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Results for “effective conductivity”

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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Heat Transfer in Adhesively Bonded Honeycomb Core Panels

The Swann and Pittman semi-empirical relationship has been used as a standard in aerospace industry to predict the effective thermal conductivity of honeycomb core panels. Recent measurements of the effective thermal conductivity of an adhesively bonded titanium honeycomb core panel using three different techniques, two steady-state and one transient radiant step heating method, at four laboratories varied significantly from each other and from the Swann and Pittman predictions. Average differences between the measurements and the predictions varied between 17 and 61% in the temperature range of 300 to 500 K. In order to determine the correct values of the effective thermal conductivity and determine which set of the measurements or predictions were most accurate, the combined radiation and conduction heat transfer in the honeycomb core panel was modeled using a finite volume numerical formulation. The transient radiant step heating measurements provided the best agreement with the numerical results. It was found that a modification of the Swann and Pittman semi-empirical relationship which incorporated the facesheets and adhesive layers in the thermal model provided satisfactory results. Finally, a parametric study was conducted to investigate the influence of adhesive thickness and thermal conductivity on the overall heat transfer through the panel.

Daryabeigi, Kamran↗
NASA NTRS1992

Two-temperature models of old supernova remnants with ion and electron thermal conduction

To investigate the potential effects thermal conduction may have on the evolution of old supernova remnants, we present the results of 1D (spherically symmetric) numerical simulations of a remnant in a homogeneous interstellar medium for four different cases: (1) without thermal conduction; (2) with both electron and ion thermal conduction assuming equal temperatures; (3) with electron thermal conduction only, following electron and ion temperatures separately; and (4) with both electron and ion thermal conduction following separate temperatures. We followed the entire evolution until the completion of the remnant bubble collapse. Our most significant result is that in remnant evolution studies concerned principally with either the shell or bubble evolution at late times, reasonable results are obtained with single-temperature models. When the electron and ion temperatures are followed separately, however, ion thermal conduction cannot safely be ignored.

Cui, Wei↗
NASA NTRS1959

Theoretical and Experimental Investigation of Heat Conduction in Air, Including Effects of Oxygen Dissociation

Solutions are presented for the conduction of beat through a semi-infinite gas medium having a uniform initial temperature and a constant boundary temperature. The coefficients of thermal conductivity and diffusivity are treated as variables, and the solutions are extended to the case of air at temperatures where oxygen dissociation occurs. These solutions are used together with shock-tube measurements to evaluate the integral of thermal conductivity for air as a function of temperature.

Hansen, C. Frederick↗
DOE OSTI2026

ThermoPI—an Online Tool to Calculate Heat Transfer Through Foam Insulation

Thermal insulation materials with ultra-low effective thermal conductivity are crucial for a multitude of applications. Over the years, a significant amount of experimental work has been dedicated to creating new insulation materials with low thermal conductivity. Similarly, substantial efforts have been made to enhance the theoretical understanding of thermal transport mechanisms in thermal insulation materials and to push the boundaries of lower thermal conductivity. However, ample room remains for enhancing the thermal resistivity of closed-cell foam insulations. To aid in the development of ultra-low effective thermal conductivity foam insulations, Oak Ridge National Laboratory has introduced a unique online tool, ThermoPI. This tool calculates not only the effective thermal conductivity but also the thermal conductivity components for porous materials, including gases, solids, and radiative, based on the materials’ structural information (e.g., porosity, pore size, gas species, pressure, solid species, pore geometry, and temperature).This tool collected and improved the existing theoretical models for thermal insulation materials’ gas, solid, and radiative thermal conductivity. These improved models have been validated with experimental data. Effective medium theory and sound velocity softening effects are considered for solid thermal conductivity. The built-in solid materials include polystyrene, polyurethane, polyethylene, and silica. Users are allowed to input new solid materials that are not built inside the tool. For gas thermal conductivity, the subcontinuum Knudsen effect is considered. In addition to several built-in gases, users can input new gases. For solid and radiative thermal conductivity calculations, several models are available to select, and the tool can determine the best model to choose based on the materials that users input. Users are also allowed to change selections manually. In addition, the tool can also calculate the effect of interfacial resistance on the overall thermal conductivity of layered materials. This work will elaborate on the tool and discuss how it can guide the development of new insulation products.

Shrestha, Som [ORNL] (ORCID:0000000183993797)↗
DOE OSTI2020

Size Effects in the Thermal Conductivity of Amorphous Polymers

Manipulating thermal conductivity through nanoengineering is of critical importance to advance technologies, such as soft robotics, artificial skin, wearable electronics, batteries, thermal insulation, and thermoelectrics. Here, by examining amorphous polymers, including polystyrene, polypropylene, polyethylene, and ethylene vinyl alcohol, using molecular dynamics simulations, we find that the thermal conductivities of amorphous polymers can be reduced below their amorphous limit by size effects. Size-dependent thermal transport in amorphous materials is decomposed into crystalline, crystalline-to-amorphous, and amorphous regimes. In the amorphous regime, the mean free path of propagating heat carriers can range from tens of nanometers to more than 100 nm, contributing 16%–36% of the total thermal conductivity. A two-channel model that combines no size effect (i.e., difusons and locons) and size effect (i.e., propagons) is proposed to account for size-dependent thermal conductivity. We also find that the presence of charged molecules in polymers can significantly affect the thermal conductivity and its size effects due to electrostatic interactions. Overall, this work provides insights into the thermal conductivity of amorphous polymers that will have a broad impact on the nano- and chemical engineering of polymers for various energy-related applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Iso-cost-performance of thermal energy storage

As thermal energy storage (TES) systems gain increasing recognition as next-generation energy storage solutions, evaluating their techno-economic performance is crucial. This perspective analyzes the cost-performance of latent heat-based TES systems and introduces the concept of iso-cost-performance—maintaining a constant cost per unit energy ($\$$/kWh) despite material degradation. Using an empirical degradation model, we show that after 1200 thermal cycles, the thermal conductivity and volumetric energy density of a paraffin-based phase change material (PCM) decreased by 36.0% and 26.1%, respectively, resulting in a 31.2% reduction in the figure of merit (FOM) and, consequently, the system cost-performance. However, by introducing thermally conductive additives to enhance effective thermal conductivity, the TES system can recover its initial FOM, achieving iso-cost-performance operation. This framework quantitatively demonstrates how degradation-mitigation strategies—such as improving thermal conductivity—can offset material degradations and maintain long-term cost-effectiveness. Beyond PCM-based TES, the proposed FOM-based approach provides a generalized pathway for cost-performance optimization across various TES technologies.

25 ENERGY STORAGE↗
DOE OSTI2020

Composite Structured Surfaces for Durable Dropwise Condensation

Dropwise condensation of steam on hydrophobic substrates has a 10X higher heat transfer coefficient compared to filmwise condensation. To promote dropwise condensation, low surface energy hydrophobic coatings (polymers) are typically utilized. The low intrinsic thermal conductivity (k < 1 W/(m · K)) of polymers, coupled with high heat transfer coefficient of dropwise condensation (100 kW/(m 2 · K)), necessitates that the coating be thin (< 1µm) in order to avoid reducing the overall heat exchanger conductance. However, thin polymeric films easily degrade. The two opposing requirements result in the need for optimization between the durability (thick coating) and the heat transfer (thin coating). To enable high thermal conductivity in thicker coatings, we develop metal-polymer structured surfaces. By using porous structures as inter-connected heat-conducting backbones that are filled with hydrophobic materials, we enable tuning of the coating effective thermal conductivity and surface energy. Three metal structures were studied; micro/nanowires, inverse opals, and sintered spheres. Heat transfer performance was calculated using three-dimensional finite element method simulations with two distinct boundary conditions; convection at the walls and isothermal walls. Interestingly, the overall conductance shows up to 40% difference depending on the boundary condition used in calculating the composite coating effective thermal conductivity. We use our model to predict the heat transfer performance as a function of metal fraction by volume and by surface area for condensation. By coupling our thermal simulations with a previously verified analytical model for predicting wetting behavior on heterogeneous surfaces, we propose a regime map to predict dropwise-to-filmwise transition. Furthermore, our work not only forms a starting point for the development of durable dropwise condensing surfaces, it identifies important considerations needed for computing effective thermal conductivity of composites.

42 ENGINEERING↗
DOE OSTI2022

Cost-efficient copper-nickel alloy for active cooling applications

It is recently proposed that active (Peltier) and passive (conductive) cooling could be combined to design highly-efficient thermal conductors as active heat sinks. Here we study the potential of low-cost copper-nickel alloys for active cooling applications. We show that for this application, copper-nickel alloys prepared using industrial-grade powders to lower the manufacturing cost, are as good as copper-nickel alloys made out of high purity metals. Furthermore, directed energy deposition, a type of additive manufacturing, can produce copper-nickel alloys which have about 10 percent lower thermoelectric power factor compared to mechanical-alloying methods but have larger thermal conductivity values, hence, possessing similar effective thermal conductivity values, reaching 403 W m -1 K -1 at 390K. Much larger values are expected at higher temperatures. The usage of additive manufacturing enables complex geometries and large-scale production.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Heat Transfer Characteristics of Particle and Air Flow Through Additively Manufactured Lattice Frame Material Based on Octet-Shape Topology

Particle-to-supercritical carbon dioxide (sCO 2 ) heat exchanger is a critical component in next-generation concentrating solar power (CSP) plants. The inherently low heat transfer between falling particles and sCO 2 imposes a challenge toward economic justification of levelized cost of electricity produced through solar energy. Introduction of integrated porous media with the walls bounding particle flow has the potential to enhance the overall particle-to-sCO 2 heat exchanger performance. Here, this paper presents an experimental study on heat transfer characterization of additively manufactured lattice frame material based on Octet-shaped unit cell with particles and air as working fluids. The lattice structures were additively manufactured in stainless steel (SS) 316L and SS420 (with 40% bronze infiltration) via Binder jetting process, where the lattice porosities were varied between 0.75 and 0.9. The mean particle diameters were varied from 266 μm to 966 μm. The effective thermal conductivity and averaged heat transfer coefficient were determined through steady-state experiments. It was found that the presence of lattice enhances the effective thermal conductivity by 2–4 times when compared to packed bed of particles alone. Furthermore, for gravity-assisted particle flow through lattice panel, significantly high convective heat transfer coefficients ranging from 200 W/m2K to 400 W/m2K were obtained for the range of particle diameters tested. The superior thermal transport properties of Octet-shape-based lattice frame for particle flow makes it a very promising candidate for particle-to-sCO 2 heat exchanger for CSP application.

14 SOLAR ENERGY↗
NASA NTRS1972

Effect of Modified Thermal Conductivity on the Temperature Distribution in the Protonosphere

At typical protonospheric electron densities, the electron mean free path is long enough that the coefficient of thermal conductivity is no longer given by Spitzer's expression. The effect on the temperature profile of using the corrected expression for conductivity is investigated. The corrected thermal conduction coefficient is density-dependent and has a more complicated temperature dependence than the coefficient applicable to higher density plasmas. The results indicate that the effect is not negligible even under quiet conditions and at low latitudes.

Mayr, H. G.↗
NASA NTRS1973

Effect of modified thermal conductivity on the temperature distribution in the protonosphere.

At typical protonospheric electron densities the electron mean free path is sufficiently long so that the coefficient of thermal conductivity is no longer given by Spitzer's expression. The effect on the temperature profile of using the corrected expression for conductivity is investigated. The corrected thermal conduction coefficient is density-dependent and has a more complicated temperature dependence than the coefficient applicable to higher density plasmas. The results indicate that the effect is not negligible even under quiet conditions and at low latitudes.

Mayr, H. G.↗
NASA NTRS1974

Space shuttle orbit maneuvering engine reusable thrust chamber. Task 13: Subscale helium ingestion and two dimensional heating test report

Descriptions are given of the test hardware, facility, procedures, and results of electrically heated tube, channel and panel tests conducted to determine effects of helium ingestion, two dimensional conduction, and plugged coolant channels on operating limits of convectively cooled chambers typical of space shuttle orbit maneuvering engine designs. Helium ingestion in froth form, was studied in tubular and rectangular single channel test sections. Plugged channel simulation was investigated in a three channel panel. Burn-out limits (transition of film boiling) were studied in both single channel and panel test sections to determine 2-D conduction effects as compared to tubular test results.

Tobin, R. D.↗
NASA NTRS1987

Investigation of dynamic ground effect

An experimental investigation of dynamic ground effect was conducted in the Univ. of Kansas wind tunnel using delta wings of 60, 70, 75 deg sweep; the XB-70 wing; and the F-104A wing. Both static and dynamic tests were made. Test data were compared to other test data, including dynamic flight test data of the XB-70 and F-104A. Limited flow visualization test were conducted. A significant dynamic effect was found for highly swept delta wings.

Chang, Ray Chung↗
NASA NTRS1996

Evaluation of high temperature superconductive thermal bridges for space borne cryogenic detectors

Infrared sensor satellites are used to monitor the conditions in the earth's upper atmosphere. In these systems, the electronic links connecting the cryogenically cooled infrared detectors to the significantly warmer amplification electronics act as thermal bridges and, consequently, the mission lifetimes of the satellites are limited due to cryogenic evaporation. High-temperature superconductor (HTS) materials have been proposed by researchers at the National Aeronautics and Space Administration Langley's Research Center (NASA-LaRC) as an alternative to the currently used manganin wires for electrical connection. The potential for using HTS films as thermal bridges has provided the motivation for the design and the analysis of a spaceflight experiment to evaluate the performance of this superconductive technology in the space environment. The initial efforts were focused on the preliminary design of the experimental system which allows for the quantitative comparison of superconductive leads with manganin leads, and on the thermal conduction modeling of the proposed system. Most of the HTS materials were indicated to be potential replacements for the manganin wires. In the continuation of this multi-year research, the objectives of this study were to evaluate the sources of heat transfer on the thermal bridges that have been neglected in the preliminary conductive model and then to develop a methodology for the estimation of the thermal conductivities of the HTS thermal bridges in space. The Joule heating created by the electrical current through the manganin wires was incorporated as a volumetric heat source into the manganin conductive model. The radiative heat source on the HTS thermal bridges was determined by performing a separate radiant interchange analysis within a high-T(sub c) superconductor housing area. Both heat sources indicated no significant contribution on the cryogenic heat load, which validates the results obtained in the preliminary conduction model. A methodology was presented for the estimation of the thermal conductivities of the individual HTS thermal bridge materials and the effective thermal conductivities of the composite HTS thermal bridges as functions of temperature. This methodology included a sensitivity analysis and the demonstration of the estimation procedure using simulated data with added random errors. The thermal conductivities could not be estimated as functions of temperature; thus the effective thermal conductivities of the HTS thermal bridges were analyzed as constants.

Scott, Elaine P.↗