Outermost Tubes Research Articles

Design and simulation of a novel reactor for the H2 storage mechanism in methylcyclohexane

The present study addresses the modeling and simulation of an intensified dehydrogenation reactor-heat exchanger system designed for a H2-based 2.0 MW mobile power plant. The reactor consisted of a multiple triple-tube system where the catalyst was packed in the middle tube while hot exhaust gases passed through the innermost and outermost tubes. A basecase design was defined and the reactor was simulated using the rigorous kinetics developed elsewhere for the MCH dehydrogenation. The parabolic differential equations of mass and heat energy were solved and the axial and radial conversion and temperature profiles were computed. Moreover, the effects of the number of longitudinal fins on the reactor tubes as well as the flow direction were also studied. With 12 number of fins on the reactor tube, the MCH conversion increased from 70.8% to 81.8%, with a corresponding increase in the average bed temperature from 569.3 K to 577.6 K and hydrogen yield from 37.05 kmol h−1 to 42.79 kmol h−1.

Numerical simulation of the thermal radiation and convective heat transfer characteristics of the second superheater in a coal-fired thermal power plant boiler

Superheaters are critical components in a coal-fired power plant because they sustain the highest tube wall (metal) temperature point in the boiler. The pressurized steam flowing in superheater tubes is mainly heated via the thermal radiation and convection from the combustion gas in the radiant zone. To prevent the bursting of superheater tubes caused by the high-temperature creep, corrosion, and thermal fatigue for proper plant operation and maintenance, accurately predicting complex heat transfer characteristics, estimating the local temperature, and assessing the heat flux of the superheater are essential. In this study, a computational fluid dynamics model of the boiler and the superheater in a coal-fired power plant was developed using radiation and turbulence models. The local metal temperature and heat flux of the superheater were evaluated by calculating the heat exchange between the combustion gas and pressurized steam flows in the tubes. The calculated values of the steam outlet temperature accurately matched the values measured using the equipment at the plant, and thermal radiation was confirmed to be dominant in the boiler. The metal temperature and heat flux of the superheater at the outermost heat transfer tubes, which receive the maximum thermal radiation, were larger than those of the superheater at the inner tubes. Therefore, the outermost tubes were determined to be under the most severe thermal conditions despite having a higher mass flow rate of steam. Additionally, the heat fluxes on the center lines of superheater tube panels were higher due to the large gap between the adjacent heat transfer tubes at the bent part.

Environmental effects, intertube interactions and σ-π bond re-hybridization in bundles of double- and triple-walled carbon nanotubes

In the literature, fine structuring and mixing of the radial breathing mode frequencies (ωRBM) due to intertube interactions (Ωint) for specific inner tubes in free-standing isolated double-walled nanotubes (DWNTs) have been addressed. Questions and arguments related to the Ωint nature and its dependence with the nanotube’s growth processes have also been addressed. Later, studies involving bundled (innermost tubes) and isolated (innermost and middle tubes) of triple-walled nanotubes (TWNTs) for diameters between 0.5 and 1.7 nm found no ωRBM fine structuring or mixing but suggested that the Ωint depends on the tube’s metallicity. Here, Raman Spectroscopy is combined with transmission electron microscopy (TEM) to show that for bundled DWNTs and TWNTs measured in a broad diameter range (0.5≤dt≤2.8 nm) three effects seem to be fundamental to explain the ωRBM behavior of their constituent tubes: interactions with the external environment, metallicity-independent Ωint and curvature effects due to σ-π bonds re-hybridization. Our results display no evidence of ωRBM mixing but show that Ωint has no apparent dependence with the growth process and that Ωint has the same nature of the tube interactions with the environment. The manuscript also discusses the thus far elusive TWNT outermost tubes ωRBM.

Experimental and numerical investigation of impact behavior of nested tubes with and without honeycomb filler

In this study, the axial impact behavior of empty and filled nested tubular structures is parametrically investigated both experimentally and numerically. The experiments are conducted by a drop test machine and the numerical studies are conducted using LS-DYNA software. The tubes are made of AL6063 aluminum alloy and a regular hexagonal honeycomb filler of ABSplus plastics. The external sizes of all specimens are held constant for meaningful comparisons. The primary objective of this study is to investigate the effects of using a honeycomb filler, increasing the numbers of tubes, and changing the lengths of the tubes on the nested tube structures. The results of single, double, triple, quadruple, and quintuple tubular structures, with and without the honeycomb filler, are compared in terms of collapse mechanism and common crashworthiness indicators, namely, peak crash force, specific energy absorption, and crash force efficiency. The results indicate that the honeycomb filler has a significant effect on the collapse mechanism of single and nested tubular structures, and the filled nested tube systems with fewer tubes are better than empty ones in terms of specific energy absorption and crash force efficiency. In addition, the energy absorption of the nested tubes could be increased in certain cases with the increase in the number of tubes and tube lengths increasing from the innermost to outermost tubes. Therefore, the double nested tube structures with a honeycomb filler are recommended for crash box designs.

Nonlocal vibrations and potential instability of monolayers from double-walled carbon nanotubes subjected to temperature gradients

Using nonlocal elasticity theory of Eringen, free transverse thermo-elastic vibrations of vertically aligned double-walled carbon nanotubes (DWCNTs) with a membrane configuration is going to be explored methodically. Accounting for nonlocal heat conduction along the side walls of DWCNTs with allowance of heat dissipation from the outer surfaces, the nonlocal temperature fields within the constitutive tubes are assessed for a steady-state regime. The van der Waals interactional forces between atoms of each pair of DWCNTs as well as the innermost and outermost tubes are displayed by laterally continuous springs whose constants are appropriately evaluated. By establishing suitable discrete and continuous models on the basis of the Rayleigh and higher-order beam theories, nonlocal in-plane and out-of-plane vibrations of thermally affected nanosystems of monolayers of DWCNTs are examined and discussed. The critical values of the temperature change are stated explicitly and the role of influential factors on this crucial factor as well as the free vibration behavior are investigated in some detail. Through conducting a fairly comprehensive numerical study, the influences of the slenderness ratio, temperature change in the low and high temperatures, number of nanotubes, small-scale parameter, intertube distance, and stiffness of the surrounding environment on the nonlocal-fundamental frequency are explained. The obtained results from this work could provide crucial guidelines for the design of membranes or even jungles of vertically aligned DWCNTs as thermal interface nanostructures.

Numerical investigation on heat transfer performance of planar elastic tube bundle by flow-induced vibration in heat exchanger

This study numerically investigated the heat transfer enhancement of planar elastic tube bundle by flow-induced vibration based on a two-way fluid structure interaction (FSI) model in the range of inlet velocity 0.2–0.5m/s. This two-way FSI calculation involved the unsteady, three-dimensional incompressible Navier–Stokes equation solved with finite volume approach and the dynamic equilibrium equation of tube bundle solved with finite element method combined with dynamic mesh scheme, which was verified by comparing with the published experimental results. Then the Nusselt number of the circumference of tube, local position, single tube and the overall tube bundle was presented. The performance evaluation criterion (PEC) was selected to study the heat transfer performance of planar elastic tube bundle. Results show that vibration frequency dominates the heat transfer enhancement at the local position near the mass-block. In the middle of tube bundle, vibration amplitude plays a significant role on heat transfer enhancement. Therefore, the average heat transfer enhancement of 12.97% and 4.58% at the middle two tubes is higher than that of 5.37% and 2.4% at the innermost and outermost tubes when the inlet velocity is 0.2m/s and 0.5m/s, respectively. In the range of inlet velocity 0.2–0.5m/s, flow-induced vibration contributes to enhancing the heat transfer of planar elastic tube bundle at low inlet velocity, resulting in the heat transfer enhancement of 8.26%, 6.07%, 5.67% and 3.91% respectively. Compared to the mechanical vibration strengthening heat transfer technology, flow-induced vibration plays an advantage on energy conversion to improve heat transfer. PEC indicates that the higher inlet velocity is sometimes more acceptable to obtain a higher heat transfer coefficient in the industry production.

A nonlocal meshless solution for flexural vibrations of double-walled carbon nanotubes

The true understanding of free vibration of double-walled carbon nanotubes (DWCNTs) plays a vital role in optimal design and dynamic control of such nanostructures. This paper is aimed to examine free flexural vibration of lengthy DWCNTs with arbitrary boundary conditions in the framework of nonlocal elasticity theory. The DWCNTs are embedded in an elastic medium and are subjected to initially axial forces. Equivalent continuum structures associated with the innermost and outermost tubes of the DWCNT are considered. The transverse and rotational interactions of the DWCNT with the surrounding elastic medium are also taken into account. The generalized equations of motion of lengthy DWCNTs are established based on the nonlocal Rayleigh beam theory. Seeking an analytical solution to the developed equations, particularly in their general form, is a very problematic task. As an alternative solution, an efficient numerical scheme is proposed. The effects of slenderness ratio, small-scale parameter, lateral and rotational stiffness of the surrounding matrix, and initially axial force on the first five natural frequencies of DWCNTs under different boundary conditions are comprehensively scrutinized.

Characterization of free vibration of elastically supported double-walled carbon nanotubes subjected to a longitudinally varying magnetic field

Free transverse vibrations of elastically supported double-walled carbon nanotubes (DWCNTs) subjected to axially varying magnetic fields are examined. Using nonlocal Rayleigh beam theory, the explicit expressions of the governing equations are obtained and then numerically solved via an efficient numerical scheme. For magnetically affected DWCNTs with simply supported, fully clamped, simple-clamped, and clamped-free ends, the flexural frequencies as well as the corresponding vibration modes are evaluated for different varying magnetic fields. The influences of the small-scale parameter and the magnetic field strength on the dominant flexural frequencies of the DWCNTs are explained and discussed. The results indicate that the vibration characteristics of DWCNTs can be significantly affected by the axially varying magnetic field. The role of variation of the axial magnetic field on the vibrational mode patterns of both the innermost and outermost tubes is also revealed. For a special applied magnetic field, the alteration from coaxial to noncoaxial vibration pattern is also reported. The obtained results display that the flexural frequencies magnify with the magnetic field strength. Generally, the variation of the magnetic field strength has more influence on the variation of the frequencies of DWCNTs with higher small-scale parameters. This matter is mainly attributed to the incorporation of the size effect into the nonlocal Lorentz forces.

Vibration analysis of elastically restrained double-walled carbon nanotubes on elastic foundation subjected to axial load using nonlocal shear deformable beam theories

In the context of nonlocal continuum theory, seeking an analytical solution to the equations of motion of stocky double-walled carbon nanotubes (DWCNTs) with arbitrary boundary conditions is a very problematic task. Thereby, proposing efficient numerical techniques for frequency analysis and optimal design of such nanostructures is of great advantageous. Herein, free transverse vibration of an elastically supported stocky DWCNT embedded in an elastic matrix under initial axial force is of interest. To this end, the equivalent continuum structures (ECSs) associated with the innermost and outermost tubes are taken into account. The interaction of the DWCNT with its surrounding matrix is modeled using lateral and rotary continuous springs. Through consideration of interlayer van der Waals forces via an equivalent spring system, the two tubes are appropriately interacted. Using Hamilton's principle, the dimensionless equations of motion of elastically supported DWCNTs are established using nonlocal Rayleigh, Timoshenko, and higher-order beam theories. The unknown fields of the equations of motion for each model are discretized in the spatial domain using reproducing kernel particle method. After tedious calculations, the set of eigenvalue equations pertinent to each model is extracted and numerically solved. The convergence checks of the proposed numerical models in predicting flexural frequencies of DWCNTs are carried out. The obtained results are also compared with those of other works and a reasonably good agreement is achieved. Through various numerical studies, the influences of slenderness ratio, ratio of the mean radius to the thickness of the ECSs, small-scale parameter, initial axial force, lateral and rotational stiffness of the surrounding matrix on the flexural frequencies of stocky DWCNTs are carefully examined for different boundary conditions. The capabilities of the proposed nonlocal models in capturing the flexural frequencies of stocky DWCNTs are discussed as well.

Triple-Coupled Intense Atmospheric Pressure Plasma Jet From Honeycomb Structural Plasma Device

The authors describe an intensified plasma jet from a honeycomb structural plasma jet array at atmospheric pressure. Despite an equally distributed gas flow, the outermost tubes did not produce strong individual plumes. Rather, the plasma plumes were drawn into the central plume, which was, in turn, amplified. As a result, the optical intensity from the 19-array plasma jet was nearly twice greater than that from a conventional single plasma jet at the maximum plasma emission condition of each device under identical applied voltages.

Raman evidence of the interaction between multiwalled carbon nanotubes and nanostructured TiO2

AbstractNanocomposites of carbon nanotubes and titanium dioxide (TiO2) have attracted much attention due to their photocatalytic properties. Although many examples in the literature have visualized these nanocomposites by electron microscopic images, spectroscopic characterization is still lacking with regard to the interaction between the carbon nanotube and TiO2. In this work, we show evidence of the attachment of nanostructured TiO2 to multiwalled carbon nanotubes (MWNTs) by Raman spectroscopy. The nanostructured TiO2 was characterized by both full‐width at half‐maximum (FWHM) and the Raman shift of the TiO2 band at ca 144 cm−1, whereas the average diameter of the crystallite was estimated as approximately 7 nm. Comparison of the Raman spectra of the MWNTs and MWNTs/TiO2 shows a clear inversion of the relative intensities of the G and D bands, suggesting a substantial chemical modification of the outermost tubes due to the attachment of nanostructured TiO2. To complement the nanocomposite characterization, scanning electronic microscopy and X‐ray diffraction were performed. Copyright © 2011 John Wiley & Sons, Ltd.

Application of nonlocal beam models to double-walled carbon nanotubes under a moving nanoparticle. Part I: theoretical formulations

The current work suggests mathematical models for the vibration of double-walled carbon nanotubes (DWCNTs) subjected to a moving nanoparticle by using nonlocal classical and shear deformable beam theories. The van der Waals interaction forces between atoms of the innermost and outermost tubes are modeled by an elastic layer. The equations of motion are derived for the nonlocal double body Euler–Bernoulli, Timoshenko and higher-order beams connected by a flexible layer under excitation of a moving nanoparticle. Analytical solutions of the problem are provided for the aforementioned nonlocal beam models with simply supported boundary conditions. The dynamical deflections and nonlocal bending moments of the innermost and outermost tubes are then obtained during the courses of excitation and free vibration. Finally, the critical velocities of the moving nanoparticle associated with the nonlocal beam theories are expressed in terms of small-scale effect parameter, geometry, and material properties of DWCNTs.

Application of nonlocal beam models to double-walled carbon nanotubes under a moving nanoparticle. Part II: parametric study

The capabilities of the proposed nonlocal beam models in the companion paper in capturing the critical velocity of a moving nanoparticle as well as the dynamic response of double-walled carbon nano- tubes (DWCNTs) under a moving nanoparticle are scrutinized in some detail. The role of the small-scale effect parameter, slenderness of DWCNTs and velocity of the moving nanoparticle on dynamic deflections and nonlocal bending moments of the innermost and outermost tubes as well as their maximum values are then investigated. The results reveal that the critical velocity increases with the slenderness of DWCNTs and the magnitude of the van der Waals interaction force. Nevertheless, the critical velocity generally decreases with the small-scale effect as well as the ratio of the mean diameter to the thickness of the innermost tube. Additionally, the predicted maximum dynamic deflections and nonlocal bending moments of the innermost and outermost tubes by using the nonlocal Euler-Bernoulli and Timoshenko beam theories are generally the lower and upper bounds of those obtained by the nonlocal higher-order beam theory (NHOBT). In the case of λ1 < 20, the use of the NHOBT is highly recommended for more realistic prediction of dynamic response of DWCNTs under a moving nanoparticle.

Low frequency vibration of multiwall carbon nanotubes with heterogeneous boundaries

Heterogeneous end constraints are imposed on multiwall carbon nanotubes (MWCNTs) by sequentially clamping one end of their originally simply supported constituent tubes. The finite element method is employed to study the vibration of such MWCNTs with an emphasis on the effect of the mixed boundary conditions. The results show that the clamping process constantly enhances the dynamic stiffness of MWCNTs, which leads to substantial frequency increase up to 50% and, in some cases, the transformation of the fundamental vibration mode. In particular, the vibration frequency is always found to be most sensitive to fixing the outermost tubes, showing the critical role of this individual tube in determining the structural stiffness of the whole MWCNTs as a coupled system.

Study of direct injection in ICP-AES using a commercially available micronebulizer associated with a reduced length torch

The most critical dimensions of an ICP torch are the distance between the outer gas inlet and the top of the intermediate tube, the magnitude of the space left between the two outermost tubes and the overall torch diameter. The reduction in the torch length while keeping constant the critical dimensions allows injecting directly the aerosol generated by a conventional pneumatic concentric micronebulizer into the plasma by simply fitting it at the torch base. This approach is presented for the first time in this paper. A MicroMist has been used in conjunction with the reduced length torch. The new system has been evaluated in terms of ICP-AES sensitivity, signal stability and matrix effects caused by nitric acid. The results have been compared against those provided by a double pass, a low inner volume cyclonic spray chamber (Cinnabar), a torch integrated sample introduction system (TISIS) and a direct injection high efficiency nebulizer (DIHEN). When using the reduced length torch, at the optimum nebulizer liquid and gas flow rates in terms of sensitivity (i.e., 60 μl min−1 and 0.15 l min−1, respectively) the MicroMist generated aerosols with droplets whose diameters were even higher than 100 μm. Therefore, the selection of this nebulizer was based on criteria other than signal stability such as robustness and lack of tip blocking problems. Despite of this, the results were not as different from those afforded by a DIHEN as expected. For the reduced length torch, the maximum signal improvement factor with respect to a double pass spray chamber was from 1.5 to about 3 depending on the liquid flow rate and the emission line. Similar limits of detection were found for both systems. Meanwhile, these ranges were from about 1.5 to 4 and from 1.3 to 7 for the TISIS and DIHEN, respectively. Note that under its optimum operating conditions, the DIHEN generated droplets with diameters above 30 μm. As expected, at low liquid flow rates, wash out times were more than 20 times shorter for the reduced length torch than for a double pass one. As regards the matrix effects, it was found that the two direct sample injection systems tested behaved alike, the interferences becoming more severe as the delivery flow rate went up. A problem found with the MicroMist associated to the reduced length torch was that the signal stability was poorer than for the remaining systems.

Computer-aided design and performance evaluation of an indirect type helical tube ultra-high temperature (UHT) milk sterilizer

A laboratory model indirect type helical tube ultra-high temperature (UHT) milk sterilizer was designed and tested. The helical triple tube heat exchanger modeling and simulation was carried out for heating milk from 90°C to 150°C in the annulus of the triple tube using steam in the innermost and outermost tubes. The holding section was designed based on an Arrhenius model to reduce B. stearothermophilus microorganisms in the milk by 8 log cycles at a sterilization temperature of 150°C with a residence time of 2.64 s. The helical double tube cooling section modeling and simulation was carried out to cool milk from 150°C to 90°C by using tap water. The lengths of heating, holding and cooling sections were found to be 2.28, 1.0 and 2.08 m, respectively. The set-up was tested for UHT processing of milk at the flow rate of 135 l/h. The microbial effectiveness of the process was tested separately with inoculated samples of B. subtilis and B. stearothermophilus microorganisms and found to be effective during processing as well as during storage of the UHT processed milk for two months. The quality attributes (i.e. color and viscosity) of the UHT processed milk were also tested during the processing and storage periods. Overall there was no marked change in quality attributes for which the processed milk can be considered unacceptable.

Exospheric model of the plasmasphere

ISEE observations indicate that after prolonged quiet periods, the saturated equatorial density decreases exponentially with the radial distance in the plasmasphere. No hydrostatic barometric models fit the saturated equatorial density profiles, since those obtained with barometric maxwellian or lorentzian kinetic models decrease more slowly with the radial distance. Orbits of trapped particles are in complete thermal equilibrium with the escaping and ballistic ones only when the Coulomb collision time is much smaller than the inter-hemispheric flight time of the ions and electrons. At large radial distances and in the outermost flux tubes (i.e. for large L-parameters), this condition is not always fulfilled. It is shown that hydrostatic exospheric models with an unsaturated and L-dependent population of trapped particles can be used to fit the observed density profiles. Such hydrostatic exospheric models with unsaturated population of trapped particles can therefore be used to construct empirical 3D models of the density distribution in the plasmasphere. At small L and low altitudes, trapped orbits are saturated, while at large distances, these orbits are almost completely depleted. The fraction η( L) of required trapped particles necessary to obtain a good fit to observed equatorial density profiles is a function of L. We have determined this function by fitting our hydrostatic theoretical equatorial electron density profiles to that observed by Carpenter and Anderson (1992) from L=2 to 8 after a prolonged period of quiet conditions. Differences between maxwellian and lorentzian hydrostatic barometric models and exospheric hydrostatic models for different values of the kappa index are presented.

Design and testing of a small scale indirect type ultra high temperature (UHT) milk sterilizer

A small scale ultra high temperature (UHT) sterilizer suitable for rural tropical cooperatives was designed and tested. An indirect, tubular type UHT sterilizer is better suited to small scale operation and low investment cost. A triple tube heat exchanger (TTHE) was designed for heating milk from 95 to 145 °C in the annulus of the triple tube using steam in the innermost and the outermost tubes. The holding section was designed to provide 4.3 s residence time to reduce B. Stearothermophilus by 8 log cycles. The cooling section was designed to cool milk from 145 to 95 °C by using water. An iterative procedure was followed to develop software to handle design calculations. Heating, holding, and cooling sections were found to be of 3, 0.6 and 3.1 m length. The set up was tested with milk for determination of heat transfer coefficients.