Radiation Heat Transfer Modes Research Articles

A methodology for measuring heat transfer coefficient and self-similarity of thermal regulation in microvascular material systems

Fluid transport through microvascular networks—a hallmark for homeostasis in living systems—has transcended to engineered materials, primarily made possible because of modern manufacturing advancements. Vascular-enabled multifunctionality, including thermal regulation and self-healing, holds great potential for extending the lifetime of structural materials and expanding the operational envelope. Prior studies on vascular-based active cooling use a “combined” heat transfer coefficient (HTC): a single parameter lumps convection and radiation effects. Although the resulting mathematical models are linear—an attractive feature for computational modeling, the combined coefficient approach may not be accurate or even applicable if the operating temperature is unknown, which is the case with many thermal regulation applications (e.g., space probes). In this paper, we illustrate the remarked limitations of the lumped approach and advocate the need to use a decoupled HTC by splitting convective and radiative heat transfer modes. We show the broad applicability of the proposed method by applying it to three material systems: glass and carbon fiber-reinforced polymer composites and an additive manufactured metal. We show, using numerical simulations, the differences in the predictions from the decoupled approach with that of the combined HTC; these differences are prominent at higher heat fluxes. Also, the decoupling has enabled us to establish a scaling law that allows transferring of solutions fields across material systems, strengthening further the validity and utility of our approach. This work's significance is two-fold. First, the research is fundamental, providing accurate measurement protocols for critical model parameters. Second, this work facilitates the development of mathematical models for vascular-based thermal regulation that are predictive even for hostile environments (which are often difficult to realize in laboratories), such as outer space.

Solar transparent and thermally insulated silica aerogel for efficiency improvement of photovoltaic/thermal collectors

Photovoltaic/thermal (PV/T) utilization has been regarded as a promising technique to efficiently harvest solar energy, but its thermal efficiency highly degrades in cold seasons because of remarkable heat loss. Although various methods, such as using air or vacuum gap, have been used to reduce heat loss of the PV/T, heat radiative loss still exists. In addition, unlike selective solar absorbers, the current PV/T absorber behaves like an infrared blackbody, showing great radiative heat loss. To overcome this drawback, a novel aerogel PV/T (referred to as “A-PV/T” hereinafter) collector based on solar transparent and thermally insulated silica aerogel is proposed, which can reduce the heat loss from both the non-radiative and radiative heat transfer modes. Experimental testing demonstrates that the thermal efficiency improvement of 25.1%-348% can be achieved for PV/T within the collecting temperature range of 35–70 °C when silica aerogel is introduced, indicating a significant efficiency enhancement. Compared with traditional PV/T (referred to as “T-PV/T” hereinafter) collector, the stagnation temperatures of the A-PV/T collector are 96.7 °C and 103.1 °C in outdoor and indoor environments, which are 27.4 °C and 25.8 °C greater, respectively, indicating a heat loss suppression of the aerogel. Moreover, simulation reveals that useful heat can hardly be provided by the T-PV/T collector in cold seasons, but the A-PV/T still exists a high solar thermal performance, showing good seasonal and regional applicability.

Numerical analysis of the radiant heating effectiveness of a continuous glass annealing furnace

• A modeling approach for industrial-scale continuous radiative heating furnaces was developed. • The spacing between the glass bottle rows affects the radiative heating effectiveness of the furnace. • Optimum row spacing was found to be 35 % of the bottle diameter. • Operating the furnace at the optimum row spacing, furnace length could be reduced by 8.5%. Radiative heat transfer is the dominant mode of heat transfer in glass annealing furnaces. Especially in electrically heated furnaces due to the nonparticipating nature of the furnace atmosphere the radiative heat transfer predominantly occurs between surfaces. In this regard, in container glass manufacturing the layout of the container glasses inside the furnace can considerably affect the radiative heating effectiveness of the furnace. In this numerical study, the effect of the spacing between the bottle rows on the radiant heating effectiveness of the furnace was numerically investigated. An industrial scale continuous annealing furnace model was used and the combined conduction – radiation heat transfer modes in the furnace were solved with the in-house developed transient solver. The convective heat transfer inside the furnace was not taken into account due to its relatively lower contribution in heat transfer. The computational cost of the numerical model is reduced with the use of a computational domain consisting of 7 bottle rows instead of full 29 bottle rows. Reverse Monte Carlo Ray Tracing method is used to solve for the surface-to-surface radiative heat transfer inside the radiant heating zone and integrated into GPU to reduce its computational cost. Further computational speed-up was achieved with the use of a 2nd order accurate radiation boundary condition implementation. It allows a time step increase of 2.5x in comparison with the first order accurate implementation for the same level of accuracy. Finally, a parametric study on the effect of the spacing between the bottle rows on the radiative heating of the bottles have been conducted. In this regard, the spacing between the bottle rows and the conveyor speed have been altered in a way that the throughput of the furnace, the number bottles processed per unit time, is kept constant. The study shows that there is an optimum spacing value between the bottle rows. For the particular bottle geometry considered in this study, the optimum row spacing value is found to be 35% of the bottle diameter. Running the furnace at this optimum value, the length of the heating zone could be reduced by 8.5% compared to the case when the furnace was operated with the bottles almost in touch with each other.

Thermal diffusivity measurement of insulating materials at high temperature with a four-layer (4L) method

This article presents a temperature-temperature thermal characterization method for the measurement of the thermal diffusivity of insulating materials at high temperature. This novel method, noted 4L, is a transient absolute measurement method based on the estimation of the transfer function at the center of a symmetrical stack composed of two specimen of an insulating material sandwiched between two conductive metallic plates. Two different direct 1D semi-analytical models were developed. The first one considers purely conductive opaque materials and the second one takes into account the coupling between radiative and conductive heat transfer modes for purely scattering semi-transparent materials. The first model was used to estimate the thermal diffusivity of two different opaque insulating materials (calcium silicate boards) up to 800 °C with accuracy better than 10%. The second model was used to estimate the thermal diffusivity of a semi-transparent insulating material (ceramic foam) up to 1000 °C with an accuracy of approximately 10%. The second model was used to identify significant estimation errors occurring if a purely conductive model is used for semi-transparent materials. The conducto-radiative model was also used to estimate an average value of the mean extinction coefficient of the semi-transparent material.

Thermal design optimization of passive cooling capability in a dry-storage system by adding wall undulations or semi-circular fins

Thermal design investigation of a novel liner inner wall surface geometry alteration of a ventilated concrete cask used for spent nuclear fuel storage is carried out through three-dimensional Computational Fluid Dynamics (CFD) simulations. The present study reveals that the new proposal returns values for the convective heat transfer coefficient that are almost three orders of magnitude higher than the base case. Hence, proving that even making such a rather simple alterations to the smooth walls geometry, i.e. adding undulations or semi-circular fins, can result in a significant convective heat transfer enhancement, therefore, further reducing the risks of structural damages caused by the dry-storage system overheating. Before conducting the design optimization study mentioned above, two validation test cases have been considered in order to examine the Reynolds Averaged Navier-Stokes (RANS) models’ capability to predict the flow under buoyancy driven and low turbulence levels such is the case in the passively cooled dry cask system. The test cases considered in the validation study sections are the experimental measurements of Betts and Bokhari for turbulent natural convection in an enclosed tall cavity and the temperature measurements in concrete-shielded spent fuel storage cask system reported by McKinnon. The models selected for the validation study are namely the k-ω SST model, the v2-f model, the low-Reynolds k-ε model, and the recently formulated elliptic blending k-ε model. It has been found that out of these models, the low-Reynolds k-ε model was found to provide a good combination in returning both; physically realistic and conservative solution. The present validation using well-resolved RANS simulations has shown that a more quantitative and detailed comparison of the predictions is required in order to clearly highlight the discrepancies between the models’ predictions in terms of velocity, turbulent kinetic energy and, in particular, convective heat transfer coefficient profiles. The commonly adopted methodology in limiting the validation process to temperature profiles comparison only, would cover up these differences as the temperature variation is affected by the combined effects of the convective and the radiation heat transfer modes.

Practical indicators for assessing the magnitudes of wall radiative flux and of coupling effects between radiation and other heat transfer modes on the temperature law-of-the wall in turbulent gaseous boundary layers

Practical indicators are derived to compare radiative and conductive fluxes and the coupling effects of conduction, convection and radiation in turbulent gaseous boundary layers. A first criterion controlling the weight of wall radiative flux compared to wall conductive flux is introduced to assess the necessity of performing radiation simulation under given flow conditions. A second criterion based on the variation of the non-dimensional temperature due to radiation (scaled in wall units) is also developed to predict whether a wall model accounting for radiation is required when coupling effects become significant. These criteria are built from turbulent channel gaseous flow field simulations for many typical conditions, based on a k-∊ model, a given turbulent Prandtl number expression and a spectral CK model for radiation. They have been validated from the comparison with corresponding fully-coupled results where turbulent fields are solved numerically with direct numerical simulations and where the radiative energy transfer is solved with a Monte Carlo approach.The obtained criterion results are then presented and thoroughly analyzed on a large set of conditions, which would not have been feasible with direct numerical simulations or large-eddy simulations. The influence of system size, Reynolds number, wall emissivity and pressure is synthesized in 2D contour plots of interest for engineers and researchers to assess the magnitude of radiation effects in wall bounded flows.

Thermal modeling of an asymmetrical totally enclosed permanent magnet integrated starter generator

Some electromechanical systems involve totally enclosed electrical machines. In these cases, forced internal air cooling, for instance by the use of fans, is nearly impossible. However, in the same time, permanent magnet machines have to expel their internal rotor losses. In this context, an accurate estimate of heat exchange transfers and heat sources are important, for instance for the determination of the temperature distribution. This can be used for future designs of the same type of machine. This paper details the thermal modeling of a totally enclosed permanent magnet machine, such as an integrated starter generator (ISG). For this purpose, a lumped parameter network is built. Thermal conduction, convection and radiation heat transfer modes are considered. Losses are calculated using both analytical expressions and (coupled) electromagnetic finite element analysis (FEA). Simulation results are compared and validated with experimental data.

Computation fluid dynamics analysis of the Reactor Cavity Cooling System for Very High Temperature Gas-Cooled Reactors

The design of passive heat removal systems is one of the main characteristics of the modular Very High Temperature Gas-Cooled Reactors (VHTR) vessel cavity. The Reactor Cavity Cooling System (RCCS) is a key heat removal system during normal and off normal conditions. The design and validation of the RCCS is necessary to demonstrate that VHTRs can survive the postulated accidents. The commercial Computational Fluid Dynamics (CFD) STAR-CCM+/V5.02.009 code was used for three-dimensional system modeling and analysis of the RCCS. Different RCCS geometries and configurations were investigated to analyze heat exchange in the VHTR cavity. Sensitivity analyses over the RCCS cavity height and cooling panel location with respect to the reactor pressure vessel (RPV) wall were performed. The objective of the present work was to use CFD tools for addressing the behavior of the RCCS following accident conditions.Heat removal from the RPV through the RCCS system during normal and off normal conditions is accomplished through radiation, convection and conduction heat transfer modes. The interaction of all three heat exchange mechanisms makes it very challenging to have an accurate description of the RCCS heat transfer dynamics during normal and transient conditions. A systematic analysis of the cavity layout was performed to address the influence of different geometrical parameters on the balance of heat transfer across the RCCS cavity. Mesh convergence was achieved with an intensive parametric study of the different geometrical configurations and boundary conditions selected. Based on the results of previous studies, the Realizable k–ε turbulence model with Two-Layer all y+ wall treatment was used for the analyses discussed in the present work. The numerical results demonstrated that the CFD analyses can resemble the behavior of the full RCCS system from an integral effect point of view, with the main physical phenomena accurately reproduced by the CFD model.

Numerical simulation on the thermal performance of a solar molten salt cavity receiver

The receiver thermal performance directly affects the efficiency, reliability and safety of the entire solar tower power system. A model combining radiation, convection and conduction heat transfer modes was developed to model a 1 MW molten salt cavity receiver with determined basic design parameters. The coupled heat transfer model was used to predict the receiver's steady-state efficiency, temperature distribution and heat losses for various flow layouts and receiver aperture lip sizes.The results show that the fraction of reflective loss (18.78%) is the largest in all heat losses, followed by radiative and convective heat losses, with conduction heat losses as the smallest part which is usually negligible. The heat loss distribution is closely related to the external tube temperature field in the cavity receiver and the specific structure of the receiver. 10 cm lips on both the top and bottom of the receiver aperture reduced the reflective heat loss by 1%. The results for different flow layouts show that center-side flow layout has the best temperature homogeneity, with less possibility of overheating in the central area. This, however, comes at the expense of increased irreversibilities and decreased efficiency. The sides-center flow layout has the worst temperature homogeneity, but the receiver efficiency is 2.06% higher. These results provide a useful reference for designing and optimizing cavity receivers.

Transient Three-Dimensional Heat Transfer Model of a Solar Thermochemical Reactor for H2O and CO2 Splitting Via Nonstoichiometric Ceria Redox Cycling

A transient three-dimensional heat transfer model is developed for a 3 kWth solar thermochemical reactor for H2O and CO2 splitting via two-step nonstoichiometric ceria cycling. The reactor consists of a windowed solar receiver cavity, counter-rotating reactive and inert cylinders, and insulated reactor walls. The counter-rotating cylinders allow for continuous fuel production and heat recovery. The model is developed to solve energy conservation equations accounting for conduction, convection, and radiation heat transfer modes, and chemical reactions. Radiative heat transfer is analyzed using a combination of the Monte Carlo ray-tracing method, the net radiation method, and the Rosseland diffusion approximation. Steady-state temperatures, heat fluxes, and nonstoichiometry are reported. A temperature swing of up to 401 K, heat recovery effectiveness of up to 95%, and solar-to-fuel efficiency of up to 5% are predicted in parametric studies.

Natural convection and radiation heat transfer of an externally-finned tube vertically placed in a chamber

A three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60° for fin surface emissivity between 0.7 and 0.8 and 55° when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important.

Experimental and numerical investigation on the pyrolysis of single coarse lignite particles

This paper reports on the mathematical modeling of the pyrolysis of single coarse lignite particles using a kinetics model coupled with a heat transfer model. The parallel reaction kinetics model of the lignite pyrolysis makes no assumptions about the activation energy distribution and the conversion of sub-reactions. The pyrolysis kinetics parameters were obtained on the basis of experimental data from thermogravimetric analysis (TGA) tests. The heat transfer model includes diffusive, convective and radiative heat transfer modes. The experimental investigations were carried out for single lignite particles in an electrically heated reactor. Measurements of the temperature and mass loss were performed during the pyrolysis in a nitrogen atmosphere. The model predictions for the temperature and mass loss histories agree well with the experimental data, verifying that the mathematical model accurately evaluates the pyrolysis of lignite particles. The effects of temperature and particle size on the pyrolysis time and final residual mass fraction were evaluated using the numerical model.

Modeling catalytic oxidation of lean mixtures of methane–air in a packed-bed reactor

Abstract A mathematical model developed for simulation of combustion of preheated lean homogeneous methane–air mixtures in an adiabatic catalytic packed bed is described. The unsteady one-dimensional model includes inter-phase heat and mass transfers, longitudinal mass dispersion in the gas phase and longitudinal conduction and radiation heat transfer modes. The solid and gas phases are assumed not to be in local thermal equilibrium. Multi-step reaction mechanisms are employed for both the surface and gas phase chemical reactions. The surface reaction mechanism for the oxidation of methane on Pt includes 36 elementary surface reactions and 11 surface adsorbed species. The gas phase reaction mechanism includes 28 elementary reactions and 21 species. The governing equations are the unsteady equations of conservation of mass, chemical species and separate energy equations for solid and gas phases. These equations were solved using the commercial CFD code ‘Fluent’ and a number of modifying subroutines especially are developed to include the equations of surface reactions in the computations. The model has been used to investigate the effects of operational conditions such as the mixture inlet temperature (600–1250 K), approach velocity (0.5–15 m/s) and equivalence ratio (0.15–0.5) on the oxidation of methane within both the catalytic and non-catalytic packed-bed reactors under adiabatic conditions. The calculated values of methane conversion showed good agreement with the corresponding available experimental data. Moreover the amount of heat release at different inlet velocities was calculated and the optimum value discussed.

Simulation of abuse tolerance of lithium-ion battery packs

A simple approach for using accelerating rate calorimetry data to simulate the thermal abuse resistance of battery packs is described. The thermal abuse tolerance of battery packs is estimated based on the exothermic behavior of a single cell and an energy balance than accounts for radiative, conductive, and convective heat transfer modes of the pack. For the specific example of a notebook computer pack containing eight 18650-size cells, the effects of cell position, heat of reaction, and heat-transfer coefficient are explored. Thermal runaway of the pack is more likely to be induced by thermal runaway of a single cell when that cell is in good contact with other cells and is close to the pack wall.

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part I: Stationary conditions

This paper describes numerical simulations, using thermal networks, of shield temperatures and radiative and conductive heat losses of a super-insulated cryogenic storage tank operating at 77 K. Interactions between radiation and conductive heat transfer modes in the shields are investigated, by calculation of local shield temperatures. As a new method, fluid networks are introduced for calculation of stationary residual gas pressure distribution in the evacuated multilayer super-insulation. Output from the fluid network is coupled to the iterative thermal network calculations. Parameter tests concern thickness and emissivity of shields, degree of perforation, residual gas sources like desorption from radiation shields, spacers and container walls, and permeation from the inner container to the evacuated insulation space. Variations of either a conductive (thickness of Al-film on Mylar) or a radiative parameter (thermal emissivity) exert crosswise influences on the radiative or conductive heat losses of the tank, respectively.

Thermal applicability of two-phase thermosyphons in cooking chambers––experimental and theoretical analysis

This paper studies the thermal applicability of two-phase thermosyphons in ovens/furnaces. The objective is to evaluate the performance of thermosyphons as heat transfer devices responsible for connecting the combustion and cooking chambers. Two different approaches were used: first, an experimental setup, which simulates an internal cross section of a convectional bakery oven cooking chamber assisted by vertical thermosyphons, was built. The experimental measurements show that the thermosyphons were able to provide a uniform temperature distribution inside the cooking chamber without the appearance of any overheated spot. In addition to the experimental setup, a theoretical lumped model was used to predict the air temperature variation and the relative importance of the convective and radiative heat transfer modes inside the cooking chamber. The net radiation heat transfer between the internal walls was determined numerically by means of a method described in [20]. The finite difference technique was applied for the computation of the non-linear differential system. The comparison between the model and the experimental data can be considered good, and the radiation was the major heat transfer mechanism inside the cooking chamber.

Signal generation mechanisms, intracavity-gas thermal-diffusivity temperature dependence, and absolute infrared emissivity measurements in a thermal-wave resonant cavity

The operating thermal power transfer mechanisms in a thermal-wave resonant cavity were explored theoretically and experimentally. Both steady-state ac (thermal-wave) and dc temperature rise were considered, and conduction and radiation heat transfer modes were found to co-exist in the cavity. By introducing controlled variable offset dc resistive heating superposed on the fixed-amplitude thermal-wave oscillation, it was also found that the thermal-diffusivity values of the intracavity gas can vary sensitively as a function of the dc temperature rise within a thin boundary layer adjacent to the cavity thermal source (a metallic Cr–Ni alloy strip). This resulted in the measurement of the temperature dependence of the thermal diffusivity of air. Furthermore, the observed dominance of thermal-wave radiation power transfer in the phase channel of the thermal-wave signal at large cavity lengths allowed the measurement of the absolute infrared emissivity of the thin Cr–Ni strip source material: ε=0.091±0.004.

A universal model for studying the performance of Carnot-like engines at maximum power conditions

A universal model for studying the performance of endoreversible heat engines with heat leak is presented. By exploiting finite-time thermodynamics, the new model allows detailed analyses of Carnot-like engines under combined modes of heat supply and/or release. Many established laws and major conclusions derived in several references are shown to represent very special cases of the new formulation. Furthermore, as a special case of the general model, the performance of endoreversible heat engines under combined conduction, convection, and radiation heat transfer modes is studied and generated results are displayed graphically.

The Efficiency at Maximum Power Output of Endoreversible Engines under Combined Heat Transfer Modes

A mathematical model for investigating the performance of endoreversible heat engines under combined conduction, convection, and radiation heat transfer modes is presented. The model is suitable to be introduced to engineering students attending a course in thermodynamics who may apply it to predicting the performance of real engines and a variety of energy conversion systems in a simplified manner. Results generated by the model show that the relative contribution of conduction/convection and radiation heat transfer modes deeply affect the efficiency at maximum power output. Moreover, a number of well-known formulae presented in several references are shown to represent special cases of the new formulation.

EMPIRICAL EVALUATION OF AN IMPORTANT APPROXIMATION FOR COMBINED-MODE HEAT TRANSFER IN A PARTICIPATING MEDIUM USING THE FINITE-ELEMENT METHOD

Abstract An approximation common to many finite-element formulations for combined-mode heal transport is investigated. Although the approximation has been employed in the past, no discussion has been provided in the open literature regarding the implications of the approximation for solution accuracy. Two combined-mode problems involving conductive, convective, and radiative heat transfer modes in one and two dimensions are solved using two finite-element formulations. The first formulation employs a traditional Galerkin method, while the second employs the approximation considered by this work. Results for the two benchmark problems are compared far accuracy and speed, and recommendations are made.