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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 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Investigation of the short argon arc with hot anode. I. Numerical simulations of non-equilibrium effects in the near-electrode regions

The atmospheric pressure arcs have recently found application in the production of nanoparticles. The distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed a one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play an important role in the operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable to the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on the current density. The model was extensively verified and validated against previous simulation results and experimental data. The Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In the case of strong anode cooling (water-cooled anode) when the anode is cold, temperature and plasma density gradients increase with current density, resulting in a decrease of the anode voltage (the absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with the current density, leading to a drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission, and the opposite trend in the VAC is observed. Here, the results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Numerical simulation of magnetized jet creation using a hollow ring of laser beams

Three dimensional FLASH magneto-hydrodynamic modeling is carried out to interpret the OMEGA laser experiments of strongly magnetized, highly collimated jets driven by a ring of 20 OMEGA beams. The predicted optical Thomson scattering spectra and proton images are in good agreement with a subset of the experimental data. Here, magnetic fields generated via the Biermann battery term are amplified at the boundary between the core and the surrounding of the jet. The simulation predicts multiple axially aligned magnetic flux ropes with an alternating poloidal component. Future applications of the hollow ring configuration in laboratory astrophysics are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

A flux-balanced fluid model for collisional plasma edge turbulence: Numerical simulations with different aspect ratios

We investigate the drift wave–zonal flow dynamics in a shearless slab geometry with the new flux-balanced Hasegawa-Wakatani model. As in previous Hasegawa-Wakatani models, we observe a sharp transition from a turbulence dominated regime to a zonal jet dominated regime as we decrease the plasma resistivity. However, unlike previous models, zonal structures are always present in the flux-balanced model, even for high resistivity, and strongly reduce the level of particles and vorticity flux. The more robust zonal jets also have a higher variability than in previous models, which is further enhanced when the computational domain is chosen to be elongated in the radial direction. In these cases, we observe complex multiscale dynamics, with multiple jets interacting with one another, and intermittent bursts. We present a detailed statistical analysis which highlights how the changes in the aspect ratio of the computational domain affect the third-order statistical moments, and thus modify the turbulent dynamics. By changing the aspect ratio and extending either the radial or the binormal direction, the present model study offers a better approximation to mimic turbulence in flux tube simulations for either very low magnetic shear or very high magnetic shear systems.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Numerical simulation of spall in lead accounting for its occurrence in solid as well as liquid phases

The capabilities of a recently-developed continuum-level model developed for the purpose of predicting the evolution of spall, including the compaction of damaged material, and applicable to the formation of spall in the material that is fully solid as well as material that is locally liquid, are evaluated against empirical data for lead. Here, the model accounts for inertial forces, elastoplastic constitutive behavior, viscosity, and surface tension. Predictions of spall layer thickness are shown to match the empirical data reasonably well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Numerical simulations of global Alfvén eigenmodes excitation and stabilization in NSTX-U

Three-dimensional nonlinear simulations of Alfvén eigenmodes in the subcyclotron frequency range demonstrate a robust physical stabilizing mechanism via modest off-axis beam injection, in agreement with experimental observations from the National Spherical Torus Experiment (NSTX-U). Experimental results from NSTX-U have demonstrated that neutral beam injection from the new beam sources with large tangency radii deposits beam ions with large pitch, which can very effectively stabilize all unstable Global Alfvén Eigenmodes (GAEs). Beam-driven GAEs have been linked to enhanced electron transport in NSTX, and the ability to control these modes will have significant implications for NSTX-U, ITER, and other fusion devices where super-Alfvénic fast ions might be present. Nonlinear simulations using the HYM code have been performed to study the excitation and stabilization of GAEs in the NSTX-U right before and shortly after the additional off-axis beam injection. The simulations reproduce the experimental finding, namely, it is shown that off-axis neutral beam injection reliably and strongly suppresses all unstable GAEs. Before additional beam injection, the simulations show unstable counter-rotating GAEs with toroidal mode numbers and frequencies that match the experimentally determined modes. Additional off-axis beam injection has been modeled by adding beam ions with large pitch and varying density. The complete stabilization occurs at less than 7% of the total beam ion inventory. New analytical theory of GAE (de)stabilization has also been derived, indicating a different interpretation for the GAE stabilization mechanism compared to previous publications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Numerical simulation of streamer evolution in surface dielectric barrier discharge with electrode-array

Atmospheric pressure surface dielectric barrier discharges (SDBDs) may be composed of streamers fast propagating along a dielectric surface in ambient air, producing reactive oxygen and nitrogen species, and inducing a force on the neutral gas, which can find applications, respectively, in plasma medicine and aerodynamics. In this work, a two-dimensional self-consistent fluid model was developed to study SDBDs with an electrode-array. Emphasis was placed on the interaction of counter-propagating streamers and discharge uniformity for different applied voltages and geometric configurations of the electrode-array. When two counter-propagating streamers collide, the streamers come to a stop within a certain (ultimate) distance between the streamer heads. Optimizing the applied voltages is a convenient way to improve uniformity, making the streamer heads reach a minimum distance between each other. Shortening the electrode spacing can simultaneously shorten the streamer length and the ultimate distance between streamers. Under certain discharge parameters (such as applied voltage), there exists an optimum electrode spacing maximizing uniformity.

42 ENGINEERING↗
DOE OSTI2014

Numerical simulation of microwave amplification in a plasma channel produced in a gas via multiphoton ionisation by a femtosecond laser pulse

This paper examines the evolution of a nonequilibrium plasma channel produced in xenon by a femtosecond KrF laser pulse. We demonstrate that such a channel can be used to amplify microwave pulses over times of the order of the relaxation time of the photoelectron energy spectrum in xenon. Using the slowly varying amplitude approximation, we analyse the propagation and amplification of an rf pulse in a plasma channel, in particular when the rf field influences the electron energy distribution function in the plasma. (interaction of laser radiation with matter. laser plasma)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Numerical simulation of a high efficiency combustion-driven HF laser with preheated fuels

We report a simulation calculation of the processes proceeding in a nozzle array and an optical cavity of a combustion-driven cw HF chemical laser with preheated fuels. Investigation results demonstrate that fuel preheating can dramatically improve the output characteristics of combustion-driven HF chemical lasers. The available output power and specific output power are improved by ∼13% and ∼46%, respectively, when fuels are preheated to 1200 K. It is shown that the fuel consumption in the combustor is reduced by ∼36%, which allows one to improve the performance of combustion-driven HF lasers and make them more compact due to fuel preheating. (paper)

42 ENGINEERING↗
DOE OSTI2019

Complementary bimodal approach to monitoring of photodynamic therapy with targeted nanoconstructs: numerical simulations

We propose a new approach to monitoring photodynamic therapy (PDT) of glioblastoma with the use of targeted nanoconstructs containing a photosensitiser (PS) [benzoporphyrin derivative (BPD)] and IRDye800 dye, antibodies for efficient accumulation of the drug in a tumour, and a chemotherapeutic agent for combined effect on tumour cells. Application of IRDye800 is determined by the shift of its absorption and fluorescence spectra to the near-IR range with respect to BPD that ensures a higher imaging depth. Monitoring of PDT is based on the simultaneous fluorescence and optoacoustic (OA) imaging. Fluorescence imaging provides visualisation of fluorescence agents with high molecular sensitivity and monitoring of the effectiveness of PDT by PS photobleaching. OA allows one to examine the vascular pattern of the tumour environment, as well as assess the depth of the tumour. IRDye800 is a better contrast agent in comparison to BPD due to red-shifted spectral characteristics and a higher fluorescence quantum yield. Monte Carlo simulations combined for OA simulations with K-wave modelling allowed the feasibility of the complementary approach to be studied and demonstrated that this combination allows one to localise a tumour with a size of 2 mm at depths from 100 μm to 2 mm. (paper)

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

Development and numerical simulation of spherical microresonators based on SiO{sub 2} – GeO{sub 2} germanosilicate glasses for generation of optical frequency combs

We propose and theoretically investigate the possibility of using spherical, whispering gallery mode microresonators based on germanosilicate glasses with different GeO{sub 2} contents for generating optical frequency combs in the dissipative soliton regime under pumping at a wavelength of 1.55 or 2 μm. The dispersion and nonlinearity of microspheres of different radii are calculated and analysed, and their optimal characteristics and the expected parameters of the output radiation are determined. It is shown that the spectral widths of optical frequency combs formed in 0.8SiO{sub 2} – 0.2GeO{sub 2} and GeO{sub 2} glass microspheres pumped at λ = 1.55 and 2 μm can be ∼200 and ∼300 nm, respectively. In these cases, in addition to the dissipative soliton with a duration of ∼100 fs, the generation of dispersive waves is also observed. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Numerical simulation of dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core in a wide range of their parameters

Dispersion and nonlinear characteristics of microstructured silica fibres with a thin suspended core surrounded by three, four or six air holes have been studied theoretically in the wavelength range 1 – 2 μm. It has been shown that, owing to strong fundamental mode confinement near the core, the Kerr nonlinearity coefficient can exceed the nonlinearity coefficient of standard telecom fibre SMF28e by two orders of magnitude. The large waveguide contribution allows for effective group velocity dispersion management. Estimates are presented that demonstrate the feasibility of using suspended core fibre exhibiting Kerr nonlinearity for generating non-classical light: a state with squeezed quantum fluctuations in one of the quadrature components of a cw laser signal at a wavelength near 1.55 μm. (paper)

36 MATERIALS SCIENCE↗
DOE OSTI2020

Numerical simulation of the beam self-cleaning process in a multimode graded-index fibre during propagation of a pump wave and a Stokes component

A model of coupled modes is proposed, which makes it possible to describe the propagation of a pump wave and a Stokes component in a multimode graded-index fibre. A decrease in the depression in the centre of the profile of the pump wave intensity distribution as a result of the influence of Kerr effects and random linear coupling of modes is demonstrated. (paper)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Numerical Simulation of Isothermal Flow Across Slant Five-Tube Bundle with Spectral Element Method Code Nek5000

The helical coil steam generator (HCSG) is a specific type of shell-and-tube heat exchanger known for having a higher heat transfer coefficient than many other designs. For this reason, they are considered in small modular reactor and high temperature reactor designs. Investigation of flow behavior in HCSGs is important for better design. In this paper we present our study for modeling the coolant flow in the primary side of the HCSG. We used Nek5000, an open source, high-order spectral element computational fluid dynamics code developed in Argonne National Laboratory. Nek5000 accepts only hexahedral mesh, which makes the meshing process for the complicated HCSG geometry very challenging. A tetrahedral-to-hexahedral meshing strategy was applied to bypass the geometric complexities. In this study large eddy simulation (LES) was performed at the Reynolds number of 9000 based on the inlet velocity and the tube diameter. The employed subgrid-scale model for LES relies on explicit filtering. First- and second-order statistics were compared to available experimental data. We report overall velocity and turbulent kinetic energy showed good agreement with particle image velocimetry data.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2024

Numerical Simulation and Experimental Comparison of System Analysis Module 1D Mixing Model for Cold Shock Transients in the Gallium Thermal-Hydraulic Mixing Facility

Abstract Liquid metals are being investigated as coolants in many advanced reactor designs because of their high thermal conductivity and effectiveness at high temperatures. However, they often pose challenges to reactor operation and safety because of the complex thermal mixing and stratification in the plenum of pool-type reactor designs. The advanced system analysis code System Analysis Module (SAM) currently under development at Argonne National Laboratory aims to develop and implement thermal mixing models to accurately capture these complex thermal fluid behaviors. In this study, the SAM thermal mixing model was compared against experimental data from the Gallium Thermal-Hydraulic Experiment facility, a scaled liquid metal test facility that uses gallium as a surrogate fluid to investigate the stratification and thermal mixing of low-Prandtl-number fluids in the upper plenum of a liquid metal-cooled reactor. Two cold shock transient cases were used: one with stable stratified flow (Ri = 32) and one with stronger thermal mixing (Ri = 0.5). The resultant temperatures were then compared with the experimental temperatures over the entire plenum to assess the ability of the mixing models to capture the thermal behavior and to better correspond mixing parameters to various flow scenarios. Generally, the zero-dimensional mixing model was more capable of capturing the bulk temperature of the component modeled assuming that an accurate mass flow rate was provided, but it was inherently unable to capture thermal gradients in space. The one-dimensional mixing model was capable of capturing that the thermal gradients provided accurate selection of the mixing coefficients. Further, the temperature at the outlet junction was compared over time for each of the mixing models with the recorded experimental temperature. The implemented mixing models demonstrated the ability to effectively capture the overall thermal behavior for stronger mixing scenarios but struggled with more stably stratified flows. It was found that a system analysis code's covering of the entire range of different operating conditions still remains a challenging task, and it is suggested that further model and closure improvements are necessary to accurately capture complex thermal mixing and stratification phenomena.

stratification↗
DOE OSTI2026

Numerical Simulation of a Natural Convection–Driven Air-Cooled Reactor Cavity Cooling System Experiment

Ensuring the efficient removal of decay heat from the reactor vessel is essential for the safety of advanced reactor technologies. Several Generation-IV concepts incorporate variations in the reactor vessel cooling systems to achieve this objective. High-temperature gas-cooled reactors utilize a reactor cavity cooling system (RCCS), a passive ex-vessel system designed to operate without active components or external power during accident conditions. The RCCS removes decay heat primarily through radiative and convective heat transfer mechanisms. Here, this study presents a comprehensive validation of a computational fluid dynamics Reynolds-averaged Navier-Stokes model for the University of Wisconsin-Madison air-cooled RCCS facility. Validation was conducted for both high- and low-power natural convection cases under a uniform heating profile. Near-wall resolution was found to be critical for accurately modeling natural convection in the RCCS; employing an all-𝑦 + wall treatment resulted in wall temperature discrepancies exceeding 50 °⁢𝐶 compared to a wall-resolved mesh. Thermal-hydraulic behaviors under natural and forced convection conditions were compared within the heated cavity and RCCS. A turbulence model sensitivity analysis indicated that low-Reynolds number k-ɛ, k-ω shear stress transport (SST), and Reynolds stress transport models produce similar wall temperature predictions. A buoyancy modeling sensitivity study revealed that the Boussinesq approximation significantly underpredicted thermal-hydraulic behavior in the RCCS. Based on these findings, modeling recommendations are provided. The validated data set along with identified sensitivities refine the modeling of natural convection in the RCCS. The information produced by this study supports RCCS design, optimization, and safety evaluations, enabling the calibration and verification of reduced-order thermal-hydraulic models.

22 - GENERAL STUDIES OF NUCLEAR REACTORS↗