RADIATION MODELING OF STATIONARY FUSED SILICA RODS AND FIBERS HEATED BY CO2 LASER IRRADIATION

This study presents a heat transfer model for a stationary fused silica rod heated by a CO₂ laser. During laser heating, the effect of fused silica being modeled to be opaque or semitransparent to laser irradiation is studied. The radiative heat transfer caused by the emission of fused silica is modeled using the zonal method, and compared to the Rosseland diffusion approximation. The spectral dependence of the fused silica absorption coefficient in semitransparent wavelengths is approximated by a two-band model. The weighted-sum-of-gray-gas (WSGG) method is used to calculate the radiative source term. The governing equation with conduction and radiation heat transfer is solved by the finite-volume method. The importance of modeling the effects of laser energy penetration below the fused silica surface during heating, especially for small diameter fibers, is discussed. The importance of radiative heat transfer in fused silica is also discussed. Around 25 K in temperature difference is observed when the diffusion approximation is used in place of the zonal method to model the radiative transfer in fused silica.     

Assessment of gas radiative property models in the presence of nongray particles

In this study, a radiation code based on the method of lines solution of the discrete ordinates method for the prediction of radiative heat transfer in nongray gaseous media is developed by incorporation of two different spectral gas radiative property models, banded spectral line-based weighted sum of gray gases (banded SLW) and gray wide band (GWB) approximation in the presence of nongray absorbing–emitting–scattering particles. The aim is to introduce an accurate and CPU efficient spectral gas radiation model, which is compatible with spectral fuel/ash particle property models. Input data required for the radiation code and its validation are provided from two combustion tests previously performed in a 300 kWt atmospheric bubbling fluidized bed combustor test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. The agreement between wall heat fluxes and source term predictions obtained by global and banded SLW models reveal that global SLW model can be converted to an accurate wide band gas model (banded SLW) which can directly be coupled with spectral particle radiation. Furthermore, assessment of GWB approximation by benchmarking its predictions against banded SLW model shows that GWB gives reasonable agreement with a higher CPU efficiency when the particle absorption coefficient is at least in the same order of magnitude with the gas absorption coefficient.     

Absorption of thermal radiation in large semi-transparent particles at arbitrary illumination of the polydisperse system

An approximate theoretical model for nonuniform absorption of the external thermal radiation in a large semi-transparent spherical particle is suggested. As applied to heat transfer problems with diffuse radiation in the wide spectral range, the asymmetric illumination of single particle is considered at each spectral interval as a uniform illumination from backward and forward hemispheres (with respect to the direction of spectral radiation flux). The Mie theory is employed in calculations for particles illuminated from a hemisphere. The modified differential approximation suggested earlier by the author is used in the case of spherically symmetric illumination. Approximate analytical relations for distribution of absorbed radiation power inside a particle are obtained. Results of calculations for typical polydisperse sprays of water and diesel fuel droplets are presented.     

Radiative heat transfer from potassium seeded water gas combustion plasma

Radiative heat transfer calculations from a potassium seeded water gas combustion plasma have been made to estimate the radiative heat losses through the walls of a MHD channel. Both molecular combustion products and seed contribute significantly to the total radiation loss from a plasma. The spectral emission properties of CO₂, H₂O, CO and potassium have been taken into account. It has been shown that the contribution of CO to heat flux is very small and, thus, can be neglected. CO₂ and H₂O are the primary contributors to the radiation from the combustion products. At MHD temperatures, 55–80% of the contribution to heat flux from the combustion products comes from bands lying up to 2.7 μm in the near infrared. It has been shown that accurate knowledge of absorption cross-section data is essential to predict the radiative heat transfer from potassium. It has been estimated that 25–30% of the total radiative heat flux is from the potassium seed.     

QUENCHING OF FLAT GLASS BY IMPINGING AIR JETS

Quenching of hot glass by impinging round air jets is analyzed. Understanding of the temperature distribution in glass is required for thermal tempering operations. In the analysis the spectral dependence of the absorption coefficient on wavelength is appropriately accounted for by solving the radiative transfer equation for the axisymmetric cylindrical geometry. Specularly reflecting boundaries are considered, and Fresnel's equations are used to predict the spectral directional reflection and transmission characteristics of the interfaces. The finite volume method is used to solve numerically the thermal energy equation. The numerical calculations have been carried out for combined forced convection and radiation cooling from the surfaces. Transient temperature distributions for parametric calculations are presented and discussed. Determination of the resulting stresses under given impingement and thermal conditions is also discussed.     

Absorption of solar radiation in ponds

Analysis is presented to predict the local rate of solar energy absorption in a pond using the radiative transfer theory. The physical model considers absorption and scattering by the water and internal reflection of radiation from the air-water interface as well as the bottom. A forward scattering approximation and a discrete-coordinate approximation of the radiative transfer equation are discussed. Numerical results for the local volumetric rate of solar energy absorption in the water are presented in the paper for a range of parameters of physical interest. The effects of the directional distribution of solar radiation incident on the water surface, the attenuation of solar radiation by the atmosphere during the diurnal cycle and the modification of the spectral radiation characteristics of water by impurities and additives on the absorption and distribution of the absorbed energy in the pond are investigated.     

Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers

Ash is produced in combustion of biomass. Some part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also can affect the thermal radiation. The effects of fly ash particles on the thermal radiation are considered in this investigation. By analyzing sampled data in an electrostatic filter, a realistic particle size distribution is found. Although the optical data on biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fe coal fly ash can be applied for the biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles on thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential on the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important on the radiative heat source and radiative heat flux to the wall whereas their absorption effect is important and can increase the radiative heat source and wall heat fluxes.     

An Estimate of the Temperature of Semitransparent Oxide Particles in Thermal Spraying

A considerable temperature difference in semitransparent oxide particles due to intensive heating in plasma spraying makes it difficult to interpret the optical measurements of their temperature. The problem of determining the bulk temperature of such particles from the experimental data on the color temperature is analyzed by using a recently proposed approximate model for the radiation transfer inside a nonisothermal refracting spherical particle. The same approximation is also employed for developing an improved model of particle heating, taking into account the radiation-conduction interaction inside the particle. Calculations for Al 2 O 3 and ZrO 2 particles in a typical plasma jet show that the color temperature of oxide particles may be less than or greater than their bulk temperature, depending on the spectral absorption coefficient of particle substance. This temperature difference during the melting of particles may reach the value of 200-300 K. A procedure for in situ evaluation of the absorption coefficient by comparison of color and brightness temperatures of molten particles is proposed.     

Combined nongray radiative and conductive heat transfer in multiple glazing taking into account specular reflection and absorption

The problem of combined nongray radiative and conductive heat transfer in multiple glazing subjected to solar irradiation is analyzed. A spectral solar model proposed by Bird and Riordan is used to calculate direct and diffuse solar irradiance. The radiation element method by ray emission model, REM², is used to analyze the spectral dependence of radiative heat transfer. Specular reflection at boundary surfaces is taken into account. The spectral dependence of radiation properties of glass such as specular reflectivity, refraction angle, and absorption coefficient is taken into account. The steady‐state temperature and heat flux distributions in the glass layer are obtained and the insulating efficiency of multiple glazing is examined. The overall heat transfer coefficients predicted by the present method are compared with those based on the JIS method. The values obtained by the present method are slightly lower than those obtained by the JIS method. To investigate the spectral variation of radiative heat flux attenuated in the glass layer, the spectral heat flux at the room‐side surface and incident radiation are compared. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 712–726, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10125     

3D modelling of radiative heat transfer in circulating fluidized bed combustors: influence of the particulate composition

A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of circulating fluidized bed (CFB) combustors. The radiative transfer equation is solved by the discrete ordinates method and Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. Empirical correlations calculate both spacial variation of solid volume fraction and temperature distribution at the wall. The model considers the influences of the particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. A very good agreement of predicted results is shown with experimental data.     

Numerical simulation of the interaction between turbulence and radiation in reactive flows

The interaction between turbulence and radiation (TRI) in reactive flows has been demonstrated experimentally, theoretically and numerically, and results from the highly non-linear coupling between fluctuations of radiation intensity and fluctuations of temperature and chemical composition of the medium. The instantaneous and the time-averaged form of the radiative transfer equation (RTE) are presented, and the TRI effects resulting from time-averaging are discussed. Methods to account for TRI in practical calculations are surveyed, and works where such methods have been employed are reviewed. These include both decoupled and coupled fluid flow/radiative transfer calculations. It is shown that the solution of the RTE using instantaneous scalar data is the most accurate way to deal with TRI, but it is computationally prohibitive for coupled problems. Hence, this approach has been mainly used to calculate the radiation intensity along lines of sight. The generation of time series of instantaneous scalar data may be accomplished using stochastic or deterministic models, which are also surveyed. Coupled fluid flow/radiative transfer problems are generally solved using the time-averaged form of the RTE or the Monte Carlo method, and rely on the optically thin fluctuation approximation, which neglects the correlation between fluctuations of the absorption coefficient and fluctuations of the radiation intensity. Experimental data and numerical calculations demonstrate that turbulent fluctuations may significantly increase the mean spectral radiation intensity in both non-luminous and luminous flames. Turbulent fluctuations contribute to decrease the flame temperature below the level observed without fluctuations, particularly for optically thick flames. The net radiative power and the fraction of radiative heat loss increase due to TRI, particularly in the case of optically thin flames. Recent direct numerical simulations provide additional insight on the role of different correlations responsible for TRI, and on how they are influenced by the optical thickness of the medium.     

偏振度对均匀折射率介质内矢量辐射传输过程中辐射熵产的影响

在半透明均匀折射率介质内矢量辐射传输过程中辐射熵传递方程及其数值模拟方法的基础上,研究了偏振度对矢量辐射传输过程中辐射熵产的影响。均匀折射率介质内辐射光束的起偏和改偏通过相距阵实现。计算结果表明:由介质内吸收发射过程的不可逆性产生的光谱辐射熵产数随着偏振度增加而减小,而由介质散射过程的不可逆性产生的光谱辐射熵产数随着偏振度增加而增加;偏振度对介质内的光谱辐射熵强度的影响很大,若不考虑偏振,光谱辐射熵强度的相对误差最大可达到18.04%;在整个系统中,光谱辐射熵产数满足热力学第二定律。     

A Comparative Exploration of Enhancing Thermal Characteristics of Natural Gas Flame by Synchronous Combustion Technique

This article is a comparative study of how the injection of micro kerosene droplets and pulverized anthracite coal particles affects soot particle nucleation inside natural gas flame and, subsequently, radiation. To this end, the yellow chemiluminescence of soot particles and IR photography were used to locate radiative soot particles and discover their qualitative distribution. The IR filter was tested with a Thermo Nicolet Avatar 370 FTIR Spectrometer for its spectral transmittance to be specified. Also, the spectral absorbance of soot particles, which are formed in flame, was measured by BOMEM FTIR. Furthermore, the variations of flame temperature, transient heat transfer, and thermal efficiency were investigated. The results indicate that, for equal heating values, kerosene droplets are more effective than coal particles in improving the radiation and thermal characteristics of natural gas flame. Also, kerosene droplets cause a higher rise in the temperature in flame downstream and make the axial flame temperature more uniform than coal particles do. In quantitative terms, when kerosene droplets were injected, the radiative heat transfer and thermal efficiency of flame were 93% and 35% higher than the corresponding values for the coal particles injection mode.     

Modeling of coupled spectral radiation,thermal and carrier transport in a silicon photovoltaic cell

Photovoltaic conversion efficiency of a crystalline silicon cell is investigated as a function of its temperature and taking into account complete thermal and irradiation operating conditions. The spectral radiative transfer problem is solved through a gray per band approach and a separated treatment of the collimated and diffuse components of radiation fluxes. The heat transfer modeling includes local heat sources due to radiation absorption and thermal emission, non-radiative recombinations and excess power release of photogenerated carriers. Continuity equations for minority carriers are solved to provide the current–voltage characteristic. A detailed analysis of the electrical and thermal behaviors demonstrates that proper adjustment and control of both thermal and surroundings radiative operating conditions are likely to provide guidelines for the improvement of photovoltaic cell performances.     

Transient flame propagation process and flame-speed oscillation phenomenon in a carbon dust cloud

A detailed numerical study was conducted to understand the transient flame propagation process and the flame-speed oscillation phenomenon in a carbon dust cloud. The modeling included the solution of a set of time-dependent conservation equations developed for the gas phase and the particle phase in a spherical coordinate. The gas-phase reactions used detailed chemistry, variable thermodynamic properties, and multicomponent transport properties. The particle-phase equations include the two-phase force interactions in the momentum equation by considering Stoke drag force and thermophoretic force resulting from the gas-phase temperature gradient. Mass and species transfer between the two phases were modeled as a result of both gas-phase and particle surface reactions. Energy transfer between the two phases, including convective, conductive, and radiative heat transfer, were included. Radiation absorption and emission by particles were both especially considered. The results show that because of the different inertia between particles and gas, a velocity slip occurs between the two phases in the region ahead of the flame front. The slip is more significant in the early flame propagation stage than in the later stage. The radiation heat losses of the hot gases and particles to the cold ambient and the radiation gain as a result of the absorption of unburned particles are both important in the present dust flame, because the characteristic time scale of the chemical reactions is longer than that of gaseous flames. Lastly, an analysis of the detailed numerical simulations shows that a slip between the gas and particle velocities is the cause of flame-speed oscillation. The slip leads to a periodic change in local particle number density in the reaction zone, which in turn changes the local fuel equivalence ratio periodically, causing the oscillation.     

Transient temperature and thermal stress profiles in semi-transparent particles under high-flux irradiation

Transient temperature and thermal stress profiles in semi-transparent spherical particles heated by concentrated solar radiation are studied by means of a theoretical model. The analysis of radiative–conductive interaction is based on the spectral radiation transfer model in a refracting and absorbing particle. The stress–strain state of the particle is described by the thermoelastic approach. An analytical self-similar solution for the particle temperature profiles and thermal stresses during the quasi-steady period of the particle heating is derived. It is shown that the circumferential tensile stress near the particle surface is a non-monotonic function of the particle radius. The range of physical parameters corresponding to the maximal tensile stress near the particle surface is determined. The model is applied to ZnO and CaCO₃ particles, which are used as reactants in industrially-relevant high-temperature processes. It is shown that tensile stresses in the selected types of particles exposed to concentrated solar radiation cannot lead to their mechanical destruction. At the same time, the considerable temperature difference and thermal stresses in non-isothermal particles can be an interesting issue in a detailed analysis of the thermal decomposition of semi-transparent particles.     

Phase-transition radiation in vapor condensation process

Energy release by radiative relaxation during water vapor condensation is considered to be responsible for phase-transition radiation in the water vapor condensation process. A two-level transition model is proposed to explain the radiative transfer mechanism for condensation radiation of water vapor. The absorption coefficient due to condensation radiation and the associated optical properties are defined in the radiative transfer equation. The spectral profile of the source function of vapor condensation radiation is examined and agreement with the experimental characteristic wavelength of condensation radiation is found.     

Flame radiation

Flame radiation is generally recognized as an important factor in fire phenomena and many combustion systems. Accurate prediction of flame radiation requires a good understanding of the radiative transport theory as well as detailed information on the radiative properties of the combustion products which generally consist of a mixture of gases plus soot particles. In this article the physics of gas radiation and its application to non-luminous flame calculations are first introduced. Subsequent formulation of luminous flame radiation incorporates properly the continuous soot emission. Effects of non-homogeneous distributions of temperature and gas partial pressures along the pathlength are discussed for both luminous and non-luminous flames. For engineering applications, useful radiation quantities such as the total emissivity, the mean absorption coefficient, and the radiation conductivity are expressed in simple analytical representations in terms of pertinent flame parameters.     

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.     

Heat transfer modelling in thermophotovoltaic cavities using glass media

Optimisation of heat transfer, and in particular radiative heat transfer in terms of the spectral, angular and spatial radiation distributions, is required to achieve high efficiencies and high electrical power densities for thermophotovoltaic (TPV) conversion. This work examines heat transfer from the radiator to the PV cell in an infinite plate arrangement using three different arrangements of participating dielectric media. The modelling applies the Discrete Ordinates method and assumes fused silica (quartz glass) as the dielectric medium. The arrangement radiator–glass–PV cell (also termed dielectric photon concentration) was found to be superior in terms of efficiency and power density.