Radiative Heat Transfer Analysis Research Articles

Numerical simulation of radiative heat transfer in indoor environments on programmable graphics hardware

The efficient use of energy for heating and cooling of indoor environments requires an accurate prediction and analysis of radiative heat transfer. Therefore it is necessary to use modern computer methods as otherwise the computational costs may become higher than for convective heat transfer, which is known to be a huge computational problem. A key problem in calculations of radiative heat transfer is the problem of mutual visibility arising in the determination of view factors. This is the same challenge that global illumination has to cope with which is one of the fundamental topics in computational graphics. The visibility problem is efficiently solved by modern graphics hardware. Therefore an OpenGL-based algorithm is developed to quickly and accurately calculate view factors for arbitrary, complex geometries. Theoretical and implementation details of the applied methods are given. We demonstrate the advantages of the developed computational method by a virtual test room for a tubular radiator, a heating of a warehouse by ceramic infrared heaters and the heat transfer in a car cabin.

Combined Mode Conduction and Radiation Heat Transfer in a Porous Medium and Estimation of the Optical Properties of the Porous Matrix

This article deals with the analysis of combined mode conduction and radiation heat transfer in a porous medium, and simultaneous estimation of the optical properties of the porous matrix. Simultaneous solution of the gas- and solid-phase energy equations encompasses local thermal nonequilibrium, while the convective heat exchange term couples the gas- and the solid-phase energy equations. A localized uniform volumetric heat generation zone is the source of heat transfer in the porous matrix. With volumetric radiative information needed in the solid-phase energy equation computed using the discrete transfer method, the solid- and gas-phase energy equations are simultaneously solved using the finite difference method. For a given set of boundary conditions and operating parameters, the computed temperature distribution serves as the exact temperature profile necessary in the estimation of parameters. In the estimation of parameters using inverse analysis, the objective function is minimized using the genetic algorithm. Effects of measurement error, number of generations, population size, crossover probability, and mutation probability are studied in regard to the accuracy of results and the computational time required. Reasonably accurate estimations of extinction coefficient, scattering albedo, and emissivity of the porous matrix are obtained.

Ablation characteristics of special concrete due to an impinging zirconium-dioxide melt jet

Jet impingement experiments were performed to investigate the ablation characteristics of special concrete, which has been developed as one of the candidate protecting materials for the EU-APR1400 ex-vessel core catcher. In order to simulate the jet impingement phenomenon owing to the reactor vessel failure during a severe core meltdown accident, the experimental facility was established and the experimental conditions were determined based on parametric studies. The special concrete specimen was manufactured in accordance with the standard procedures, and its microstructures and physicochemical properties were analyzed to verify the requirements for the qualification. An induction melting technique in a cold crucible was employed to generate the zirconium-dioxide melt as a simulant of the corium melt. The special concrete was ablated uniformly over the impact area by jet impingement, and the average ablation depth was measured to be 4.33mm. The average ablation rate in depth was evaluated as 1.59mm/s using the temperature measurements of the specimen. As compared with the predictions by the models based on the convective and radiative heat transfer analysis, both the measured ablation rate and depth were estimated appropriately within the bounds of their limits. However, the convective heat transfer model turned out to predict the ablation characteristics of the special concrete more reasonably during the jet impingement even though some water content within the special concrete could lead to a sudden generation of the steam layer through which the material ablation is attenuated substantially by the radiative heat transfer.

Performance of cylindrical plastic solar collectors for air heating

A theoretical and experimental study including the combined convective and radiative heat transfer analysis of a flexible cylindrical type solar air-heater for agriculture crops dehydration as well as heating processes is presented. The solar collector is manufactured from LDPE films acting as a black absorber with a back insulation and double transparent covers sealed together along its edges. The collector is to be blown with a flow of pressurized air. The experiments are carried out with solar collectors of circular shapes having 0.5m diameter and solar collectors of elliptic shapes having 0.55m and 0.65m major axis. Energy balance of the cover, absorber and air yield three simultaneous quadratic algebraic equations in the three unknowns namely, cover, absorber and outlet air temperatures. A computer program is written for calculating the outlet temperature using the Newton–Raphson method and the collector thermal efficiency in terms of its diameter, length, mass flow rate, inlet temperature and solar insolation. Moreover the Nusselt number between the absorber and the heated air is determined experimentally in relation with the Reynolds number. Comparisons between the experimental data and the theoretical methods for the collector efficiency demonstrate a good agreement. In addition of this, the present experimental results of Nusselt number are correlated and compared with a correlation of another authors.

2-D Transient Radiative Heat Transfer Analysis on the Slab in a Walking-Beam-Type Reheating Furnace

A two-dimensional mathematical heat transfer model for the prediction of temperature distribution within the slab has been developed by considering the thermal radiation in the walking-beam-type reheating furnace chamber and transient heat conduction in the slab, respectively. The steel slabs are heated up through the preheating, heating, and soaking zones in the furnace. Heat transfer characteristics and temperature uniformity of the slab is investigated by changing hot gas temperature. Comparison with the in-situ experimental data show that the present heat transfer model works well for the prediction of thermal behavior of the slab in the reheating furnace. The skid mark severity decreases with an increase in hot gas temperature. Keywords: Reheating Furnace, Thermal Radiation, Transient heat conduction

Numerical and Optical Analysis of Solar Power Level Adaptable Solar Reactor

Solar energy is an abundant renewable energy resource that can be used to provide high process heat necessary to run thermochemical processes for production of various solar fuels and commodities. In a solar reactor, sunlight is concentrated into a receiver through a small opening called the aperture. However, obtaining and maintaining semiconstant high temperatures inside a solar reactor is a challenge. This is because the incident solar radiation can fluctuate depending on the position of the sun and the weather conditions. For fixed aperture size reactors, changes in incident solar flux directly affect the temperature inside the reactor. This paper presents a novel solar reactor with variable aperture mechanism that is designed and manufactured at our lab. Radiation heat transfer analysis of this reactor concept is studied via Monte Carlo (MC) ray tracing. MC ray tracing module is coupled to a steady-state one-dimensional energy equation solver. Energy equation is solved for the wall and gas, accounting for the absorption, emission, and convection. Incoming direct flux values for a typical day are obtained from National Renewable Energy Lab database. Results show that for a perfectly insulated reactor, the average temperature of the working fluid may be kept appreciably constant throughout the day if aperture diameter is varied between 3 cm and 1.5 cm for incoming fluxes starting with 400 W/m2 at 05:12 a.m., reaching peak value of 981 W/m2 at noon, and eventually receiving 400 W/m2 at 6:58 p.m., which can make the solar reactor run about 13 hr continuously at 1500 K semiconstant temperature.

Lattice Boltzmann Method Applied to Radiative Transport Analysis in a Planar Participating Medium

This article deals with the extension of the usage of the lattice Boltzmann method (LBM) to the analysis of radiative heat transfer with and without conduction in a one-dimensional (1-D) planar participating medium. A novel lattice needed for the calculation of the volumetric radiation spanned over the 4π spherical space has been introduced. The LBM formulation is tested for three benchmark problems, namely, radiative equilibrium, nonradiative equilibrium, and a combined mode conduction–radiation problem in a planar geometry. In the combined mode problem, with radiative information known from the proposed lattice structure, the energy equation is also formulated and solved using the LBM. The D1Q2 lattice is used in the energy equation. For validation, in problems 1 and 2, the LBM results are compared with the finite-volume method (FVM), while in problem 3, the LBM-LBM results are compared with the LBM-FVM in which FVM is used for the computation of radiative information. Comparisons are made for the effects of the governing parameters such as the extinction coefficient, the scattering albedo, and so on, on heat flux and emissive power (temperature) distributions. LBM results are found to be in excellent agreement with the benchmark results.

Radiative heat transfer analysis in modern rocket combustion chambers

Radiative heat transfer is analyzed for subscale and fullscale rocket combustion chambers for H2/O2 and CH4/O2 combustion using the P1 radiation transport model in combination with various Weighted Sum of Gray Gases Models (WSGGMs). The influence of different wall emissivities, as well as the results using different WSGGMs, the size of the combustion chamber and the coupling of radiation and fluid dynamics, is investigated. Using rather simple WSGGMs for homogeneous systems yields similar results as using sophisticated models. With models for nonhomogeneous systems the radiative wall heat flux (RWHF) decreases by 25–30 % for H2/O2 combustion and by almost 50 % for CH4/O2 combustion. Enlarging the volume of the combustion chamber increases the RWHF. The influence of radiation on the flow field is found to be negligible. The local ratio of RWHF to total wall heat flux shows a maximum of 9–10 % for H2/O2 and 8 % for CH4/O2 combustion. The integrated heat load ratio is around 3 % for H2/O2 and 2.5 % for CH4/O2 combustion. With WSGGMs for nonhomogeneous systems, the local ratio decreases to 5 % (H2/O2) and 3 % (CH4/O2) while the integrated ratio is only 2 % (H2/O2) and 1.3 % (CH4/O2).

Numerical Analysis of Spatial Unsteady Regimes of Conjugate Convective-Radiative Heat Transfer in a Closed Volume with an Energy Source

Unsteady regimes of convective-radiative heat transfer in a cubic enclosure with finitely thick heat-conducting walls in the presence of a constant-temperature energy source have been modeled mathematically under the conditions of convective heat exchange with the environment. A mathematical model has been formulated in dimensionless variables “vector potential–vorticity vector–temperature;” the model was realized numerically by the finite-difference method. An analysis of radiative heat transfer has been made on the basis of the surface-radiation approximation with the balance method in Polyak’s version. Three-dimensional temperature and velocity fields and dependences for the average Nusselt number have been obtained; they reflect the influence of the reduced emissivity factor of interior surfaces of enclosing walls, of the relative thermal conductivity, and of the unsteadiness factor on the flow regimes and heat transfer.

Normalization of Various Phase Functions for Radiative Heat Transfer Analysis in a Solar Absorber Tube

Normalization of various phase functions is considered for accurately predicting radiative heat transfer. A solar absorber tube filled with anisotropic scattering working medium is used as an example. Analysis of a previous normalization technique shows that while it does conserve scattered energy exactly after discrete-ordinates method (DOM) discretization, the overall asymmetry factor of the phase function is distorted, leading to substantial changes in overall scattering effect. A new normalization technique that conserves asymmetry factor and scattered energy simultaneously is investigated. The impact of lack of asymmetry factor conservation is analyzed for both the Legendre polynomial and the Henyey–Greenstein phase function approximations. Variations of medium optical thickness, scattering albedo, asymmetry factor, and side-wall emissivity are scrutinized to determine the effects of said parameters on wall heat flux and energy absorbing rate inside the absorber tube. Side-wall heat flux is found to increase with increases in asymmetry factor, optical thickness, and wall emissivity, and with decreases in scattering albedo. Energy absorbing rate profiles are found to depend greatly on optical thickness and scattering albedo.

Dynamic Modeling of a Solar Reactor for Zinc Oxide Thermal Dissociation and Experimental Validation Using IR Thermography

A dynamic numerical model of a solar cavity-type reactor for the thermal dissociation of ZnO is formulated based on a detailed radiative heat transfer analysis combining the Monte Carlo ray-tracing technique and the radiosity enclosure theory. The quartz window is treated as a semitransparent glass layer with spectrally and directionally dependent optical properties. Model validation is accomplished by comparison with experimental results obtained with a 10-kW solar reactor prototype in terms of cavity temperatures, reaction extents, and quartz window temperature distribution measured by IR thermography. The solar-to-fuel energy conversion efficiencies obtained experimentally are reported, and the various energy flows are quantified.

Development of the concept of mean temperatures in the analysis of radiative heat transfer in an inhomogeneous non-isothermal non-gray medium

Concepts of an emission mean temperature and a transmission mean temperature are introduced and shown to be effective and necessary in characterizing the radiative emission and transmission of an inhomogeneous non-isothermal non-gray medium. Based on a two zone model for a CO2/H2O/N2 mixture, the mathematical behavior of these two mean temperatures and their dependence on the temperature and the partial pressure of the absorbing gases of the two zones are illustrated. Results show that the average temperature is not appropriate to be used in the prediction of radiative emission and transmission in practical engineering systems.

Numerical analysis of natural convection and radiation heat transfer from various shaped thin fin-arrays placed on a horizontal plate-a conjugate analysis

Steady state natural convection and radiation heat transfer from various shaped thin fin-arrays on a horizontal base plate has been numerically investigated. A conjugate analysis has been carried out in which the conservation equations of mass, momentum and energy for the fluid in the two fin enclosure are solved together with the heat conduction equation in the fin and the base plate. Heat transfer by radiation is also considered in analysis. The heat transfer coefficient has been determined for each of the fin array considered in the present study at the same base and the same total area. The results of the analysis show that there are some important geometrical factors affecting the design of fin arrays. Taking into consideration these factors, an optimum fin shape that yields the highest average heat transfer coefficient has been determined.

Postflight Thermal Protection System Analysis of Hayabusa Reentry Capsule

The thermal response of the ablative thermal protection system of the Hayabusa capsule is calculated along the most probable reentry trajectory by using the integrated analysis method. The analysis method consists of an axisymmetric nonequilibrium Navier–Stokes flow solver and a radiative heat transfer analysis code loosely coupled to an ablation thermal response code. The calculated results are compared with the flight data obtained at the present time. It is found that a reasonable agreement is obtained between the calculated surface temperature and the temperature values estimated from the DC-8 airborne observation and the ground observation. In contrast, the calculated surface recession overestimates the measurement along the wall surface by a factor of three. It is also found that the density distributions inside the ablator can be reproduced reasonably both at the stagnation point and at the downstream region by taking account of the conventional surface reaction set and nitridation reaction with a laminar boundary condition. Especially for the thickness of the char layer at the downstream region, calculation overestimates the flight data when the effect of turbulence due to the pyrolysis gas is considered, whereas it underestimates the flight data when the effect of surface roughness of the ablative thermal protection system is taken into account.

Evaluation of optical properties and thermal performances of different greenhouse covering materials

An inverse method was conducted to obtain the spectral optical properties of four greenhouse covering materials, (Low Density Polyethylene (LDPE), Polyolefin (PO), Polyvinylchloride (PVC) and Fused Silica Glass). Diffuse reflectance and transmittance of the covering materials were measured using spectrophotometric method; the complex index of refraction in the range between 0.22 and 25μm was deduced by inverse calculation using Radiative Element Method by Ray Emission Model (REM2). At longwave radiation, the optical constants of opaque glass material were found by utilizing Kramers–Kronig method resulting good correlation with results obtained by other investigations. A rigorous model for radiative heat transfer analysis to an agricultural greenhouse was developed. The greenhouse covering material was analyzed as a non-gray one-dimension plane-parallel medium subjected to solar and thermal irradiation using REM2. Specular reflectance and diffuse incident irradiation were treated at the boundary surfaces and absorption and emission were taken into account. Thermal performance was evaluated for the above mentioned covering materials.

Phase-Change Radiation: Enhanced Infrared Absorption in Vapor–Liquid Water Mixtures

Phase-change radiation is studied in water clouds undergoing liquid–vapor phase transitions. Radiant energy transmission in the midinfrared () spectrum is measured in vapor–liquid mixtures of water at 1 atm and 60°C. Monte Carlo analysis of radiative heat transfer is performed to assess the contribution of infrared absorption associated with evaporative phase transition, for which a theoretical description is developed. The probability of absorptive, phase-change radiative relaxation is found to be on the order of per collision between a water vapor molecule and a liquid water droplet, which leads to a surface absorption efficiency of order 0.02 to 0.06 in the spectrum. Thereby, absorption of midinfrared radiation is slightly enhanced by evaporative absorption in water clouds.

Radiative heat transfer analysis in pure scattering layers to be used in vacuum insulation panels

Radiative heat transfer in the pure scattering layers in vacuum insulation panels is analyzed using Monte Carlo method for two cases; for isotropic scattering and the other for pure backward scattering. The result for isotropically-scattering medium agrees well with the diffusion solution within 1% error. This, indeed, shows that the diffusion approximation can be used as a good approximation in the tested cases. The results also suggest that the radiative heat transfer in the vacuum insulation panels can be effectively reduced by increasing the optical thickness of the core material and/or decreasing the surface emissivity by inserting more radiation shields. When the pure backward scattering is applied, significant reduction is found in the radiative heat transfer rate compared with the isotropic scattering case. It is shown that radiative thermal conductivity of the scattering layer is decreased to nearly half for the backward scattering medium when the optical thickness is large and the wall emissivity is high. Thus, the materials with highly backward scattering are recommended to be used as the core or insertion material between the radiation shields to enhance the performance of the vacuum insulation panels.

Analysis of Radiative Heat Transfer in an Indirect-Fired Strip Annealing Furnace based on Integral Equations

A dynamical model of a tunnel furnace for strip annealing is proposed. The continuous furnace has a uniform cross section and is divided into several heating zones, where the local temperatures are used as model inputs. The presented model is tailored to control and optimization applications and takes into account the most important underlying physical phenomena, i.e., the energy balance of the strip and the heat transfer due to radiation. For determining the radiative heat transfer, radiation balances in the form of integral equations are used. Approximate solutions of these equations are obtained by means of the method of weighted residuals. To facilitate an accurate representation of the strip temperature, the strip motion is described based on Lagrangian coordinates, which is especially useful for capturing the temperature transition at welded joints between consecutive strips. The capabilities of the model are explored in simulation studies.

Radiation Heat Transfer Analysis in a Semitransparent Single Crystal with Specular Surfaces: Application of REM2

In this work, to numerically analyze radiation heat transfer in the Czochralski (CZ) single crystal growth of oxide, the radiation element method by ray emission model (REM2) was considered for application. Here, REM2 was improved so that the radiation heat transfer through a transparent specular interface between media with arbitrary three-dimensional configurations and different refractive indices can be analyzed. The improved method was verified of its validity by comparing its results with the numerical results of previous works. Then, the effects of the optical thickness and refractive index of oxide crystals on the temperature distributions in the crystals were numerically investigated, considering a simplified model of the CZ furnace.

Combined Convective-Radiative Thermal Analysis of an Inclined Rooftop Solar Chimney

A model for the combined spectral radiative and convective heat transfer analysis of solar chimneys is developed. The radiation part of this model is based on the spectral distribution of the solar heat flux and spectral radiative properties of solar chimney components. Two approaches are used for the convective part of this model, empirical correlations and a CFD analysis. The empirical correlations are based on the stack effect correlation for airflow motion and a convective heat transfer correlation for the heat transfer coefficient. The empirical correlations are used to obtain an initial estimation of surface temperatures, which are then used in the CFD model to determine an improved estimation of the heat transfer coefficients and airflow rate. Iterating between the spectral radiative and the CFD models resulted in a converged set of values for the solar chimney airflow rate and its thermal characteristics. The model is used to predict the airflow rate for various configurations and solar irradiances of solar chimneys.