598 research outputs found
Numerical Investigations of Coherent Structures in Axial Flow in Single Rod-Channel Geometry
No full textIn order to ramp up the conversion ratios and burn-up of nuclear reactors, it is inevitable to go to tightly packed fuel rods in the reactor. These nuclear reactors with tightly packed rod-bundles are characterized by interesting flow patterns, different from the ones encountered in regular channel and pipe flows. The correct prediction and control of the flow distribution is essential for the reactor design and safety assessment, and has been an active area of research in reactor thermal-hydraulics. Apart from the axial flow of coolant parallel to the rod bundles, there exists cross-flow between the sub-channels. The cross-flow promotes homogeneous enthalpy distribution and enhanced mixing between the coolant flowing in the sub-channels. Turbulent mixing is an important phenomenon, which influences the flow and temperature patterns in the rod bundles. Large-scale coherent structures along with transverse flow pulsations have been identified in the rod-rod and rod-wall gap regions. This large-scale structure has a quasi-periodic behavior and is considered an important factor for high mixing-rate. The aim of this work is to get a better understanding of the flow in a rod-bundle. This is done by performing numerical investigations on a simplified rod-channel geometry. The Unsteady Reynolds Averaged Navier Stokes equations are solved using the Computational Fluid Dynamics software OpenFOAM. Extensive benchmark and validation studies were done in order to determine the simulation technique that offers a good balance between computational cost and accuracy. The flow dynamics and the transport and mixing of a passive scalar due to the coherent structures are studied. Different turbulence models were used to study their effect on flow dynamics, and no major differences were observed. Following this, the computationally cheaper k-epsilon turbulence model with wall functions was chosen for the simulations. The time required for flow development in this geometry was significantly higher than that in regular turbulent channel or pipe flow. This led to different results than the ones observed in the experiments and in previous simulation results published in the literature. It was concluded that the flow in the experiments was not fully developed and that probably not enough time was used to allow flow development in the previous simulations. Our results indicate that the shear-layer becomes thinner and the number of structures decreased with flow development, which would explain the higher number of structures found in previous simulations. High values of velocity fluctuations and the kinetic energy due to these fluctuations indicated the presence of structures in the near-gap region. Large-scale three-dimensional counter-rotating sledge-shaped structures were observed via the flow visualization of resolved velocity. These structures were not only restricted to the gap region, but encompassed the entire flow domain. The high periodicity and stability of these structures indicate that they are not turbulence structures. The effect of gap-size on the coherent structures was studied, and this study suggested that the presence of more than one mechanism for the formation of these structures. A critical gap-size was obtained, at which the intensity of the structures has a maximum value, and a cut-off gap size was identified, at which a transition takes place between the two mechanisms. The coherent structures were found to play a significant role in both the transport and mixing of the passive scalar. The contribution were similar to that of the turbulent diffusion. The simulations indicate that the effect of the coherent structures on the transport and mixing of a passive scalar is of the same order of magnitude of the effect of the turbulent diffusion.Chemical EngineeringRadiation, Radionuclides & ReactorsApplied Science
Inertial effects on non-spherical particle rotation on turbulent channel flow
No full textWe investigated the rotation of non-spherical particles (rod-like and disk-like) in turbulent channel flow with focus on inertial effects. A direct numerical simulation (DNS) with an Eulerian-Lagrangian approach was performed. A wide range of particle aspect ratios, λ, ranging from 0.01 to 50 were considered for Stokes numbers St equal to 1 and 30. In the particle reference frame, statistical results reveal the importance of shape effect on the particle rotation. The rods (λ > 1) are spinning (rotation about their symmetry axis) more than tumbling (rotation about other axes) whereas disks (λ < 1) behave oppositely. With increasing particle inertia, i.e. higher St, the preferential tumbling of the disks and the spinning of the rods are reduced. We ascribe these observations to the varying degree of alignment of the particle symmetry axis with the fluid vorticity vector
Estimation of the Rod Velocity in Wood using Multi-frequency Guided Wave Measurements
No full text(c) The Author/sThis study presents a new approach for measuring the acoustic “rod velocity” in wood using guided wave measurements. The approach fits the acoustic guided wave longitudinal L(0,1) wave mode dispersion curve, through experimental guided wave phase velocity measurements taken over a range of frequencies. The rod velocity is obtained by measuring the phase velocity of the fitted L(0,1) wave mode dispersion curve at zero frequency. This technique is used to obtain rod velocity measurements for cylindrical wood and aluminium samples. The same approach was also performed on resonance measurements at a wide range of harmonics. These rod velocities are then compared to acoustic velocities obtained using the traditional time of flight and resonance methods.FALS
Remarkably Rich Variety of Nanostructures and Order-Order Transitions in a Rod-Coil Diblock Copolymer
No full textA remarkably rich variety of nanophase-separated structures and various order-order transitions are observed in a series of low-molecular weight (MW) rod-coil block copolymers (BCPs) with the rod blocks of different lengths (L-Rod's). The rod-coil diblock copolymer studied herein is poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS), in which PMPCS is a rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior. When the polymerization degree of the PMPCS rod block (N-Rod) is less than 32 (L-Rod < 8 nm), the PMPCS block is always amorphous in the entire temperature range. And the corresponding PDMS-b-PMPCS BCPs with N-Rod from 11 to 29 and the volume fraction of the PMPCS rod (f(Rod)) from 43% to 67% self-assemble into various equilibrium nanostructures after annealed at temperatures above the glass transition temperatures of the PMPCS blocks. When N-Rod = 11 and f(Rod) = 43%, the BCP forms a lamellar structure (LAM); when N-Rod = 15 and f(Rod) = 51%, the BCP forms a double gyriod structure (GYR) ; when N-Rod = 20 and f(Rod) = 57%, the BCP forms a GYR structure after annealed below 180 degrees C and transforms to the Fddd structure after annealed above 180 degrees C; when N-Rod = 29 and f(Rod) = 67%, the nanostructure of the BCP after annealed below 180 degrees C is hexagonally packed cylinders (HEX) and changes to a body centered cubic structure (BCC) after annealed above 180 degrees C. When N-Rod > 32 (L-Rod > 8 nm), the PMPCS rod block is amorphous at low temperatures and transforms to a stable columnar LC phase after annealed at high temperatures. Correspondingly, the PDMS-b-PMPCS BCP with N-Rod = 44 and f(Rod) = 75% forms a HEX structure after annealed at lower temperatures at which the PMPCS block is amorphous, and the nanostructure transforms to LAM after the sample is annealed at higher temperatures at which the PMPCS block enters into the LC phase. Therefore, only by a small change of the rod length in the low-MW PDMS-b-PMPCS rod-coil BCPs, various nanostructures including LAM, GYR, Fddd, HEX, and BCC are obtained. In addition, by increasing annealing temperatures, GYR-to-Fddd and HEX-to-BCC transitions are observed in the BCPs with the amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC PMPCS block undergoes an isotropic-to-LC phase transformation.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000321793300022&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Polymer ScienceSCI(E)EI21ARTICLE135308-53164
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Durability Study of Membrane Electrode Assembly for Heavy-Duty Fuel Cells
No full textPolymer electrolyte membrane fuel cells (PEMFCs) are a zero emission replacement for heavy duty applications due to their range, energy density and fast refueling times.[1] In 2020, the U.S. Department of Energy (DOE) lunched the Million Mile Fuel Cell Truck (M2FCT) consortium to fund fuel cell R&D to meet heavy duty truck standards.[2] Durability studies focusing on heavy-duty applications for advanced materials testing under the M2FCT consortium have been extensively explored and continue to be analyzed to standardize the evaluation of next generation fuel cell materials.
This study combines in situ electrochemical characterization with ex situ analysis of single cell PEMFCs membrane electrode assemblies (MEAs) to predict long-term durability for heavy-duty applications. Various accelerated stress test (AST) parameters were analyzed to determine the stressors affecting the long-term durability. Local degradation resulting from repeated high voltage to low current cycles was analyzed by testing state-of-the-art materials. High potential holds at various conditions were analyzed to determine membrane chemical degradation. Repeated wet and dry cycles were performed to test the membrane mechanical durability. In situ electrochemical analysis include mass activity, electrochemical surface area, hydrogen crossover, and polarization curves were collected and compared among the MEAs. Ex situ analysis includes quantification of membrane thinning at end of life and fluoride emission rate measurement for water effluent throughout the test was conducted to study the membrane degradation.
Acknowledgement:
This work was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy (EERE), US DOE through the Million Mile Fuel Cell Truck (M2FCT) consortium, technology managers G. Kleen and D. Papageorgopoulos.
References:
David A. Cullen, K. C. Neyerlin, Rajesh K. Ahluwalia, Rangachary Mukundan, Karren L. More, Rodney L. Borup, Adam Z. Weber, Deborah J. Myers, and Ahmet Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation. Nat. Energy, 2021. 6(5): 462-474.
DOE Launches Two Consortia to Advance Fuel Cell Truck and Electrolyzer R&D. 2020; Available from: https://www.energy.gov/eere/articles/doe-launches-two-consortia-advance-fuel-cell-truck-and-electrolyzer-rd
VIBRATIONAL OPTICAL ACTIVITY IN SUCSTITUTED CYCLOHEXENES
No full textAuthor Institution:Raman optical activity (ROD) between 1300 and and vibrational circular dichroism (VCD) between 2800 and for (+)-P-menth-l-ene and (+)-p-menth-l-en-9-01. In addition, VCD of (+)-limonene in the CH-stretching region is also considered together with the ROD spectrum of (-)-limonene available in the literature. All three molecules have the same ring structure and differ only in the para-position. Correlations of the VCD and ROD data as well as VCD-ROD correlations lead to the conclusion that ring methylene modes make the dominant contributions to the vibrational optical activity in the regions investigated
Retinitis Pigmentosa GTPase Regulator (RPGR) protein isoforms in mammalian retina:insights into X-linked Retinitis Pigmentosa and associated ciliopathies
No full textMutations in the cilia-centrosomal protein Retinitis Pigmentosa GTPase Regulator (RPGR) are a frequent cause of retinal degeneration. The RPGR gene undergoes complex alternative splicing and encodes multiple protein isoforms. To elucidate the function of major RPGR isoforms (RPGR 1-19 and RPGR ORF15), we have generated isoform-specific antibodies and examined their expression and localization in the retina. Using sucrose-gradient centrifugation, immunofluorescence and co-immunoprecipitation methods, we show that RPGR isoforms localize to distinct sub-cellular compartments in mammalian photoreceptors and associate with a number of cilia-centrosomal proteins. The RCC1-like domain of RPGR, which is present in all major RPGR isoforms, is sufficient to target it to the cilia and centrosomes in cultured cells. Our findings indicate that multiple isotypes of RPGR may perform overlapping yet somewhat distinct transport-related functions in photoreceptors
Precise synthesis of a rod-coil type miktoarm star copolymer containing poly(n-hexyl isocyanate) and aliphatic polyester
No full textWell-defined hydroxyl end-functionalized poly(n-hexyl isocyanate), PHIC-(OH)(2) and PHIC-(OH)(3), as rod-type macroinitiators were synthesized by the Cu-catalyzed azide-alkyne cycloaddition reactions of azido end-functionalized PHIC with ethynyl alcohol derivatives. The PHIC-(OH)(2) and PHIC-(OH) 3 were suitable macroinitiators for the ring-opening polymerization of L-LA and epsilon-CL leading to the synthesis of novel rod-coil type miktoarm star copolymers, PHIC-b-PLLA(2), PHIC-b-PLLA(3), PHIC-b-PCL2, and PHIC-b-PCL3, with controlled molecular weights, narrow polydispersities, and controlled arm numbers. Additionally, the thermal and solution properties of the obtained miktoarm star copolymers along with the corresponding block copolymers, PHIC-b-PLLA and PHIC-b-PCL, were characterized by TGA, DSC, and DLS analyses
Author Correction: Grooved electrodes for high-power-density fuel cells
No full textCorrection to: Nature Energy. Published online 25 May 2023. This paper was originally published under a standard Springer Nature license (© The Author(s), under exclusive licence to Springer Nature Limited). It is now available as an open-access paper under a Creative Commons Attribution 4.0 International license, © The Author(s). The error has been corrected in the online version of the article
FEM analysis on the "self-sharpening" behavior of tungsten fiber/metallic glass matrix composite long rod
No full textIn the present manuscript the geornetrical model of tungsten fiber/metallic glass matrix (WF/MG) composite long rod is established based on the microstructure of composite, and a modified coupled thermo-mechanical constitutive model is employed to describe the high strength and high shear sensitivity of metallic glass (MG) matrix. Then the finite element method (FEM) simulations on the penetration of WF/MG composite long rod into the steel targets are conducted by integrating with related penetrating tests. The effects of impact velocity, target strength and initial shape of rod nose on the "self-sharpening" property of rod and the consequent penetrating performance are discussed in detail. Corresponding analysis shows that due to the high shear sensitivity of MG matrix, a significant "self-sharpening" behavior occurs in the composite rod, and the rod nose maintains as a sharp shape during the penetration. Comparatively, for-the tungsten heavy alloy (WHA) rod, the nose blunts as a "mushroom" shape due to the relatively high plasticity of material. The "self-sharpening" behavior leads to a relatively lower resistance for the rod, and thus the composite rod achieves a much better penetrating performance compared with the WHA one. Moreover, the impact velocity and the target strength play an important role for the "self-sharpening" property and the consequent penetrating performance of the WF/MG composite long rod, whereas the initial nose shape has a relatively slight effect. (C) 2015 Elsevier Ltd. All rights reserved.Science and Technology Development Foundation [2015B0201025]; key subject "Computational Solid Mechanics" of China Academy of Engineering Physics; National Outstanding Young Scientists Foundation of China [11225213]; National Natural Science Foundation of China [11221202]SCI(E)[email protected]
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