Radiative Heat Transfer Properties Research Articles

Elevating high-temperature insulation performance of silica aerogels enabled by innovative surface-structured opacifiers

Silica aerogel, recognized as an exceptional material for thermal insulation, has gained considerable attention in thermal protection. However, its inherent infrared transparency compromises its insulating performance at high temperatures. To address this limitation, incorporating micron-sized opacifier particles has emerged as a crucial strategy. This study introduces a novel opacifier particle, distinguished by surface protrusions inspired by the ’structural absorption’ mechanism, and assesses its extinction capabilities. Then, a thorough analysis was conducted to understand how the shape, volume, and numbers of these surface protrusions influence the radiative heat transfer properties of silica aerogels. The findings reveal that the presence of protrusions on the opacifier surface significantly enhances its extinction capacity, consequently reducing the radiative thermal conductivity of silica aerogels. Notably, cylindrical protrusions demonstrate the most effective extinction properties compared to spherical protrusions and rectangular protrusions. Additionally, for opacifier particles with cylindrical protrusions, there exists an optimal number of 12 protrusions that minimize the radiative thermal conductivity of the aerogel. At 1300 K, the radiative thermal conductivity of the silica aerogel doped with opacifiers featuring 12 protrusions is 22.6 % lower than that of the silica aerogel doped with opacifiers lacking protrusions. Moreover, it was observed that maintaining a constant diameter of cylindrical protrusions while increasing their height diminishes the radiative thermal conductivity of silica aerogel. However, the effect on the radiative thermal conductivity becomes less significant beyond a certain protrusion height. The outcomes of this study provide significant insights into the innovation of opacifiers.

Thermal analysis on a novel natural convection flat plate collector with perforated tilted transparent insulation material parallel slats (TIM-PS) in Mediterranean climate

The purpose of this research is the evaluation of heat transfer by natural convection in an innovative flat plate solar collector operating in natural convection mode. The design consists of inclined, perforated slats of transparent insulation material installed between the absorber and the glass cover. Hourly climatic data on the four specific days of solstices and equinoxes was used to investigate the behavior of the novel collector design in the city of Monastir; Tunisia. A comprehensive process, ANSYS FLUENT, was implemented to properly evaluate the thermal performance of the collector. In order to obtain accurate thermal properties of the smart collector façade unit containing the TIM-PS to exploit in the simulation, a validated three-dimensional finite volume model developed using the CFD software was used to solve the conductive, convective and radiative heat transfer properties of the system. We have explored in depth the effect of optical characteristics of the TIM-PS on the collector performance. Present research focuses on the analysis of thermal efficiency profile of flat plate collector with innovative façade in comparison with conventional collector for different climatic conditions. The analysis indicated that innovative façade collector’s efficiency is highly influenced by solar intensity. It was found that TIM-PS façade can effectively reduce the convective heat losses through the front of collector. The simulation results show that perforated tilted TIM-PS offer better performance than conventional façade of collector during all the seasons of the year. Simulations performed for one year predict that with careful selection of the parallel slats properties, the new façade system can generate an efficiency up to 83% in June, 76% in September, 53% in March and 42% in December. The results of the thermal evaluation of this research will help guide the future development of this system, as we have demonstrated that the innovative façade collector can be used all year round. In conclusion, the TIM-PS have great potential and a very wide range of applications in the field of flat plate solar collector.

A new WSGG radiation model of CO/CO2 mixed gas for solar-driven coal/biomass fuel gasification

Gasification driven by solar energy with CO2 is an ideal way of low-carbon resource utilization. However, there is a lack of research on the radiation heat transfer process which is important in gasification simulation. In this study, we developed a new the weighted-sum-of-gray-gases (WSGG) model to calculate the radiation heat transfer properties of CO and CO2 mixtures in solar-driven coal/biomass fuel gasification. Benchmarked against the statistical narrow-band model (SNB) of the EM2C laboratory, the WSGG model is suitable for the temperature range of 400–2500 K and the path length range of 0.001–60 m. This study also explored the effect of the CO/CO2 molar ratio on the overall emissivity of the mixture. Furthermore, the model introduces a pressure term into the emissivity calculation process and broadens the pressure range (1 bar, 5 bar, 45 bar). For the first time, the WSGG model is applied to the case where the H/C element ratio is 0, and the fluctuating temperature distribution case (1000 – 2000 K) is analyzed, which is suitable for coal/biomass fuel gasification. In addition, this study calculated the one-dimensional radiation transfer equation. The results show that the average radiation source term difference between the new WSGG and the benchmark SNB model is within 5 % in common solar gasification engineering conditions (5 bar, 5 m). Meanwhile, this study also clarified the effect of pressure on the radiation heat transfer with different temperatures.

A weighted-sum-of-gray soot-fractal-aggregates model for nongray heat radiation in the high temperature gas-soot mixture

Soot, a product of insufficient combustion, is usually in the form of aggregate. The multi-scattering of soot fractal aggregates has been proved to play an important role in studying the soot radiative properties, which is rarely considered in predicting the radiative heat transfer in combustion flame. In the present study, based on the Rayleigh-Debey-Gans-Fractal-Aggregate (RDG-FA) theory, a model, named Weighted Sum of Gray Soot fractal Aggregates (WSGSA), used to calculate the radiative properties of nongray soot fractal aggregates in gas-soot mixture is developed by combining the features of full-spectrum k-distribution (FSK) model and Weighted-Sum-of-Gray-Gases (WSGG) model. Four gray soot fractal aggregates obtained from reordering in the form of the k-distribution at Gauss-Legendre integral points are selected to represent the nongray soot fractal aggregate, while corresponding weighting factors are obtained by fitting the radiation data obtained by the line-by-line (LBL) model with soot radiation data. Then, the WSGSA model is systematically validated by studying the radiative heat transfer properties in a one-dimensional plane-parallel slab system only with soot aggregates. The maximum relative discrepancies of emittance under different temperatures and different path lengths are less than 12%, while the maximum relative discrepancies of radiative heat source term and radiative heat flux are both less than 11% with path length less than 3 m. Moreover, combined with the WSGG model, the performance of the WSGSA model is further validated by predicting the radiative heat transfer properties in the mixture consisted of gases and soot aggregates, and acceptable results are also obtained. All the results reveal that the developed model can be applied to predict the radiative heat transfer in the mixture containing gases and soot aggregates, even in the case of significant changes in temperature and species concentration.

Development in the Heat Transfer Properties of Nanofluid Due to the Interaction of Inclined Magnetic Field and Non-Uniform Heat Source

In the current scenario a new mathematical model is designed and examined for the unsteady course of nanofluid through permeable vertical surface due to the interaction of inclined magnetic field. Radiative heat transfer properties is included assuming the Cogley radiation, dissipative heat energy due to the conjunction o magnetic field i.e., Joule dissipation and the space and time-dependent heat source/sink amplifies the study as well. Depending upon todays need in various industries the implementation of nanofluid is vital. Therefore, present study involves the behavior of both metal and oxide nanoparticles in the base fluid kerosene. Involvement of transformation rules the problem is converted into nonlinear set of ODEs and further these are solved employing approximate analytical technique such as Variational Iteration Method (VIM). The characteristics of various flow parameters are analyzed via graphs and the numerical simulation along with the validation of the result is obtained through tables. The comparative study brings out the convergence criterion of the methodology adopted herein. However, the favorable results are; the fluid temperature augments with increasing nanoparticle volume fraction and suction enriches both the fluid velocity and temperature whereas injection retards it significantly.

Experimental investigation on degradation of heat transfer properties of a black chromium-coated aluminium surface solar collector tube

ABSTRACTIn our earlier research, we have studied the surface radiation property degradation of the solar collector receiver tube material [Logesh, K., R. Ganesh, I. Saran Raj, V. Ramesh, and B. Tharun Raj. 2017. “Experimental Investigation on Radiation Heat Transfer Properties Degradation of Aluminium Solar Receiver Tube Material.” International Journal of Ambient Energy. doi:10.1080/01430750.2017.1335230]. In this study, the examination is expanded to degradation property of the selective surface, black chromium-coated aluminium material. Black chrome is mainly used for its high corrosion resistance. The selective surface has been examined for its emissivity (ε) and absorptivity (α) changes due to its sunlight exposure at elevated temperatures. The exposure duration ranges from 240 to 960 h. In the selected range of exposure, the properties are measured in four intervals. The emissivity (ε) is found to decrease, whereas the absorptivity (α) gets increased for coated than the uncoated surfaces for the same duration of exposure. The examination is also carried out to find the nature of the heat transfer properties determined by its temperature. For every duration of exposure, the properties are measured at 10 temperature intervals.

Ab-Initio Calculation of Spectral Absorption Coefficients in Molten Fluoride Salts with Metal Impurities

Molten salts have been proposed as coolants for numerous advanced reactor designs. It is envisioned that these reactors, both fluoride-salt–cooled high-temperature reactors and molten-salt–fueled reactors will operate at high temperatures, where the radiative heat transfer properties of the salts may be required for accurate heat transfer analysis. Experimental challenges have prevented the measurement of absorption coefficients in most salts. In an attempt to fill this gap in data, the Vienna Ab-Initio Simulation Package is used in the present research to calculate the absorption coefficient resulting from photoelectric interactions in numerous molten salts. Ab-initio molecular dynamics is used to generate the amorphous structures of a variety of salts. The pure halide salts LiF, FLiNaK, and FLiBe, are shown to be optically clear through a wide portion of the electromagnetic spectrum. Conversely, the transition metal fluoride salt KF-ZrF4 is shown to be substantially opaque. As chromium is a known impurity of concern from the corrosion of steels in reactor environments, the effect on absorption of low levels of chromium in an otherwise transparent salt is investigated and found to significantly increase absorption at relevant wavelengths.

Linear Parameter-varying Approach for Modeling and Control of Rapid Thermal Processes

In the present paper, a new approach is presented to model and control single wafer rapid thermal processing (RTP) systems. In the past decade, RTP has achieved acceptance as the mainstream technology for semiconductor manufacturing. Thermal processing is one of the most efficient ways to control the phase-structure properties. Moreover, the time duration of RTP systems reduces the so-called thermal budget significantly compared to the traditional methods. RTP implementation is based on the use of light from heating lamps to provide a heat flux. This process is highly nonlinear due to the radiative heat transfer and material properties. By invoking the first principles-based models, we develop in this paper a linear parameter-varying (LPV) model to directly account for the nonlinearities within the system. The model is then discretized into a high-order LPV model; thereafter, principal component analysis (PCA) method is utilized to reduce the number of LPV model’s scheduling variables, followed by the use of proper orthogonal decomposition (POD) for model order reduction. Using the reduced order model, we then design a gain-scheduled controller to satisfy an induced L2 gain performance for tracking of a temperature profile and show improvement over other controller design methods suggested in the literature.

Experimental investigation on radiation heat transfer properties degradation of (black nickel-coated aluminium) solar receiver tube material

ABSTRACTIn our previous research, we have studied the surface property degradation of the uncoated receiver tubes of solar water heater at elevated temperatures Logesh et al. [2017. “Experimental Investigation on Radiation Heat Transfer Properties Degradation of Aluminium Solar Receiver Tube Material.” International Journal of Ambient Energy 1–6]. In this study, the investigation is extended to property degradation of selective surface, black nickel-coated aluminium material. The selective surface has been investigated for its absorptivity (α) and emissivity (ϵ) changes due to its exposure to sunlight at elevated temperatures. The duration of exposure ranges from 240 to 960 h; the properties are measured at four intervals in the selected range of exposure. The investigation is also carried out to find the temperature dependence nature of the heat transfer properties. The properties are measured at 10 temperature intervals for every period of exposure. The absorptivity (α) is found to be increased for the coated surface than the uncoated, whereas the emissivity (ϵ) gets decreased for the same period of exposure.

Near-field radiative heat transfer in graphene plasmonic nanodisk dimers

Near-field thermal radiation mediated by surface plasmons in parallel graphene nanodisk dimers is studied using a semianalytical model under the electrostatic approximation. The radiative heat transfer between two disks as a function of the distance between them in coaxial and coplanar configurations is first considered. Three regimes are identified and their extents determined using nondimensional analysis. When the edge-to-edge separation is smaller than the disk diameter, near-field coupling and surface plasmon hybridization lead to an enhancement of the radiative heat transfer by up to four orders of magnitude compared to the Planck blackbody limit. A mismatch in the disk diameters affects the plasmonic mode hybridization and can either diminish or enhance the near-field radiation. Destructive interference between eigenmodes that emerge when the relative orientation between disks is varied can induce a twofold reduction in the radiative heat transfer. In all configurations, the radiative heat transfer properties can be controlled by tuning the disk size/orientation, the substrate optical properties, and graphene's doping concentration and electron mobility.

Dispersive Properties of Mesoporous Gold: van der Waals and Near-Field Radiative Heat Interactions

We present a theoretical calculation of the dispersive interaction of mesoporous Au sponges. The ability to manipulate the dielectric function of the sponge by varying its porosity is applied to the calculation of the Hamaker constant using Lifshitz theory and the near radiative heat transfer properties. By using an effective medium approximation the dielectric function of the mesoporous sponge is calculated. As the porosity increases, our calculations show that the Hamaker constant decreases significantly. This is also used to calculate the van der Waals force between a gold nanoparticle and a gold mesoporous sponge as a function of the porosity. Also, using the fluctuating electrodynamics formalism, the change in the local electromagnetic density of states in the near field is calculated. As an application, the near field radiative heat absorbed by a SiC nanoparticle at a fixed distance from a Au sponge is calculated for different values of the porosity. The maximum heat transfer is achieved for high porosities near a metal–insulator percolation transition.

セメントクリンカー粉塵粒子群のホットモデル単色輻射物性特性

セメントクリンカー粉塵粒子群のホットモデル単色輻射物性特性

Enhancement of Natural Convection by Carbon Nanotube Films Covered Microchannel-Surface for Passive Electronic Cooling Devices.

Owing to the outstanding properties of thermal conduction, lightweight, and chemical durability, carbon nanotubes (CNTs) have revealed promising applications in thermal management materials. Meanwhile, the increasingly popular portable electronics and the rapid development of space technology need lighter weight, smaller size, and more effective thermal management devices. Here, a novel kind of heat dissipation devices based on the superaligned CNT films and underlying microchannels is proposed, and the heat dissipation properties are measured at the natural condition. Distinctive from previous studies, by combining the advantages of microchannels and CNTs, such a novel heat dissipation device enables superior natural convection heat transfer properties. Our findings prove that the novel CNT-based devices could show an 86.6% larger total natural heat dissipation properties than bare copper plate. Further calculations of the radiation and natural convection heat transfer properties demonstrate that the excellent passive cooling properties of these CNT-based devices are primarily caused by the reinforcement of the natural convection heat transfer properties. Furthermore, the heat dissipation mechanisms are briefly discussed, and we propose that the very high heat transfer coefficients and the porous structures of superaligned CNT films play critical roles in reinforcing the natural convection. The novel CNT-based heat dissipation devices also have advantages of energy-saving, free-noise, and without additional accessories. So we believe that the CNT-based heat dissipation devices would replace the traditional metal-finned heat dissipation devices and have promising applications in electronic devices, such as photovoltaic devices, portable electronic devices, and electronic displays.

Experimental investigation on surface radiation properties’ degradation of solar collector receiver tube material

ABSTRACTIn this study, we have investigated the degradation characteristics of surface radiation heat transfer properties of stainless steel 304. The emissivity (ε) and absorptivity (α) of stainless steel has been measured for a surface unexposed to sunlight and that exposed to sunlight for various time intervals (240, 480, 720, and 960 h). The temperature dependence of these properties is also measured for the temperature range of 30–300°C at 10 equal intervals. It is observed that the emissivity of the material increased when it is exposed to sunlight and the temperature dependence is very strong beyond 180°C. The absorptivity of the material increases first and then starts to decrease with exposure time. The combined effect of decrease in absorptivity and increase in emissivity leads to reduction in aS/E ratio, which results in a higher rate of radiation loss for exposed surfaces when compared to that for the unexposed surfaces.

A Review on Sodium Boiling Phenomena in Reactor Systems

Sodium boiling phenomena in nuclear reactors have been reviewed in the context of the renewed interest in sodium-cooled fast reactors. This paper presents all properties that influence sodium boiling behavior, including thermodynamic and transport properties, as well as the typical composition of reactor-grade sodium, the surface wetting, radiative heat transfer properties, and noncondensable behavior. Starting from these properties, the tendency for high superheat is explained, together with the reasons that the problem of superheat can be neglected for reactor systems. The peculiar boiling behavior of sodium in assemblies is explained on the basis of the temperature profile. This leads us to conclude that a typical slug flow pattern prevails for sodium boiling. The boiling heat transfer for pool film boiling is also given, deducing that the critical heat flux phenomena for sodium boiling in reactor systems is mainly related to dryout and not to the departure from nucleate boiling. The correlations that exist for the minimum film-boiling temperature are discussed in light of their applicability to liquid sodium. Although there are already a large amount of data, gaps in the current understanding of sodium are highlighted.

Two Methods for Solving Radiative Transfer in a Two-Dimensional Graded-Index Medium

Two methods are presented for the study of radiative heat transfer in a two-dimensional graded-index semitransparent medium. One is a combination of the cell model and the ray-tracing technique, and the other is a combination of the linear refractive-index cell model and the discrete curved-ray-tracing technique. The medium is discretized into many cells. The constant-refractive-index assumption is used in the first model and the linear approximation of refractive index is used in the second model. We have compared the results and the calculation efficiency of these two methods, as well as the heat transfer at radiative equilibrium. The results of the two methods show little difference. We also studied the radiative heat transfer properties of the medium, and the results show that the refractive-index distribution has great influence on the temperature field. That is, the medium with various types of graded index has a higher or lower temperature than the medium with a constant refractive index. The medium also has a flat temperature field or a sharper temperature field than the constant-refractive-index medium. This is due to internal total reflection in the graded-index medium.

FE Simulation of Vacuum Hot Bulge Forming Process of BT20 Ti-Alloy Cylindrical Workpiece

The vacuum hot bulge forming has been used in aerospace industry to manufacture cylindrical workpiece with improved mechanical properties and reduced fabrication cost. Vacuum hot bulge forming is based on the material soften and the stress relaxation theory. Different from other metal forming techniques, deformation of the workpiece takes place well below yield point and the amount of plastic deformation is directly relaxed to heating temperature and holding time. In this paper, a two-dimension thermo-mechanical coupled finite element model was developed. In this model, nonlinear radiation heat transfer and thermal physical properties of material depending on temperature were considered. This paper carried out numerical simulation of vacuum hot bulge forming of BT20 Ti-alloy cylindrical workpiece by using finite element software MSC.Marc. The temperature field, deformation field and stress field of hot bulge forming of BT20 Ti-alloy cylindrical workpiece were calculated. Numerical simulation results were accorded with experimental ones, which provided for the practice production as theory bases.

Modeling the propagation of drying and decomposition fronts in wood

Decomposition of moist wood is modeled using the shrinking unreacted-core approximation for a finite rate of reaction and the assumption of a thermally controlled evaporation of moisture across an infinitely thin front at constant temperature. The one-dimensional, quasi-steady (along the char layer) equations also take into account convective, conductive, and radiative heat transfer and different physical properties for char, dry wood, and moist wood. The use of realistic values for all the input parameters results in acceptable agreement between predicted and measured weight losses for 0.04-m-thick beech wood particles (external radiative heat fluxes between 40 and 80 kW/m 2 and initial moisture contents between 0 and 47 wt% on a dry basis). A parametric analysis indicates that, apart from a linear dependence on the moisture content, the characteristic times are especially affected by water evaporation in the case of thick samples. It has also been found that assuming a coincident front for both wood decomposition and moisture evaporation, often done in gasification/combustion models, could be applied with sufficient accuracy only for thick samples with high moisture contents and subjected to severe thermal heating.

Radiative Heat Transfer Properties of Electro-Controllable Fluids

Abstract Active control of radiation heat transfer in liquids can be accomplished with the use of a class of fluids referred to here as electro-controllable (EC) fluids. EC fluids in general consist of a colloidal suspension of polarizable, micron-size particles dispersed in a carrier fluid with an appropriate dielectric constant. When an electric field is applied, the particles redistribute in the fluid, changing from a uniformly dispersed configuration to a tightly organized chain formation that follows the lines of the electric field, thus causing a change in the thermal radiation transport. In an example application, experiments are conducted and models are developed for thermal radiation transmittance through a composite window featuring a central layer of EC fluid. The specific EC fluids of this study are made of micron-sized Zeolite particles suspended in a light Silicone oil carrier fluid of appropriate dielectric strength. The incident thermal radiation ranged in wavelength between 500 nm and 800 nm, and the strength of the applied electric field ranged from 100 V/mm to 500 V/mm. The models are applicable for both the dispersed organizational state and the field induced chained state. Absorption was demonstrated to be the fundamental radiation transport property enabling the control process. The EC fluid transmittance predicted by these models are compared to the data obtained by experimental measurement demonstrate very good agreement.

セラミックタイルのふく射熱伝導連成解析

The radiative heat transfer is a dominant heat transfer mechanism at high temperature in the insulation materials of the thermal protection system (TPS) of reentry vehicles because of the high porosity of the materials. It has been well known that serious problems come out when a simple heat conductive analysis is conducted with a thermal conductivity measured by a usual static method and that one should take into account the coupling of radiative and conductive heat transfer in the thermal analysis of such insulations. A finite element code for the combined radiation-conduction analysis is developed. A series of experiments with ceramic tile insulator is also conducted to obtain the fundamental radiative heat transfer properties and to examine the validity of the developed FE program.