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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Experimental characterization of pressure drops and channel instabilities in helical coil SG tubes

Helical tube heat exchangers provide better heat transfer characteristics, an improved capability to accommodate stresses due to thermal expansions and a more compact design with respect to straight tube heat exchangers. For these advantages they are considered as an option for the Steam Generator (SG) of many new reactor projects of Generation III+ and Generation IV. In particular, their compactness fits well with the requirements of Small-medium Modular Reactors (SMRs) of integral design, where all the primary system components are located inside the reactor vessel. In this framework, thermal hydraulics of helical pipes has been studied in recent years by Politecnico di Milano in different experimental campaigns. Experiments have been carried out in a full-scale open loop test facility installed at SIET labs in Piacenza (Italy)), to simulate the SG of a typical SMR. The facility includes two helical pipes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. Following recently completed experimental campaigns dedicated to pressure drops and density wave instabilities, this paper deals with a new experimental campaign focused on both pressure drops (single-phase flow and two-phase flow, laminar and turbulent regimes) and flow instabilities. The availability of a large number of experimental data, in particular on two-phase flow, is of fundamental interest for correlation development, model validation and code assessment. Two-phase pressure drops have been measured in adiabatic conditions, ranging from 200 to 600 kg/m{sup 2}s for the mass flux, from 30 to 60 bar for the pressure and from 0.1 to 1.0 for the flow quality. The channel characteristics mass flow rate - pressure drop has been determined experimentally in the range 10-40 bar, varying the mass flow rate at a fixed value of the thermal flux. In addition, single-phase pressure drops have been measured in both laminar and turbulent conditions. Density wave instabilities have been studied at mass flux from 100 to 400 kg/m{sup 2}s and pressure from 10 to 20 bar, to confirm the particular behavior of the stability boundary in helical geometry at low pressure and low mass flow rate. Finally, starting from the unstable regions identified from the experimental channel characteristics, Ledinegg type instabilities have been investigated to drawn stability maps with complete stable and unstable regions in the dimensionless plane N sub-N pch. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Pressure drop measurements across a helical coil steam generator configuration - 15997

The primary heat transport loop heat exchanger within a Very-High Temperature Reactor (VHTR) is most commonly a Helical Coil Steam Generator (HCSG). This design has been regarded as having increased heat transfer within a smaller design in comparison to other heat exchangers. The fluidic heat transfer properties of the HCSG is still under evaluation in tangent with HTRs due to its complex design. Heat transfer between these layers relies heavily on their relative location and fluidic properties. A model was constructed to study the pressure field on a rod s surface as well as across multiple layers of helically facing tubes. Experiments focused on evaluating the impact relative geometry played on the overall pressure drop of the test section with inlet fluid velocity ranging from 0 to 3 m/s. A study to compare the experimental results with the Gaddis-Gnielinski Correlative theoretical values of pressure drop in tube bundles shows that the complex helical geometry results in pressure drops similar to that of fully developed rod bundle although the test facility is comprised of two half rod helical coils. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2017

Visualization of flow interaction in a channel flow produced by a pair of helically coiled rods

Helically coiled tubes have been considered in the design of steam generators or heat exchangers of next generation nuclear power plants. Its compactness, efficiency, and the capability to absorb thermal expansion are desirable for nuclear reactor designs. However, the complex flow generated by the helically coiled geometry can result in flow induced vibration (FIV). In order to predict the FIV, there have been efforts using Computational Fluid dynamics (CFD)4, 5 to be coupled with multi-mechanics simulations. However, it is required to validate CFD to have confidence in capturing the flow structure characteristics of the transient complex flow behavior and fluctuation. In the present study, a simplified helically coiled steam generator (HCSG) design experiment was built acquiring data for CFD validation purpose. It is not perfect representation of the multiple rods, however, the current test facility produced transient data of the flow interaction between the rods and flow fluctuations near the rods. Particle Image Velocimetry (PIV) was applied to visualize the turbulent flow in a channel with slant rods. The two dimensional turbulent flow fields were analyzed. The velocity components were defined as below. u = U + u' (1) v = V + v' (2) where u is the lateral velocity on x-axis, v is the vertical velocity on y-axis, U and V are the mean velocities, and u' and v' are the fluctuation. Reynolds number, Re, was calculated using the rod diameter, d = 15.9 mm, and the upstream approach velocity, V{sub 0} = 0.5 m/s. Re = V{sub 0} d/η (3) where η is the kinematic viscosity of water. The experiment was conducted at Re ≅ 9,000 considering the currently reasonable computing power. Flow visualization in a flow channel produced by helically coiled rods was performed using PIV. The mean velocity field and streamlines were presented in this paper. The vortical flow structures in the cavity between rods were observed. Because of the asymmetric geometry, the flow profile was skewed. Additional measurements and transient analysis will be conducted to produce benchmark data for FIV study using CFD. Also, a test facility with a 5-rod heat exchanger design is operating to produce more data as a further work.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Sulfono-g-AApeptides as a New Class of Nonnatural Helical Foldamer

Foldamers offer an attractive opportunity for the design of novel molecules that mimic the structures and functions of proteins and enzymes including biocatalysis and biomolecular recognition. Herein we report a new class of nonnatural helical sulfono-g-AApeptide foldamers of varying lengths. The crystal structure of the sulfono-g-AApeptide monomer S6 illustrates the intrinsic folding propensity of sulfono-g-AApeptides, which likely originates from the bulkiness of tertiary sulfonamide moiety. The two-dimensional solution NMR spectroscopy data for the longest sequence S1 demonstrates a 10/16 right-handed helical structure. Optical analysis using circular dichroism further supports welldefined helical conformation of sulfono-g-AApeptides in solution containing as few as five building blocks. Future development of sulfono-g-AApeptides may lead to new foldamers with discrete functions, enabling expanded application in chemical biology and biomedical sciences.

2D NMR spectroscopy↗
DOE OSTI2014

Generalized helicity and Beltrami fields

We propose covariant and non-abelian generalizations of the magnetic helicity and Beltrami equation. The gauge invariance, variational principle, conserved current, energy–momentum tensor and choice of boundary conditions elucidate the subject. In particular, we prove that any extremal of the Yang–Mills action functional 1/4 ∫{sub Ω}trF{sub μν}F{sup μν}d{sup 4}x subject to the local constraint ε{sup μναβ}trF{sub μν}F{sub αβ}=0 satisfies the covariant non-abelian Beltrami equation. -- Highlights: •We introduce the covariant non-abelian helicity and Beltrami equation. •The Yang–Mills action and instanton term constraint lead to the Beltrami equation. •Solutions of the Beltrami equation conserve helicity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Flow structures of the cross-flow over a five-layer helically coiled steam generator geometry

The helically-coiled heat exchanger (HCSG) offers advantages over straight tubes such as compactness in geometry, increased heat transfer coefficients, and capability to absorb thermal expansion. In this study, the time-resolved velocity field data of shell-side crossflow over a five-layer helically coiled steam generator geometry was obtained using Particle Image Velocimetry (PIV) between adjacent rods in four regions with up to 5000 frames per second (5 kHz) at approximately Re u = 3600. The slant rod bundle created different flow patterns in different regions. Proper Orthogonal Decomposition (POD) revealed high-energy-mode flow structures. Due to the confined flow channel geometry, the large flow structures were not advected resulting in no strong POD mode paring. Continuous Wavelet Transform (CWT) visualized the characteristics of flow fluctuation in time and frequency domain simultaneously, which ranged in Strouhal number, S u , between 0.03 and 1.17. In the wake regions of the rods, flows changed in both pattern and magnitude over time. Once a pattern formed, it continued for relatively shorter than straight tube arrays, before changing the pattern again. The histograms of the POD time coefficients presented this multimodal flow characteristics. The current study focuses more on whole flow field analyses in the selected regions rather than local point-wise analyses. Finally, this study revealed that various Strouhal numbers can exist in the helically coiled heat exchanger geometry, which was not observed in straight tube arrays. In particular, Strouhal numbers smaller than 0.1 do not seem to be caused by vortex shedding but seem to be the result of the combination of complex geometry and possible multimodal trends. Based on the current observation, further works would involve intensive instantaneous point-wise analyses on the interactions between important locations such as separation points, stagnation points, shear layers, etc.

42 ENGINEERING↗
DOE OSTI2015

Silicon-Copper Helical Arrays for New Generation Lithium Ion Batteries

The helical array (with 10 atom % Cu) exhibits 3130 mAh g–1 with 83% columbic efficiency and retains 83% of its initial discharge capacity after 100th cycle. Homogeneously distributed interspaces between the helical arrays accommodate high volumetric changes upon cycling and copper atoms form a conductive network to buffer the mechanical stress generated in the electrode while minimizing electrochemical agglomeration of Si. Also, ion assistance is believed to enhance the density of the helices at the bottom thus increasing the adhesion

anode↗
DOE OSTI2026

Helical Complex Ladder Polymer with Amplification of Asymmetry

Here, we report a Cu–salen helical complex ladder polymer (HLP-Cu) built from covalently fused Cu(Salen) units. Point chirality from enantiopure diamines directs the formation of a rigid, π-conjugated helix. Compared with a planar analogue (LLP-Cu) and the Cu(Salen) monomer, HLP-Cu exhibits up to 5-fold stronger circular dichroism and a maximum dissymmetry factor of 7.4 × 10 –3 . Titration experiments suggest a cooperative amplification of asymmetry arising from extended conjugation and helical geometry during helix formation. Time-dependent density functional theory calculations are employed to support this hypothesis, showing that increasing conjugation length and helical order enhance the rotational strength of contributing excited states via elevated magnetic transition moments and improved alignment of electric and magnetic transition moments. Our findings provide a new polymer platform for designing functional chiral materials with tunable chiroptical and spin-dependent properties.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Nonlinear stability of magnetic islands in a rotating helical plasma

Coexistence of the forced magnetic reconnection by a resonant magnetic perturbation (RMP) and the curvature-driven tearing mode is investigated in a helical (stellarator) plasma rotated by helical trapped particle-induced neoclassical flows. A set of Rutherford-type equations of rotating magnetic islands and a poloidal flow evolution equation is revisited. Using the model, analytical expressions of criteria of spontaneous shrinkage (self-healing) of magnetic islands and sudden growth of locked magnetic islands (penetration of RMP) are obtained, where nonlinear saturation states of islands show bifurcation structures and hysteresis characteristics. Considering radial profile of poloidal flows across magnetic islands, it is found that the self-healing is driven by neoclassical viscosity even in the absence of micro-turbulence-induced anomalous viscosity. Effects of unfavorable curvature in stellarators are found to modify the critical values. The scalings of criteria are consistent with low-{beta} experiments in the large helical device.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Effect of the helicity injection rate and the Lundquist number on spheromak sustainment

The dynamics of the magnetic relaxation process during the sustainment of spheromak configurations at different helicity injection rates is studied. The three-dimensional activity is recovered using time-dependent resistive magnetohydrodynamic simulations. A cylindrical flux conserver with concentric electrodes is used to model configurations driven by a magnetized coaxial gun. Magnetic helicity is injected by tangential boundary flows. Different regimes of sustainment are identified and characterized in terms of the safety factor profile. The spatial and temporal behavior of fluctuations is described. The dynamo action is shown to be in close agreement with existing experimental data. These results are relevant to the design and operation of helicity injected devices, as well as to basic understanding of the plasma relaxation mechanism in quasi-steady state.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Tunable magnetic helicity in Mn{sub 1−x}Fe{sub x}Ge: A Monte Carlo simulation

Motivated by recent experiments on B20 compound Mn{sub 1−x}Fe{sub x}Ge, we explore the variations of spin structures and magnetic helicity γ{sub m} in Mn{sub 1−x}Fe{sub x}Ge as a function of concentration x, using Monte Carlo simulation. We propose a simple spin model including hybrid Dzyaloshinskii-Moriya (DM) interactions with different signs in FeGe and MnGe. The results reveal a series of spin structures with helicity γ{sub m} varying continuously from negative values to positive values upon decreasing x (0.0 ≤ x ≤ 1.0). The simulated x-dependence of the spin structures is consistent with experimental observations, suggesting the validity of the present hybrid model. This study sheds light on control of magnetic helicity in helimagnets, by mixing crystals with different DM interactions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Helicity sensitive terahertz radiation detection by dual-grating-gate high electron mobility transistors

We report on the observation of a radiation helicity sensitive photocurrent excited by terahertz (THz) radiation in dual-grating-gate (DGG) InAlAs/InGaAs/InAlAs/InP high electron mobility transistors (HEMT). For a circular polarization, the current measured between source and drain contacts changes its sign with the inversion of the radiation helicity. For elliptically polarized radiation, the total current is described by superposition of the Stokes parameters with different weights. Moreover, by variation of gate voltages applied to individual gratings, the photocurrent can be defined either by the Stokes parameter defining the radiation helicity or those for linear polarization. We show that artificial non-centrosymmetric microperiodic structures with a two-dimensional electron system excited by THz radiation exhibit a dc photocurrent caused by the combined action of a spatially periodic in-plane potential and spatially modulated light. The results provide a proof of principle for the application of DGG HEMT for all-electric detection of the radiation's polarization state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

TWIST, WRITHE, AND HELICITY IN THE INNER PENUMBRA OF A SUNSPOT

The aim of this work is the determination of the twist, writhe, and self-magnetic helicity of penumbral filaments located in an inner sunspot penumbra. For this purpose, we inverted data taken with the spectropolarimeter on board Hinode with the SIR (Stokes Inversion based on Response function) code. For the construction of a three-dimensional geometrical model we applied a genetic algorithm minimizing the divergence of B-vector and the net magnetohydrodynamic force, consequently a force-free solution would be reached if possible. We estimated two proxies to the magnetic helicity frequently used in literature: the force-free parameter {alpha}{sub z} and the current helicity term h{sub c{sub z}}. We show that both proxies are only qualitative indicators of the local twist as the magnetic field in the area under study significantly departs from a force-free configuration. The local twist shows significant values only at the borders of bright penumbral filaments with opposite signs on each side. These locations are precisely correlated to large electric currents. The average twist (and writhe) of penumbral structures is very small. The spines (dark filaments in the background) show a nearly zero writhe. The writhe per unit length of the intraspines diminishes with increasing length of the tube axes. Thus, the axes of tubes related to intraspines are less wrung when the tubes are more horizontal. As the writhe of the spines is very small, we can conclude that the writhe reaches only significant values when the tube includes the border of an intraspine.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Study of magnetic helicity injection in the active region NOAA 9236 producing multiple flare-associated coronal mass ejection events

To better understand a preferred magnetic field configuration and its evolution during coronal mass ejection (CME) events, we investigated the spatial and temporal evolution of photospheric magnetic fields in the active region NOAA 9236 that produced eight flare-associated CMEs during the time period of 2000 November 23-26. The time variations of the total magnetic helicity injection rate and the total unsigned magnetic flux are determined and examined not only in the entire active region but also in some local regions such as the main sunspots and the CME-associated flaring regions using SOHO/MDI magnetogram data. As a result, we found that (1) in the sunspots, a large amount of positive (right-handed) magnetic helicity was injected during most of the examined time period, (2) in the flare region, there was a continuous injection of negative (left-handed) magnetic helicity during the entire period, accompanied by a large increase of the unsigned magnetic flux, and (3) the flaring regions were mainly composed of emerging bipoles of magnetic fragments in which magnetic field lines have substantially favorable conditions for making reconnection with large-scale, overlying, and oppositely directed magnetic field lines connecting the main sunspots. These observational findings can also be well explained by some MHD numerical simulations for CME initiation (e.g., reconnection-favored emerging flux models). We therefore conclude that reconnection-favored magnetic fields in the flaring emerging flux regions play a crucial role in producing the multiple flare-associated CMEs in NOAA 9236.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Properties of magnetic helicity flux in turbulent dynamos

We study the flux of small-scale magnetic helicity in simulations of driven statistically homogeneous magnetohydrodynamic turbulence in a periodic box with an imposed large-scale shear. The simulations show that in the regime of strong dynamo action the eddy-scale magnetic helicity flux has only two significant terms: advective motion driven by the large-scale velocity field and the Vishniac-Cho (VC) flux which moves helicity across the magnetic field lines. The contribution of all the other terms is negligible. The VC flux is highly correlated with the large-scale electromotive force and is responsible for large-scale dynamo action, while the advective term is not. The VC flux is driven by the anisotropy of the turbulence. We derive analytical expressions for it in terms of the small-scale velocity or magnetic field. These expressions are used to predict the existence and strength of dynamo action for different turbulent anisotropies and tested against the results of the simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Hypermagnetic helicity evolution in early universe: leptogenesis and hypermagnetic diffusion

We study hypermagnetic helicity and lepton asymmetry evolution in plasma of the early Universe before the electroweak phase transition (EWPT) accounting for chirality flip processes via inverse Higgs decays and sphaleron transitions which violate the left lepton number and wash out the baryon asymmetry of the Universe (BAU). In the scenario where the right electron asymmetry supports the BAU alone through the conservation law B/3−L{sub eR} = const at temperatures T > T{sub RL} ≅ 10 TeV the following universe cooling leads to the production of a non-zero left lepton (electrons and neutrinos) asymmetry. This is due to the Higgs decays becoming more faster when entering the equilibrium at T = T{sub RL} with the universe expansion, Γ{sub RL} ∼ T > H ∼ T{sup 2}, resulting in the parallel evolution of both the right and the left electron asymmetries at T < T{sub RL} through the corresponding Abelian anomalies in SM in the presence of a seed hypermagnetic field. The hypermagnetic helicity evolution proceeds in a self-consistent way with the lepton asymmetry growth. The role of sphaleron transitions decreasing the left lepton number turns out to be negligible in given scenario. The hypermagnetic helicity can be a supply for the magnetic one in Higgs phase assuming a strong seed hypermagnetic field in symmetric phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Advancing local helicity injection for non-solenoidal tokamak startup

Experiments on the A ~ 1 PEGASUS ST are advancing the physics and technology basis of Local Helicity Injection (LHI). LHI injects helicity with relatively intense electron current sources in the plasma edge. It creates high toroidal current, toroidally-averaged tokamak-like plasmas that have been efficiently transitioned to Ohmically driven tokamak plasmas. Tradeoffs between physics and engineering goals are tested with LHI systems on the low-field-side and the high-field-side of PEGASUS, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive, respectively. An extensive LHI source development campaign comparing active arc sources, passive and gas-effused electrode sources lead to the selection of active arc sources for present and next-step LHI deployments. LHI plasmas with net toroidal current I p =0.225 MA, T e >100 eV, and n e ~ 10 19 m -3 are attained to date. A predictive 0D power-balance model describes experimental I p (t) and partitions the active current drive sources. High-frequency MHD activity is found to be present during LHI current drive, in addition to n=1 modes previously found in NIMROD simulation and experiment. A new system of reduced MHD activity was detected where n=1 activity is suppressed, LHI CD efficiency improves, and long-pulse plasmas are sustained with V IND ~ 0.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

One-loop QCD and Higgs bosons to partons processes using six-dimensional helicity and generalized unitarity

We combine the six-dimensional helicity formalism of Cheung and O'Connell with D-dimensional generalized unitarity to obtain a new formalism for computing one-loop amplitudes in dimensionally regularized QCD. With this procedure, we simultaneously obtain the pieces that are constructible from four-dimensional unitarity cuts and the rational pieces that are missed by them, while retaining a helicity formalism. We illustrate the procedure using four- and five-point one-loop amplitudes in QCD, including examples with external fermions. We also demonstrate the technique's effectiveness in next-to-leading order QCD corrections to Higgs processes by computing the next-to-leading order correction to the Higgs plus three positive-helicity gluons amplitude in the large top-quark mass limit.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗