Radiative Thermal Conductivity Research Articles

A Monte Carlo method to solve for radiative effective thermal conductivity for particle beds of various solid fractions and emissivities

A method is described to find the effective thermal conductivity due to radiation (krad) for groups of particles at packed and less than packed states. Unlike most previous studies, the method does not rely on the assumption of a unit cell or absorption and scattering coefficients to derive krad. In this method, radiation is modeled with a 3D Monte Carlo ray tracing code, steady state particle temperatures are found with a particle-particle heat exchange simulation, and krad is found with a comparison to heat conduction in an isotropic solid of the same geometry. This leads to the dimensionless Exchange Factor (FE), allowing krad to be calculated at any temperature and particle radius. The key result is a model for FE over the entire range of emissivities from 0.3 to 1 and solid fractions from 0.25 to the fully packed state of 0.64. FE results are compared to previous models, with agreement shown in some cases but a large disagreement found for low solid fractions. The krad results are combined with the Zehner and Schlünder model for solid and fluid conduction, providing an equation for the full effective thermal conductivity.

A fractal-skeleton model of high porosity macroporous aluminum and its heat transfer characterizes

Macroporous metal foams are used as energy exchange materials and have been widely used in the industry. In this paper, the finite element method is used to calculate the equivalent thermal radiation conductivity and equivalent thermal conduction equivalent conductivity of the fractal-skeleton heat transfer model, and the heat transfer characteristics of the two components are analyzed. The results show that during thermal conduction progress, energy transfer mainly occurs at the junction of walls and holes perpendicular to the direction of heat flow. While during the thermal radiation process, the smaller the cell wall thickness and the larger the porosity, the larger the proportion of the heat radiation conductivity in the total heat conductivity. When the structural size is equal to the characteristic wavelength, the scattering coefficient is rapidly reduced, so that the radiation heat transfer effect is improved. Finally, the cause of this phenomenon is further explained by the micro-mechanism theory.

Characterization of high temperature optical spectra of glass melts and modeling of thermal radiation conductivity

AbstractTwo innovative experimental techniques for measuring the high temperature near infrared optical spectra of glass melts are compared. The critical experimental features of both techniques, one based on transmission and the other based on emittance measurements, are reviewed. Typical results of both techniques, including high temperature spectra and values for the Rosseland mean absorption coefficient and thermal radiation conductivity versus temperature for similar glass melts, are compared. The study is focused on sulfate fined soda lime silicate glass melts colored with iron oxide and chromium oxide and on the effect of the glass redox state on the thermal radiation conductivity. It is shown that essentially different measuring principles provide consistent results for similar glass melt types, that is, colors. Using the high temperature spectra of a large variety of (colored) glasses, a new semi‐empirical model is developed for predicting the Rosseland radiation conductivity of arbitrary sulfate fined soda lime silicate glass melts, colored with iron oxide and chromium oxide. By separating the effects of (a) the temperature‐dependent redox state, (b) the high temperature changes in ligand field strengths and (c) the glass matrix, the model reliably predicts the Rosseland radiation conductivity, with a chemical analysis of the glass as input only.

Parameter analysis and wall effect of radiative heat transfer for CFD-DEM simulation in nuclear packed pebble bed

In the heat transportation of core of high temperature gas-cooled nuclear reactor (HTGR), radiative heat transfer plays a significant role in the CFD-DEM simulations. The numerical investigation is conducted for parameter analysis and wall effect of the thermal radiation. A cell model is presented to discuss the effects of temperature and pebble size. The radiation effective conductivity is directly proportional to pebble diameter and cube of the temperature. For engineering cases, the emissivity on radiation is linear approximately. In the bulk region without wall effect, the radiative thermal conductivity is inversely proportional to the packing density. The effect of solid conductivity and gas absorption can be neglected for common gases with forced convection. With uniform continuum model and discrete particle simulation, the radiative conductivity is inversely proportional to the pebble sphericity and directly proportional to the integral of the radial distribution and radiation interaction function. And radiation characteristics in wall and near-wall region are different from that of bulk region.

A matrix model of particle-scale radiative heat transfer in structured and randomly packed pebble bed

Thermal radiation in pebble bed is very hard to compute accurately and efficiently since it occurs between the surfaces no matter how far they are. Herein, radiative heat flux and effective thermal conductivity were derived mathematically by a matrix model to present the thermal radiation between particles in pebble beds. For structured packing, the transient radiative heat transfer is proven to be similar to thermal diffusion. This study proposed a correlation to predict the radiation exchange factor under different porosity. Radiative effective thermal conductivity was found to grow slightly with void fractions. For random pebble beds, a regression model of the multi-layer neural network was trained by large datasets to compute the view factor matrix efficiently. With the trained model and GPU acceleration, it is feasible to perform real-time-simulation of full-range radiations inside large-scale pebble beds, even for real reactors (e.g. HTR-PM). A thermal radiation simulation of the decay heat removal of a real reactor adopted this model as a demonstrative application and showed that the highest temperature is still within the design limitation of the packed bed.

Comparison of kinetic theory and fluctuational electrodynamics for radiative heat transfer in nanoparticle chains

In chains of closely spaced nanoparticles supporting surface polaritons, near-field electromagnetic coupling leads to collective effects and super-Planckian thermal radiation exchange. Researchers have primarily used two analytical approaches to calculate radiative heat transfer in these systems: fluctuational electrodynamics, which directly incorporates fluctuating thermal currents into Maxwell's equations, and a kinetic approach where the dispersion relation provides modes and propagation lengths for the Boltzmann transport equation. Here, we compare results from the two approaches in the isotropic dipole limit in order to identify regimes in which kinetic theory is valid and to explain differing results in the literature on its validity. Using both methods, we calculate the diffusive radiative thermal conductivity of nanoparticle chains. We show that kinetic theory is valid and matches predictions by fluctuational electrodynamics if both the surface polariton propagation lengths are greater than the particle spacing and surface polaritons are the dominant contributors to radiative heat transfer.

Thermal conductivity of evacuated expanded perlite measured with guarded-hot-plate and transient-hot-wire method at temperatures between 295 K and 1073 K

The effective thermal conductivity of two evacuated expanded perlite powders has been measured at temperatures between 295 K and 1073 K. Since conduction via the gas phase is suppressed, thermal transport occurs only via solid conduction and thermal radiation. Due to thermal contact resistances between the powder particles, solid conduction is very small and radiative heat transport dominates, especially at high temperatures. Applying the guarded-hot-plate (GHP) method to optically thick specimens, the true effective thermal conductivity λeff, which is the sum of solid thermal conductivity λs and radiative thermal conductivity λr from the diffusion model, has been measured. After plotting λeff as a function of third power of absolute temperature and calculating the regression line, λs is obtained from the intercept of the line, and the extinction coefficient for thermal radiation E is determined from its slope. The resulting values are λs = (6.4 ± 1.5) ∙ 10−3 W m−1 K−1 and E = (1600 ± 40) m−1 for the first perlite powder and λs = (3.1 ± 1.3) ∙ 10−3 W m−1 K−1 and E = (5700 ± 350) m−1 for the second. With an effective thermal conductivity below 0.01 W m−1 K−1 up to a mean sample temperature of 473 K, the second material is suitable to realize an economic evacuated powder insulation for medium-temperature applications up to approximately 673 K at the hot side. Both materials have also been investigated with the transient-hot-wire (THW) method. This technique has the advantage of shorter measurement time, but underestimates the effective thermal conductivity according to numerical calculations from literature, especially for samples with 1000 m−1≤E≤ 10 000 m−1. This leads to an apparent extinction coefficient Eapp>E from the λeff vs. T3 plot. Using the same procedure as above, the THW measurements deliver λs = (3.9 ± 2.7) ∙ 10−3 W m−1 K−1 for the first perlite powder and λs = (2.2 ± 1.4) ∙ 10−3 W m−1 K−1 for the second, which agrees with the values obtained from the GHP method. The apparent extinction coefficients are Eapp = (2170 ± 110) m−1 and Eapp = (6730 ± 370) m−1, which corresponds to an overestimation by 35.6% and 18.1%, respectively. Both results are in good agreement with the numerical calculations from literature, which have now been verified experimentally for the first time. Because such calculations can in principle be used to correct experimental THW data, it is possible to extend the applicability of the THW method to materials with 1000 m−1≤E≤ 10 000 m−1, i.e. near the limit of radiation diffusion.

Full‐Field Modeling of Heat Transfer in Asteroid Regolith: 1. Radiative Thermal Conductivity of Polydisperse Particulates

AbstractCharacterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperature amplitudes are directly related to the thermal properties of the materials on the surface. We describe a numerical model for studying the thermal conductivity of particulate regolith in vacuum. Heat diffusion and surface‐to‐surface radiation calculations are performed using the finite element (FE) method in three‐dimensional meshed geometries of randomly packed spherical particles. We validate the model for test cases where the total solid and radiative conductivity values of particulates with monodisperse particle size frequency distributions (SFDs) are determined at steady‐state thermal conditions. Then, we use the model to study the bulk radiative thermal conductivity of particulates with polydisperse, cumulative power law particle SFDs. We show that for each polydisperse particulate geometry tested, there is a corresponding monodisperse geometry with some effective particle diameter that has an identical radiative thermal conductivity. These effective diameters are found to correspond very well to the Sauter mean particle diameter, which is essentially the surface area‐weighted mean. Next, we show that the thermal conductivity of the particle material can have an important effect on the radiative component of the thermal conductivity of particulates, especially if the particle material conductivity is very low or the spheres are relatively large, owing to non‐isothermality in each particle. We provide an empirical correlation to predict the effects of non‐isothermality on radiative thermal conductivity in both monodisperse and polydisperse particulates.

Оптические свойства отечественного сетчатого стеклоуглерода и его основы

The paper presents the results of experimental studies and mathematical modeling of the optical properties of glassy carbon and domestic reticulated foam materials based on it. Since the optical properties of the surface are studied on dense samples, dense samples were previously created, identical in physical properties to glassy carbon - the basis of highly porous cellular carbon materials. From the experimentally measured the spectral hemispherical reflectivity of the surface of the samples under its normal illumination and by the Kramers-Kronig relations the spectra of optical constants of glassy carbon - the refractive indices and absorption, as well as a number of their derivative characteristics were determined. For them, simple approximating relations are given in the paper. The obtained spectral data was incorporated into the previously developed optical statistical simulation model of ultra-porous reticulated foam materials, which is based on a rigorous electromagnetic theory and allows you to take into account both the features of their microstructure and physical processes that occur in such systems at different spa-tial and temporal scales. The results of the calculation of local spectra, the scattering phase function, and radiation thermal conductivity are presented for the reticulated glassy carbon foam, which has wide prospects for use as a structural and heat-shielding material. Some additional features of the mathematical model are demonstrated also.

Thermal radiation in systems of many dipoles

Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwell's equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we unify different theoretical descriptions of these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and SiO2 nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems. We validate our calculations by comparison to the exact solution for a one-dimensional particle chain, and we demonstrate that the dipolar approximation significantly underpredicts the exact results at separation distances less than the particle radius.

Micro-nano scale heat transfer mechanisms for fumed silica based thermal insulating composite

Developing nanoporous thermal insulation materials is an effective method for solving energy consumption problems. In this study, fumed silica based thermal insulating composite was prepared using a novel dry molding method. Thermal conductivity was measured using a guarded hot plate method in the steady state. The results indicated that this composite exhibited excellent thermal insulating performance, achieving an ultra-low thermal conductivity of 0.0205 W/mK at 100 °C. By analyzing the microstructure using field emission scanning electron microscopy (FESEM), we found that the fiber surface was modified by a 12 μm-thick layer of fumed silica particles that reduced solid heat transfer. However, at high temperatures, thermal insulating performance decreased rapidly, and thermal conductivity reached up to 0.119 W/mK at 500 °C. Based on this problem, a heat transfer model was constructed to explain micro-nano scale heat transfer mechanisms of this composite. The results indicated that gas-solid coupled thermal conductivity occupied a large proportion of 78.3% at 100 °C, but the value was only 0.015 W/mK. As the serving temperature increased, radiative heat transfer gradually occupied the dominant position in total heat transfer. Radiative thermal conductivity reached up to 0.101 W/mK at 500 °C, which was approximately 93.3% of total thermal conductivity. Therefore, it was apparent that the increase in radiative heat transfer was the main cause of the decline in thermal insulating performance at high temperatures. Additionally, the effect of the fiber mass ratio on thermal conductivity was investigated at various temperatures. Empirical and simulated data were compared, and the results revealed that the heat transfer model was basically consistent with experimental findings, achieving a mean error of only 4.78%.

An approximate methodology to simulate combined conduction-radiation heat transfer for multi-layer insulator

A quasi-analytical methodology was developed to model combined conduction-radiation heat transfer through the thickness of a reflective multi-layer insulator. This methodology was validated by the experimental result. It could be used for earlier steps of the design process of the high-temperature multi-layer insulators. Traditionally, radiation thermal conductivity approximation is used in the earlier stages of design. Despite the accuracy of this approach in steady-state cases, it yields to some unacceptable errors when the thermal load is transient. It was shown that old methodology can not predict maximum temperature and time of occurrences in acceptable margins. The developed model originated from the radiation thermal conductivity approximation. But unlike the primitive one, the developed model seems acceptable in transient cases. This model is based on consideration of thermal emittance through the thickness of the insulator. It can predict the maximum temperature of the structure and occurrences time with an error of less than 4%.

A comprehensive review on nanofluid operated solar flat plate collectors

The impact of population explosion and continuous upsurge on energy demand has resulted in the intimidating depletion of fossil fuel resources, increased environmental pollution, and elevated production and consumption cost. Hence, in the past two decades the demand for renewable energy has escalated. The solar energy is the most trending topic when talking about renewable energy sources, because of its ease of availability, reduced dependence on foreign fuels and negligible maintenance. This can be directly harnessed unlike other renewable energy sources. A solar flat plate collector converts the radiant solar energy from the sun into thermal energy; usually, copper or aluminium is used as heat absorbing material. However, to further enhance the performance and thermophysical properties of the heat exchanger liquids of flat plate solar collectors like radiative heat transfer and thermal conductivity, the nanofluids are used. The use of nanofluids as an innovative type of working fluids is reasonably a new development in solar flat plate collectors. They are prepared by mixing low concentration of solid particles, sized 1–100 nm with the base fluid. The objectives of this review paper is to recapitulate the investigations carried in the field of solar flat plate collectors using a range of nanofluids, the performance analysis of various flat plate collectors using numerous nanofluids and the challenges faced in developing an efficient thermal collector using nanofluids. Furthermore, the article discusses the opportunities for future research.

Thermal Conductivity of Low Density Polyethylene Foams Part I: Comprehensive Study of Theoretical Models

Polymeric foams are one of the most applicable thermal-insulation materials due to their low thermal conductivity, high mechanical properties, and low cost. Optimization of thermal-insulation performance of polymeric foams needs a theoretical model in order to predict the overall thermal conductivity. So far, several theoretical approaches are presented in this regard but to the best knowledge of the authors, there is no comprehensive investigation on comparing the proposed models. Therefore, the study of validity of the theoretical models in comparison with the experimental results is one of the main goals of the present study. Low density polyethylene (LDPE) foams are selected as the case study due to the wide application range. Different models to predict the overall conductivity of the foam based on conduction through the combined gas and solid phases (λgs) as well as radiation thermal conductivity (λr) are presented. The results indicate that the best model is a model in which λgs is calculated using Gibson and Ashby model and λr is obtained using Williams and Aldao model based on the root mean square (RMS) parameter. The results show that the theoretical error of this model is smaller than 10%.

Near-field radiative heat transfer in multilayered graphene system considering equilibrium temperature distribution.

In the present work, the near-field radiative heat transfer of a multilayered graphene system is investigated within the framework of the many-body theory. For the first time, the temperature distribution corresponding to the steady state of the system is investigated. Unique temperature steps are observed near both boundaries of the system, especially in the strong near-field regime. By utilizing the effective radiative thermal conductance, the thermal freedom of heat flux in different regions of the system is analyzed quantitatively, and the cause of various temperature distributions is explained accordingly. To characterize the heat transfer ability of the whole system, we evaluate the system with two heat transfer coefficients (HTC), transient heat transfer coefficient (THTC), and steady heat transfer coefficient (SHTC). A unique many-body enhancement is observed, which causes a red-shift of resonance peak corresponding to graphene surface plasmon polaritons. Furthermore, a three-body enhancement of SHTC emerges thanks to the relay effect and the complexity of the system. The regime of heat transport can be tuned by changing the chemical potentials of graphene and undergoes a transition from diffusive to quasi-ballistic transport in the strong near-field regime.

New Constraints on the Thermal Conductivity of the Upper Mantle From Numerical Models of Radiation Transport

AbstractTo address uncertainties in the values and mathematical form of the radiative thermal conductivity krad in the mantle, we developed new models for the transport, scattering, and absorption of thermal radiation in semitransparent multiphase polycrystalline assemblages. We show that the Rosseland diffusion equation correctly describes the diffusion of thermal radiation and infer the form of the effective spectral coefficients through numerical experimentation. We show that the scattering coefficient depends on the grain size and on interphase contact statistics in complicated ways, but that simplifications can be employed in practice. The effective opacity of a composite random material is a harmonically weighted mixture in the limit of infinitely large grain size and an arithmetically weighted mixture in the limit of infinitesimal grain size. Using existing absorption spectra for major upper mantle minerals, we estimate krad as a function of temperature, grain size, and petrology. In mantle assemblages, the scattering effect is important for small grain sizes (<1 mm), but the grain size effect on the effective opacity of a multiphase medium is important for grain sizes up to 10 cm. We calculate that upper mantle krad is about 2–3.5 W·m−1·K−1 for a representative mean grain size range of 0.01 to 1 cm. This translates to a total thermal conductivity of 5.5–7 W·m−1·K−1. Application of our model to the cooling of oceanic lithosphere shows that krad increases net cooling by about 25%.

Effect of Thermal Radiation and Variable Thermal Conductivity on Magnetohydrodynamics Squeezed Flow of Carreau Fluid Over a Sensor Surface

Effect of Thermal Radiation and Variable Thermal Conductivity on Magnetohydrodynamics Squeezed Flow of Carreau Fluid Over a Sensor Surface

Threshold cell diameter for high thermal insulation of water-blown rigid polyurethane foams

Water-blown rigid polyurethane foams manufactured in eco-friendly manners attract great attentions for applications in various industrial products. Especially, the polyurethane foams are widely applicable as thermal insulation materials for LNG carrier, electronic appliances, pipes, and building. Heat transfer mechanisms in foamed materials have strong relationships with gaseous molecules in cells, solid parts, and cellular morphologies. In this study, thermal conductivity of the water-blown rigid polyurethane foams was investigated by controlling the cellular morphologies using different types of surfactant molecules and gelling catalysts. The cell sizes were controlled from 551 μm to 153 μm by varying surfactants and gelling catalysts. The small cell sizes showed low radiative thermal conductivity due to the high number of thick cell walls and struts to obstruct the photon transport process. More importantly, there was a clear cell threshold size (230 μm) in variations of the overall thermal conductivity value (about 24.3 mW/m K) of polyurethane foams, and the delayed slowly weaken thermal insulation property was also noted in the cell sizes less than 230 μm.

Enhancing thermal conductivity and near‐infrared radiation reflectance of poly(ε‐caprolactone)/poly(lactic acid)‐based nanocomposites by incorporating hexagonal boron nitride

This work aims to develop multifunctional biodegradable ternary nanocomposites with heat dissipation capability. The effects of adding hexagonal boron nitride (hBN), regarding phase morphology, near‐infrared radiation (NIR) reflectance and thermal conductivity of poly(ε‐caprolactone) (PCL) and poly(lactic acid) (PLA) blends were investigated. The hBN nanoparticles were selectively localized in the PCL phase. Scanning electron microscopy showed that the increased hBN concentration in the PCL/PLA blend led to a more fibrillar PLA phase and, after compression molding, its morphology presented a co‐continuous structure. Adding hBN nanoplatelets increased the reflectance in the NIR region by 184%, the thermal conductivity of the PCL/PLA by 400%, and increased the blend's thermal stability. The co‐continuous morphology could favor the heat flow, because it promotes the confinement of the hBN nanoplatelets in the PCL phase, favoring the conductive thermal pathway formation. POLYM. COMPOS., 40:3464–3471, 2019. © 2019 Society of Plastics Engineers

Change in internal energy of thermal diffusion stagnation point Maxwell nanofluid flow along with solar radiation and thermal conductivity

Abstract This paper concerns the characteristics of heat and mass transfer in upper convected Maxwell fluid flow over a linear stretching sheet with solar radiation, viscous desperation and temperature based viscosity. After boundary layer approximation, the governing equations are achieved (namely Maxwell, upper convected material derivative, thermal and concentration diffusions). By using the self-similarity transformations the governing PDEs are converted into nonlinear ODEs and solved by RK-4 method in combination with Newton Raphson (shooting technique). The effects of developed parameters on velocity, temperature, concentration, fraction factor, heat and mass diffusions are exemplified through graphs and tabular form and are deliberated in detail. Numerical values of fraction factor, heat and mass transfer rates with several parameters are computed and examined. It is noticed that the temperature is more impactable for higher values of radiative heat transport, thermal conductivity and viscous dissipation. The comparison data for some limiting case are acquired and are originated to be in good agreement with previously published articles.