Radiative Heat Transfer Calculations Research Articles

A systematic analysis of energy-saving potential of spectrum-selective radiative cooling materials in buildings

Passive cooling of buildings through radiative cooling (RC) is considered an effective method for reducing the energy consumption of air conditioning. In recent years, spectrum-selective RC materials (RCMs) have received extensive attention owing to their low solar absorptivity and high thermal emissivity in the atmospheric window. The energy-saving performance of RCMs is related to their own properties and local meteorological conditions, building type, building form, and energy use patterns. Although several studies have explored the performance of RCMs, most existing studies have considered a single building type and meteorological condition, and simple building forms that differ significantly from real buildings. Hence, the application analyses of RCMs to buildings are not sufficiently comprehensive. This study focused on the energy-saving potential of RCMs for building applications, and presented a systematic framework to analyze the energy-saving potential of RCMs under different conditions. DeST, which is embedded with a long-wave radiative heat transfer calculation module based on the equivalent sky temperature, was adopted as the simulation software. It can accurately calculate the radiative heat transfer in different wavelength bands, thus making the simulation results more accurate. Based on the calculation results of large-scale cases considering four major factors, namely climate, building type, construction year, and installation location of RCMs, the energy-saving performance of RCMs in different cases was analyzed, and the return on investment is performed, and more comprehensive suggestions were provided for applying RCMs in future buildings.

Determination of interparticle radiative heat transfer and radiation absorption for ellipsoidal particle beds using Monte Carlo ray tracing

The calculation of radiative heat transfer between particles in particle-based solar systems presents significant challenges in terms of computational cost and accuracy. This study addresses these challenges by employing the discrete element method to generate random particle packings and the Monte Carlo ray tracing method to investigate the influence of particle shape and volume fraction (ϕ) on interparticle radiation transfer and the absorption of incident radiation. The results reveal that, at low volume fractions, the shape and orientation of ellipsoidal particles have a pronounced impact on direct radiative transfer. Oblate particles exhibit the highest average radiation distribution factor (RDF), and the dispersion of RDF data is more prominent for oblate and elongated prolate particles compared to spheres. As ϕ increases to 0.35, the contribution of indirect transfer from neighboring particles becomes significant for oblate particles, resulting in a reduction in the dispersion of RDF data. Furthermore, when ϕ reaches 0.55, the decisive factor in determining RDF becomes the shielding effect of particles positioned between the emitting and receiving particles. The average RDF tends to converge among particles of different shapes, although oblate and elongated prolate particles still exhibit considerably larger RDF dispersion compared to spherical and near-spherical particles. Additionally, the effective absorptivity of the particle bed for incident radiation decreases as the aspect ratio of particles increases at all volume fractions. Moreover, the transmitted fraction demonstrates an exponential relationship with the depth, and beds composed of spherical particles exhibit better radiation transmittance compared to beds containing non-spherical particles.

Detailed modeling of the radiative feedback in the large eddy simulation of a lab-scale heptane pool fire

This article proposes a detailed modeling of gas/soot radiation for large eddy simulation (LES) of sooting pool fires with a focus on the radiative heat feedback through the fuel vapor dome. The numerical model combines a non-adiabatic steady laminar flamelet (SLF) model, a two-equation acetylene/benzene-based soot production model, the Rank-Correlated Full-Spectrum K (RCFSK) and a presumed filtered density function (FDF) approach to model the subgrid-scale (SGS) interactions between turbulence, chemistry, soot production and radiation. This LES model is applied to simulate 15 cm heptane pool fire. The reliability of the RCSFK and the acetylene/benzene soot model for heptane pool fire scenarios is investigated through a priori analysis. First, decoupled radiative heat transfer calculations along lines of sight, representative of heptane pool fires and covering a wide range of optical thicknesses, demonstrate that the RCFSK computes the absorption of the fuel vapor dome with a similar accuracy as the narrow-band correlated-k (NBCK) model. Second, simulations of laminar coflow diffusion flame are used to extend the acetylene/benzene-based soot production model, previously validated for C1-C3 hydrocarbons, to n-heptane flames. LES of the 15 cm heptane pool fire reproduces with fidelity the flame structure in terms of mean and fluctuating temperatures and mean soot volume fraction (SVF). Simulations with and without considering heptane vapor radiation evidences its effects on the radiative outputs and the heat feedback. The radiative contribution of heptane vapor noticeably enhances the radiative loss to the surroundings and the peak of axial radiative flux. On the hand, the heat feedback to the pool surface is predicted within the experimental uncertainty when this contribution is taken into account with the radiative component being reduced by more than 10 %.

Thermal radiation at the nanoscale and applications

There has been a paradigm shift from the well-known laws of thermal radiation derived over a century ago, valid only when the length scales involved are much larger than the thermal wavelength (around 10 μm at room temperature), to a general framework known as fluctuational electrodynamics that allows calculations of radiative heat transfer for arbitrary sizes and length scales. Near-field radiative heat transfer and thermal emission in systems of sub-wavelength size can exhibit super-Planckian behavior, i.e., flux rates several orders of magnitude larger than that predicted by the Stefan–Boltzmann (or blackbody) limit. These effects can be combined with novel materials, e.g., low-dimensional or topological systems, to yield even larger modifications and spectral and/or directional selectivity. We introduce briefly the context and the main steps that have led to the current boom of ideas and applications. We then discuss the original and impactful works gathered in the associated Special Topic collection, which provides an overview of the flourishing field of nanoscale thermal radiation.

Effect of different HITRAN databases on the calculation of radiative heat transfer of H2O and CO2 in a one-dimensional planar layer

The performance of HITRAN 2020 and 2016 databases on calculating radiative heat transfer of H2O and CO2 was evaluated in this study. The main spectral bands and lines contributing to the discrepancy in accuracy between the HITRAN 2020 and 2016 databases were determined. For the calculation of radiation of H2O, the HITRAN 2020 database showed higher accuracy than the HITRAN 2016 database in the isothermal/homogeneous cases. The maximum relative error of radiative heat flux of the HITRAN 2020 was reduced by 3.93%, due to the addition of 157, 165, 110 and 121 spectral lines in the 3425–3450 cm−1, 3600–3625 cm−1, 3950–3975 cm−1 and 4050–4075 cm−1 wavebands, respectively. Whereas, for the isothermal/homogeneous CO2, the difference of the average relative error in radiative source calculated by the HITRAN 2020 and 2016 databases was less than 0.1%. For the air and oxy-fuel combustion cases, the HITRAN 2020 showed a better performance than the HITRAN 2016 on predicting radiative heat transfer. The maximum relative error of radiative heat flux of the HITRAN 2020 was reduced by 4.32%. This was mainly ascribed to the increase in the number of the spectral lines of H2O in the HITRAN 2020 in the wavebands of 3425–3450 cm−1, 3600–3625 cm−1, 3950–3975 cm−1 and 4050–4075 cm−1 at high temperatures.

Study of the roughness effect on the normal spectral emissivity of GH3044

The normal spectral emissivity is a significant parameter characterizing the radiative capacity of the object's surface. However, the normal spectral emissivity of superalloys varies with the rough state of the surface, causing unacceptable errors in radiation thermometry and heat transfer calculation for aero-engines and gas turbines. A computational method for estimating the effect of roughness on the normal spectral emissivity of superalloys was established based on constructing randomly rough surfaces. Through this method, the effect of roughness on four emissivity models commonly used in multispectral radiometric thermometry (MRT) was computationally evaluated, and the relationship between roughness and spectral emissivity was quantitatively analyzed. In addition, the normal spectral emissivity of GH3044 superalloy with different rough surfaces at 300 K and high temperatures was experimentally measured and compared with the result calculated by this computational method. Their maximum relative error was close to ± 5 % for the rough surface without emissivity oscillation. This study promotes understanding the mechanism of normal spectral emissivity varying with roughness.

Calculation of Radiative Heat Transfer in a Composite Z-Pinch

Calculation of Radiative Heat Transfer in a Composite Z-Pinch

View factor for radiative heat transfer calculations between triangular geometries with common edge

View factor for radiative heat transfer calculations between triangular geometries with common edge

Evaluation of spectral line mixing models and the effects on high pressure radiative heat transfer calculations

At higher pressures, the spectral lines in a gas spectrum can become more broadened and shifted due to collisional effects, leading to increased spectral line mixing. In the present study, we have reviewed different line mixing models for modeling the absorption spectra for three gases of CO2, CO, and H2O and applied these models to simulate radiative spectra for pressures within approximately 5–100 bar. The simulated data were compared with the yet-published high-pressure spectral experimental data as well as experimental data published in the literature. It has been found that the empirical “pseudo-Lorentz” line shape model considering the line mixing effects shows the best performance for modeling high-pressure gas spectra for the tested conditions, which is easy to be implemented and relatively accurate, thus is recommended for high-pressure radiative heat transfer calculations in engineering applications. Based on the selected “pseudo-Lorentz” line mixing model, we have constructed a high-pressure absorption coefficients database for CO2, CO, and H2O for pressure ranging from 1 bar to 80 bar. The newly generated database considering the spectral line mixing effects was tested for radiative heat transfer calculations, and the results were compared with the ones without considering the line mixing effects. The significance of ignoring the line mixing effects on radiative heat transfer calculations under different pressures was evaluated with one-dimensional radiative heat transfer benchmark cases. The high-pressure absorption coefficients database is available from the corresponding author upon request.

Building thermal environment solution model and radiation heat transfer calculation model based on Gebhart absorption coefficient

The building walls temperatures are different, and there are radiation heat transfers between the walls, which have the characteristics of multiple reflections and multiple absorptions. The Gebhart absorption coefficient considers the radiation energy both directly absorbed by the wall and indirectly absorbed by other walls multiple times reflections. The absorption coefficient Gij is used to represent the proportion of radiation energy absorbed by surface j from the radiation energy of surface i. Through employing Gebhart absorption coefficient, the Gebhart absorption coefficient matrix is constructed based on the wall angle factors and the wall radiation emissivity. The building thermal environment solution model is established, which can calculate the indoor air temperature and wall temperature. The wall radiation heat transfer calculation model is built to calculate the net radiation heat transfer of the walls. An experimental bench was carried out to verify the above theoretical calculation results. The research results showed that the theoretical calculated values of the air temperatures of the building thermal environment solution model were basically consistent with the measured values, and the deviation was within 0.25°C. The deviation between the calculated values of the wall temperatures and the measured values was basically within 1.0°C. The error between the calculated values of the wall radiation heat transfer calculation model and the measured values varied within a small range. The Gebhart absorption coefficient is more appropriate in characterizing radiation heat transfer mechanism of the walls, and it provides a new way and thought for the analysis of radiation heat transfer on building wall.

Improved Monte Carlo method for radiative heat transfer in semitransparent media with BRDF surface

This paper proposes an improved Monte Carlo method (IMCM) for the radiative heat transfer solution in semitransparent media with BRDF surface for the first time. This method not only keeps the strong points of high precision and excellent applicability to the calculation of radiative heat transfer in complex shape scattering media with BRDF surface, but also overcomes the shortcomings of time-consuming for huge amounts of energy beams tracing in the Monte Carlo method. In the IMCM we developed, the energy beams of reflection or scattering can be worked out by proportional iteration accumulation. The scattering and reflection process only need to be sampled twice rather than lots of scattering and reflection energy beams tracing. This improvement not only retains the high accuracy of the MCM, but also greatly improves the calculation efficiency. A number of radiative heat transfer problems in semitransparent media enclosure with BRDF surface are studied. The effects of absorption coefficient, wall emissivity and scattering characteristics on radiative heat transfer are analyzed. Results indicate that the IMCM is a very efficient method with high precision for solving radiative heat transfer.

Radiative heat transfer calculations using full spectrum k-distribution method for benchmark test cases

In the present work, the full spectrum k-distribution method (FSK) has been adopted to calculate the radiative transfer in the presence of participating gaseous medium within an enclosure. The radiative properties of the medium is obtained from the HITEMP-2010 database. Further, the radiative properties have been assembled into a monotonically increasing function using the FSK. Moreover, a look-up table has been developed for these properties at different thermodynamic states and a multi-dimensional linear interpolation technique for unavailable thermodynamic states of gases. Furthermore, the FSK method is extended for mixture of gases using different mixing models such as superposition, multiplication and hybrid models. The radiation transfer equation (RTE) is solved by finite angle method to calculate the wall heat fluxes and the divergence of radiative heat flux for various test cases in the categories of homogeneous and non-homogeneous medium having different conditions of temperature and mole-fraction. The multiplication mixing model produces most accurate results among the mixture models used here. The results obtained from FSK has been validated against benchmark solution of line by line method (LBL). The FSK method has been also successfully applied to non-homogeneous gaseous medium for single gas or mixture of gases with almost LBL accuracy at extremely less computational cost.

Numerical investigation to evaluate the effects of gravity and pressure on flame structure and soot formation of turbulent non-premixed methane-air flame

In this study, a turbulent non-premixed (diffusion) methane-air flame has been investigated computationally to analyze the influences of pressure and gravity on flame structure and sooting characteristics between 1 and 10 atm. The simulation has been conducted in a 2-D axisymmetric computational domain using the finite volume-based computational fluid dynamics (CFD) code. The interaction of turbulence and chemistry is modeled by considering the steady laminar flamelet model (SLFM) and the GRI Mech 3.0 chemical mechanism. The radiative heat transfer calculation is carried out by considering the discrete ordinate (DO) method and the weighted sum grey gas model (WSGGM). The semi-empirical Moss-Brookes model is considered to calculate soot. The impact of gravity on flame and sooting characteristics are evaluated by comparing the normal-gravity flames with the zero-gravity flames. The effect of soot and radiation on flame temperature is also examined. The results show a close agreement with the measurement when both soot and radiation are included in the numerical modeling. The rates of soot formation, surface growth, and oxidation increase with increased operating pressure, regardless of gravity. Zero-gravity flames have a higher soot volume fraction, a wider soot-containing zone, a higher CO mass fraction, and a lower flame temperature than normal-gravity flames while maintaining constant pressure. In normal-gravity flames, the CO mass fraction decreases with pressure, whereas it increases with pressure rise in flames of zero gravity. Flames of zero gravity appear taller and broader compared to the flames of normal-gravity for a fixed pressure. An increase in pressure significantly reduces the flame length and width in normal-gravity flames. However, the pressure elevation has little effect on the shape of a zero-gravity flame. The outcomes of the present study will assist in fully understanding the combustion and sooting characteristics of turbulent diffusion flames that will help design and develop high-efficiency, pollutant-free combustion devices and fire suppression systems for space application.

A machine learning based line-by-line absorption coefficient model for the application of atmospheric carbon dioxide remote sensing

Space-based remote sensing, which is used to infer CO2 concentration from the satellite-measured atmospheric spectral absorption signals, is an effective way to obtain CO2 concentration data for greenhouse gas monitoring. The next-generation greenhouse gases monitoring satellites mainly address the challenge of improving the spatial and temporal resolutions of observations, which will dramatically increase the computational power required for the CO2 retrievals. One of the bottlenecks is the high computational cost for the radiative heat transfer calculations, which involve inefficient high-resolution (usually requires a line-by-line spectral resolution) spectral modeling. Therefore, developing a fast and accurate spectral modeling method becomes necessary to tackle this problem. In the present study, we presented a machine learning based line-by-line absorption coefficient calculation (prediction) method for CO2 in the applications of atmospheric remote sensing. By training an artificial neural network with data randomly generated from a line-by-line CO2 absorption coefficient look-up table, a compact, accurate and efficient absorption coefficient prediction model can be developed, which only takes the CO2 thermodynamic states as input. The proposed method has been tested by developing an absorption coefficient prediction model for the CO2 1.6 μm spectral band, which was later used to simulate the measured spectra for clear-sky conditions from the Greenhouse gases Observing SATellite (GOSAT) for several different locations around the world. Results have shown that the model is both accurate and efficient. In addition, the same approach has been applied to fit the absorption coefficient tables provided by the Orbiting Carbon Observatory (OCO)-2 mission and an accurate prediction model was also presented.

Numerical simulation of radiant floor cooling systems using PCM for naturally ventilated buildings in a hot and humid climate

This study constructs a coupled simulation model of heat balance analysis and computational fluid dynamics (CFD) for naturally ventilated buildings with installed phase change materials (PCMs), and demonstrates a modeling method for the thermal storage effect of PCM in the simulation. Then, the constructed model was applied to a sensitivity analysis to clarify the key parameters of a radiant floor cooling system using PCMs. The simulation was conducted for the target building, where the radiant floor cooling system was installed, and the results were validated with field measurements in the hot and humid climate of Indonesia. The results showed that the coupled simulation model can evaluate both the temperature and heat balance of PCMs in naturally ventilated buildings, where air temperature and air flow distribution are not uniform, with higher accuracy. The temperature and heat flux on the floor surface showed good correlations between the simulation and measurement, with the root mean square errors of 0.3–0.5 °C and 4.0 W/m2, respectively. In particular, coupling with radiative heat transfer calculation increased the simulation accuracy in terms of floor surface and PCM temperatures by 0.3 °C when the surface temperature difference between the floor and the other surfaces, i.e., the wall and ceiling, was considerable owing to the thermal storage effect of the PCMs. The following sensitivity analysis showed that a PCM thickness of 6 mm and ventilation during nighttime are the optimum settings for the radiant floor cooling system under the given hot and humid conditions.

Improvements, extensions, and validation of the Urban Weather Generator (UWG) for performance-oriented neighborhood planning

Policymakers have to assess and optimize neighborhood-scale planning aspects to profit from climate mitigation and adaptation measures due to various thermal environmental concerns. There are many models to evaluate the local thermal environment, but their outputs or complexity limit their application in performance-oriented neighborhood planning. In this study, the Urban Weather Generator (UWG) was updated to meet the needs of neighborhood planners. The model improves the radiation, vegetation, and convective heat transfer calculation processes to better match the actual physical representation of the neighborhood. Besides, the functions of the model are extended to calculate the mean radiant temperature (MRT) and water temperature for the evaluation of outdoor human thermal comfort and blue infrastructure effects. A new graphical interface also makes it easier for planning decision-makers to start quickly. Then, the prediction results of the updated model are evaluated with the neighborhood observations of the hot and humid regions of Guangzhou and Nanning. The air temperature, relative humidity (RH), and MRT results revealed that this novel model performs quite consistently and demonstrated the reliability and computational efficiency of the UWG model. In the early stages of thermal environment evaluation, the UWG model can be used for performance-oriented neighborhood planning.

Assessment of narrow-band and full spectrum gas radiation methods in a real industrial glass furnace configuration

This work is dedicated to a comparison of various methods of gaseous flame radiation in a three-dimensional configuration representative of a glass furnace studied at Saint Gobain Research Paris. For this purpose, several narrow band and full spectrum models of flame radiation are assessed against LBL calculations. Among narrow band models, the recently proposed l-distribution method is shown to be more accurate than the k-distribution method. Among global models, the RC-SLW technique provides a high accuracy and can be confidently used for radiative heat transfer calculations in three-dimensional high temperature configurations.

Study on the Influence of Globe Thermometer Method on the Accuracy of Calculating Outdoor Mean Radiant Temperature and Thermal Comfort

With global warming and the rapid development of urbanization, the outdoor thermal environment is deteriorating. More and more research focuses on the outdoor thermal environment and thermal comfort. The globe thermometer method is widely used in more than half of the outdoor thermal environment research studies, but there is a large error compared with the six-direction method. In order to explore the accuracy of the results of the globe thermometer method and its impact on the subsequent thermal comfort indicators, this study carried out a year-round comparative experiment under multiple working conditions outdoors in cold areas to explore the impact of meteorological parameters such as shortwave radiation, wind speed, and wind direction on the results of the globe thermometer method. The results show that the continuous increase of shortwave radiation reduces the accuracy of the black bulb thermometer to less than 60%, and the instantaneous change of wind speed will make the deviation of the mean radiation temperature obtained by the globe thermometer method exceed 5 °C. The influence of the mean radiation temperature obtained by the globe thermometer method on the thermal comfort index is mainly reflected in the working condition of a high temperature and strong radiation in summer. Taking the six-direction method as the standard, this study gives the scope of application of the globe thermometer method; and taking the human body calculation model of PET as an example, a universal optimization method for detailed division of radiation heat transfer calculation is proposed, so that it can get more accurate and rigorous conclusions in the evaluation of outdoor complex radiation environment.

An Improved nongray-wall emissivity-absorptivity model for the Full-Spectrum Correlated K-distribution method

In practical combustion chambers, the emissivity of wall materials in general varies spectrally and inappropriate treatments may lead to significant errors in modeling radiative heat transfer. To reduce the error of the Full-Spectrum Correlated K-distribution (FSCK) method in practical problems involving nongray walls, a nongray-wall model is proposed by reordering the wall emissivity and absorptivity separately, in which the Planck function at the wall temperature and an effective mean temperature is used as the weight function of wall emissivity and absorptivity, respectively. As a result, this new model preserves the wall emission and considers the spectral radiative property of the wall. Radiative heat transfer calculations in 1D and 3D configurations bounded by nongray walls are simulated to evaluate the accuracy of this new model. The results show that this new model is in general more accurate than the recently developed nongray-wall model (Liu et al., Int. J. Heat Mass Transf. 2021), especially in cases where the wall-to-wall radiative heat transfer plays an important role. It is also showed that this new model is less sensitive to the choice of the effective mean temperature and the medium blackbody emission-averaged temperature is recommended.

A New Method Based on Artificial Neural Network for Radiative Heat Transfer Calculation: Comparison with Benchmark Numerical Solutions in Homogeneous and Heterogeneous Media

A New Method Based on Artificial Neural Network for Radiative Heat Transfer Calculation: Comparison with Benchmark Numerical Solutions in Homogeneous and Heterogeneous Media