynamics simulations, demonstrating that the best binder, HESFWYLPHQSY, has the highest binding energy with (7, 6) SWCNT in the peptide pool. Computational calculations also show that the selected peptides preferentially bind to large-diameter tubes and the peptide HSNWRVPSPWQL has the best discernment on chirality. Chiral selectivity was examined by UV-vis-NIR and photoluminescence spectroscopy, showing that HSNWRVPSPWQL could shift the SWCNT size distribution to larger diameter and enrich (10, 3), (9, 5), (12, 2), (11, 4), and (10, 6). This work presents chirality-preferential dispersion of SWCNTs by phage-displayed peptides for the first time, and offers new prospects of understanding the mechanism of peptide–CNT binding and producing “designer” peptide for CNT surface recognition.

Structure and electronic properties of the nanopeapods—One dimensional C60O polymer encapsulated in single-walled carbon nanotubes

The structure–property relationship of the nanopeapods—one dimensional (1D) C60O polymer encapsulated in single-walled carbon nanotubes (SWCNTs)—is studied by means of the self-consistent field crystal orbital method. The calculations show that the nearest distance between the two constituents is within Van der Waals interaction scope in the most stable two peapods. The SWCNT sizes affect not only the stability but also the electronic structures of the peapods. The peapods with larger tube diameters keep the semiconductive and metallic properties of the corresponding pristine SWCNTs. These combination systems are stiffer than the corresponding SWCNTs due to larger Young's moduli. The magnitude order of the calculated mobility of charge carriers is in the range of 102–105cm2V−1s−1 for the peapods, indicating that the combined systems may be good high-mobility electronic materials.

Electro-optical properties of zigzag and armchair boron nitride nanotubes under a transverse electric field: Tight binding calculations

The electro-optical properties of zigzag and armchair BNNTs in a uniform transverse electric field are investigated within tight binding approximation. It is found that the electric field modifies the band structure and splits band degeneracy where these effects reflect in the DOS and JDOS spectra. A decrease in the band gap, as a function of the electric field, is observed. This gap reduction increases with the diameter and it is independent of chirality. An analytic function to estimate the electric field needed for band gap closing is proposed which is in good agreement with DFT results. In additional, we show that the larger diameter tubes are more sensitive than small ones. Number and position of peaks in DOS and JDOS spectra for armchair and zigzag tubes with similar radius are dependent on electric field strength.

Conductive Monolithic Catalysts: Development and Industrial Pilot Tests for the Oxidation of o-Xylene to Phthalic Anhydride

We present herein results from a campaign (>1500 h) of o-xylene oxidation runs in a 1 in. single-tube technical fixed-bed pilot reactor (i.d. = 24.6 mm) loaded with 16 washcoated (V2O5/TiO2) monolithic catalysts with Al honeycomb supports (each 10 cm long) and operated at typical industrial conditions for PA (phthalic anhydride) production. The highly conductive monolithic supports afforded substantially reduced axial T-gradients in comparison with reference runs in the same pilot reactor loaded with conventional egg-shell ring catalyst pellets, the maximum T-difference with the salt bath being halved at the same hot spot temperature (∼440 °C) and the mean bed temperature being about 20 °C higher. Temperature gradients were still moderate at an o-xylene feed concentration of 80 g/Nm3 (at Qair = 4 N m3/h), which represents an upper limit for industrial PA packed-bed reactors loaded with the same V2O5/TiO2 catalyst coated onto ring pellets. Operation at higher o-xylene feed contents (up to 95 g/Nm3) was found feasible. The Al honeycomb catalysts were successfully unloaded at the end of the runs. The strong (∼2-fold) enhancement of radial heat transfer rates associated with the adoption of highly conductive monolith catalyst supports is thus herein demonstrated at a fully representative industrial scale for the first time. It can be exploited, for instance, either to increase the o-xylene feed concentration, possibly above 100 g/Nm3 (and the PA productivity accordingly), within a retrofitting strategy, or to design new reactors with larger tube diameters, resulting in reduced investment costs.

Control of carbon nanotube handedness using a supramolecular chiral surface

Sorting diameter and handedness of carbon nanotubes still appears as an important challenge in nanotechnology. In this context, supramolecular structures formed by self-assembled chiral molecules deposited on well-defined metal surfaces can be used to discriminate the two isomers of carbon nanotubes. Calculations are carried out to determine the adsorption energy of nanotube enantiomers on alaninate coated Cu(110) surface. The results show a significant discrimination of the L and R handed isomers by such a surface and an additional selectivity in terms of small and large tube diameters.

Flooding Experiments with Steam and Water in a Large-Diameter Vertical Tube

An experimental study on flooding in a large-diameter tube is being conducted. In a countercurrent, two-phase-flow system, flooding can be defined as the onset of flow reversal of the liquid component that results in cocurrent flow. Flooding can be perceived as a limit to two-phase countercurrent flow, meaning that pairs of liquid and gas flow rates exist that define the envelope for stable countercurrent flow for a given system. Flooding in the AP600 pressurizer surge line can affect the vessel refill rate following a small-break loss-of-coolant accident. Analysis of hypothetical severe accidents with current simplified flooding models shows that these models represent the largest uncertainty in steam generator tube creep rupture. During a hypothetical station blackout scenario without auxiliary feedwater recovery, should the hot leg become voided, the pressurizer liquid will drain to the hot leg, and flooding may occur in the surge line. Experiments were conducted using a 76.2-mm (3-in.)-diam tube along with subcooled water and superheated steam as the working fluids at atmospheric pressure. Water flows down the inside of the tube as an annulus while the steam flows upward in the middle. Water flow rates vary from 0.00022 to 0.00076 m3/s (3.5 to 12 gal/min), and the water inlet temperature is ˜70°C. The steam inlet temperature is ˜110°C. It was found that a larger steam flow rate was needed to achieve flooding for a lower water flow rate and for a higher water flow rate. These unique data for flooding in steam-water systems in large-diameter tubes will reduce uncertainty in flooding models currently utilized in reactor safety codes.

Numerical Simulation Analysis of the Ultimate Bearing Capacity of the Circular Steel Tube K-Joints Reinforced with the Concrete

In order to solve the insufficient bearing capacity of the large-diameter circular steel tube K-Joints, the chord in the joint zone is filled with the concrete. The reinforcement of bearing capacity that the concrete makes to K-Joints is researched with the finite element numerical simulation method, in consideration of the material nonlinearity and the geometric nonlinearity. The numerical computation of bearing behavior is employed to eighteen groups of the large-diameter circular steel tube K-Joints and reinforced ones with the concrete (RK-Joints). The failure styles and the influencing factors of bearing capacity are analyzed with RK-Joints. The result shows that the bearing capacity of K-Joints is enhanced significantly by the concrete filled in the chord in the joint zone. The load-deformation curves reveal the changing regularity that the bearing capacity of joint follows the relevant parameters. It can provide reference for the engineering application of RK-Joints.

The effects of the evaluation method on the average heat transfer coefficient for a mini-channel tube bundle

Heat transfer performance of tube bundles, which are widely used in practical applications, was investigated. Experimental and numerical studies for tube bundles were performed with a tube diameter of 1.5 mm and a shell-side Reynolds number of 3,000 ⩽ Re ⩽ 7,000. The average convective heat transfer coefficients of the tube bundles in most previous investigations were based on the difference between the bulk fluid temperature and the average surface temperature of the tubes. However, the tubular heat exchanger design process uses the concepts of overall thermal resistance and the log-mean-temperature-difference (LMTD), which is defined as the temperature difference between two working fluids. This paper examines the variation of the shell-side convective heat transfer coefficients of tube bundles caused by the two different evaluation methods. The comparison showed that the average Nusselt number evaluated based on the LMTD method is 22.6% smaller than that based on the surface temperature method. This shows that it is necessary to consider the discrepancy in the heat transfer coefficient definition for proper design of heat exchangers. The Zukauskas correlation developed for larger diameter tubes and higher Reynolds numbers are still in good agreement with experimental data for the present small diameter tube bundle within a discrepancy of 4.7%.

Study on the Buckling of Steel Columns with one Special-Shaped Section

One large-diameter and non-circular steel tube was adopted in Henan TV tower (China). This special cross-section consists of three flat plates welded to three arc plates one by one. This paper studies the critical local buckling behavior of steel plates by using the finite element analysis method. Initial geometric imperfections and residual stresses presented in steel plates, material yielding and strain hardening were taken into account in the nonlinear analysis. An experimental study was performed to verify the capacity ability of this special steel tube. Based on the results obtained from the nonlinear finite element analyses and experiments, a set of design recommendations are provided for ensuring the safety of this special tube in Henan TV tower.

Ultrasound assessment of the motion patterns of human flexor digitorum superficialis and profundus tendons with speckle tracking

The purposes of our study were to correlate ultrasonographically measured and joint angle estimated excursions of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) tendons of the hand and to estimate the relative motion of FDS and FDP while gripping cylinders of standard diameter in normal human subjects. Thirty wrists from 15 human subjects were imaged with an ultrasound scanner. Speckle tracking was used to measure the excursion of the FDS and FDP tendons. The tendon excursions necessary to grip three differently sized acrylic tubes were measured and correlated with the corresponding finger joint angles. The FDP/FDS excursion ratio was calculated. The Pearson's correlation coefficient between the FDS excursion and MP + PIP joint angle was 0.61. The Pearson's correlation coefficient between the FDP + FDS excursion and the DIP + PIP + MP joint angle was 0.67. The FDP/FDS excursion ratio was smaller for larger excursions (gripping a smaller diameter tube) and larger for small excursions (gripping a larger diameter tube, P < 0.01). These data suggest that speckle tracking may be a useful method to discriminate the relative motion of flexor tendons, which in turn may be relevant in evaluating tendon function, for example after tendon injury.

Investigation of convection-based energy systems using supercritical carbon dioxide as a working fluid

Carbon dioxide is the best retrofit to meet the future demand on long-term environmental friendly working fluids. The volumetric efficiency of supercritical carbon dioxide is very high, which makes it a promising working fluid in convection-based energy systems with high efficiency and small volume. Here, the natural convection of supercritical carbon dioxide driven only by temperature difference is studied in circulation loops. The Reynolds number of the flow is about 104 when the temperature difference is only 20 K, about two orders of magnitude greater than that of water. Furthermore, the heat transfer rate is about 3 times as great as that of water. These results demonstrate the potential of carbon dioxide as a working fluid in solar thermal conversion, nuclear power and waste heat utilization, etc. The influence of the tube diameter on the flow and heat transfer characteristics is discussed. Both the velocity and the Nusselt number are greater in the loop with a larger tube diameter where flow reversal occurs periodically. It is found that flow reversal degrades the system efficiency of the natural circulation loop. Therefore, the optimization about the geometric configuration of convection-based energy systems using carbon dioxide as a working fluid does exist and is very important for their safe and effective operation. Copyright © 2010 John Wiley & Sons, Ltd.

A Model for the Strength of Yarn-like Carbon Nanotube Fibers

A model for the strength of pure carbon nanotube (CNT) fibers is derived and parametrized using experimental data and computational simulations. The model points to the parameters of the subunits that must be optimized in order to produce improvements in the strength of the macroscopic CNT fiber, primarily nanotube length and shear strength between CNTs. Fractography analysis of the CNT fibers reveals a fibrous fracture surface and indicates that fiber strength originates from resistance to nanotube pull-out and is thus proportional to the nanotube-nanotube interface contact area and shear strength. The contact area between adjacent nanotubes is determined by their degree of polygonization or collapse, which in turn depends on their diameter and number of layers. We show that larger diameter tubes with fewer walls have a greater degree of contact, as determined by continuum elasticity theory, molecular mechanics, and image analysis of transmission electron micrographs. According to our model, the axial stress in the CNTs is built up by stress transfer between adjacent CNTs through shear and is thus proportional to CNT length, as supported by data in the literature for CNT fibers produced by different methods and research groups. Our CNT fibers have a yarn-like structure in that rather than being solid, they are made of a network of filament subunits. Indeed, the model is consistent with those developed for conventional yarn-like fibers.

Optical and Vibrational Properties of Toroidal Carbon Nanotubes

Toroidal carbon nanotubes (TCNTs), which have been evaluated for their potential applications in terahertz communication systems, provide a challenge of some magnitude from a purely scientific perspective. A design approach to TCNTs, as well as a classification scheme, is presented based on the definition of the six hollow sections that comprise the TCNT, slicing each of them to produce a (possibly creased) planar entity, and projecting that entity onto a graphene lattice. As a consequence of this folding approach, it is necessary to introduce five- and seven-membered rings as defect sites to allow the fusing together of the six segments into final symmetric TCNTs. This analysis permits the definition of a number of TCNT geometry families containing from 108 carbons up to much larger entities. Based on density functional theory (DFT) calculations, the energies of these structural candidates have been investigated and compared with [60]fullerene. The structures with the larger tube diameters are computed to be more stable than C(60) , whereas the smaller diameter ones are less stable, but may still be within synthetic reach. Computational studies reveal that, on account of the stiffness of the structures, the vibrational frequencies of characteristic low-frequency modes decrease more slowly with increasing ring diameter than do the lowest optical excitation energies. It was found that this particular trend is true for the "breathing mode" vibrations when the diameter of the tubes is small, but not for more flexible toroidal nanotubes with larger diameters.