燃烧产物辐射特性的误差对炉内辐射传热计算精度的影响

本文用离散坐标法对含吸收散射性介质矩形空腔内的３维辐射传递过程进行了模拟，并编写了相应的数值计算程序。利用该程序分析了介质的吸收系数、散射系数、相函数、光谱特性及壁面灰渣沉积层黑度的不确定性对矩形燃烧室内烟气温度及热流计算精度的影响。结果表明计算精度很大程度上取决于燃烧产物辐射特性的取值精度，特别是壁面灰渣沉积层黑度的取值精度。在煤粉燃烧室中，介质的散射不宜忽略。     

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.     

A comparison of radiative characteristics for fly ash and coal

Numerical results for extinction efficiency, phase function, albedo and asymmetry factor are presented for clouds of spherical particles illuminated by black body radiation. the refractive indices (m = m₁ − im₂) used were m₁ = 1.5 with m₂ varied from 0 to 0.024 in steps of 0.006 and m = 1.6 −i0.6, chosen to be representative of fly ash and coal. averaged scattering data are given for use in radiative transfer modelling of pulverised fuel furnaces. the results are seen to depend on the product of the mean particle size and the radiation temperature. the scattering parameters of fly ash are shown to vary significantly with absorption index m₂ for small particles, but the choice of refractive index is not critical for large particles.     

Modelling of scattering and absorption coefficients for a polydispersion

An adequate treatment of the thermal radiation heat transfer mechanism is essential to a mathematical model of the combustion process or to design a combustion device. Predictive tools using flux models, such as the discrete transfer method, the discrete ordinates method and the spherical harmonics method, that solve the radiative heat transfer equation, require as input the values of the absorption and scattering coefficients of the participating media. Such coefficients must be evaluated in an expedite fashion since computational fluid dynamics and radiative flux models are extremely time demanding by themselves. In this work, a curve fitting approach to the Mie theory is used to evaluate the above-mentioned coefficients for intermediate and large particles, ensuring a compromise between accuracy and computational economy. The same coefficients for small particles are calculated using power series to represent the Mie coefficients accurately and economically. Predictions with the present models were performed for soot, carbon particles and fly ash and are presented herein. The results have proved that the models proposed in this work are computationally much faster than the prohibitive Mie theory calculations: reductions in computing times as high as three-hundred fold. Additionally, the referred models allow for the achievement of very accurate results: a relative error between approximated values and the corresponding Mie exact solution almost always below 5%.     

TRANSIENT RADIATION HEAT TRANSFER WITHIN A NONGRAY NONISOTHERMAL ABSORBING-EMITTING-SCATTERING SUSPENSION OF REACTING PARTICLES UNDERGOING SHRINKAGE

ABSTRACT

A nonisothermal, nongray, absorbing, emitting, and anisotropically scattering suspension of reacting particles exposed to concentrated thermal radiation is considered. The steam gasification of coal is selected as the model thermochemical reaction. The unsteady energy equation that couples the radiative heat flux with the chemical kinetics is solved by means of a numerical model that incorporates Monte Carlo ray tracing, the finite-volume method, and an explicit Euler time integration scheme. Two modeling approaches are applied: (1) a quasi-continuous model that assumes a homogeneous medium and utilizes its macroscopic radiative properties (absorption and scattering efficiencies and scattering phase function), and (2) a particle-discrete model that assumes an ensemble of randomly positioned particles and traces the interaction of radiation with each particle by geometric optics. Temperature profiles and reaction extent are computed using both approaches. The quasi-continuous approach is superior in accuracy at the expense of lower spatial resolution, while the particle-discrete approach gives detailed information for every single particle in the suspension at the expense of larger stochastic errors.     

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.     

Modeling the radiation of anisotropically scattering media by coupling Mie theory with finite volume method

A new mathematical model and code for radiative heat transfer of particulate media with anisotropic scattering for 2-D rectangular enclosure is developed. The model is based on the coupling of (i) finite volume method for the solution of radiative transfer equation with (ii) Mie equations for the evaluation of scattering phase function. It has not been done before to the authors’ best knowledge. The predictions were compared against the only found results, published 15 years ago. For those results the S-N discrete ordinates method for the solution of radiative transfer equation and the Legendre polynomials expansions for the evaluation of scattering phase function were used. The agreement between the results is very good. The advantages of new model and code are in their straight forward application to any given particles parameters without the need for previously designed analytical expression for scattering phase function. In addition, that analytical expression, with generated expansion coefficients, is restricted and can be used only for that particular case of particle parameters. The new model was applied to the solid particles of several various coals and of an ash and the series of 2-D predictions are performed. The effects of particle size parameter and of scattering albedo on radiative heat flux and on incident radiation were analyzed. It was found that the model developed is reliable and very accurate and thus suitable for extension towards: (i) 3-D geometries, (ii) mixtures of non-gray gases with particles as well as for (iii) incorporation in computational fluid dynamics codes.     

NUMERICAL SIMULATION OF THE GAS FLOW AND MASS TRANSFER BETWEEN TWO COAXIALLY ROTATING DISKS

A problem of combined conductive and two-phase radiative heat transfer in a two-dimensional rectangular enclosure with two-phase (gas-particles) media is analyzed. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is studied. Its nonlinear integrodifferential RTE is solved using the discrete ordinates method (DOM, or so-called S N method). To validate the program, we compare the solution in a two-dimensional rectangular black enclosure with others. The DOM is then applied to the unsteady thermal development in two-phase media contained in a rectangular enclosure. A parametric study is performed by changing the gas and particle absorption coefficients, particle number density, particle emissivity, wall emissivity, and aspect ratio of the enclosure. The results confirm a significant effect of the two-phase radiation on the thermal development in the geometry. However, it is found that the conduction is predominant near the hot wall.     

Effect of light scattering on the performance of a direct absorption solar collector

Recently, a solar thermal collector often employs nanoparticle suspension to absorb the solar radiation directly by a working fluid as well as to enhance its thermal performance. The collector efficiency of a direct absorption solar collector (DASC) is very sensitive to optical properties of the working fluid, such as absorption and scattering coefficients. Most of the existing studies have neglected particle scattering by assuming that the size of nanoparticle suspension is much smaller than the wavelength of solar radiation (i.e., Rayleigh scattering is applicable). If the nanoparticle suspension is made of metal, however, the scattering cross-section of metallic nanoparticles could be comparable to their absorption cross-section depending on the particle size, especially when the localized surface plasmon (LSP) is excited. Therefore, for the DASC utilizing a plasmonic nanofluid supporting the LSP, light scattering from metallic particle suspension must be taken into account in the thermal analysis. The present study investigates the scattering effect on the thermal performance of the DASC employing plasmonic nanofluid as a working fluid. In the analysis, the Monte Carlo method is employed to numerically solve the radiative transfer equation considering the volume scattering inside the nanofluid. It is found that the light scattering can improve the collector performance if the scattering coefficient of nanofluid is carefully engineered depending on its value of the absorption coefficient.     

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.     

生物质垃圾与煤混烧飞灰颗粒的微观形态特征及能谱研究

利用扫描电镜(SEM)和能谱(EDX)结合联用技术直观地观察了生物质垃圾与煤混合压制燃料的飞灰微观形态特征,分析了其主要组成元素及其质量分数.实验结果表明,飞灰颗粒的形态多样,以不规则形态居多.不同粒径颗粒之间存在逐级吸附的现象.在本实验观察的所有飞灰颗粒的能谱分析中都发现了K元素的存在.实验发现,生物质与煤混烧排放钾的主要存在形式是氯化钾.     

EFFECT OF RADIATIVE TRANSFER ON THE THERMOPHORETIC PARTICLE DEPOSITION AROUND A CIRCULAR CYLINDER IN A UNIFORM CROSS FLOW

We investigated the effects of radiative transfer on the particle deposition around a circular cylinder in a cross flow. The fluid is transparent to radiation, and radiating particles are suspended in flow. The finite volume method was applied to analyze radiative transfer in the flow utilizing the nonorthogonal coordinate system. The radiative transfer coupled with convection and mass transfer is reasonably predicted by the method. The results are in good agreement with those of boundary layer analysis and available experimental data. We discuss the effects of the conduction-to-radiation parameter, optical thickness, scattering albedo, and wall emissivity on the heat transfer and particle deposition.     

Stability and thermophysical properties of fly ash nanofluid for heat transfer applications

Improving the performance of heat transfer fluids is altogether significant. The best approach for improving the thermal conductivity is the addition of nanoparticles to the base fluid. In the present study, specific heat, dynamic viscosity, and thermal conductivity of water-based Indian coal fly ash stable nanofluid for 0.1% to 0.5% volume concentration in the temperature range of 30 to 60°C has been investigated. To evaluate an average particle diameter of 11.5 nm, the fly ash nanoparticles were characterized with scanning electron microscopy and dynamic light scattering. Using zeta potential, the stability of nanofluid in the presence of surfactant Triton X-100 was tested. Thermal conductivity and viscosity of fly ash nanofluid increased, while specific heat decreased as volume concentration increased. The effect of temperature on the fly ash nanofluid was directly proportional to its thermal conductivity and specific heat and inversely proportional to viscosity.     

煤、灰粒子的辐射特性

本文在粒子为光滑、均质假设的基础上，利用Ｌｏｒｅｎｔｚ－Ｍｉｅ理论计算了煤粉粒子、煤灰粒子的衰减系数、散射系数和吸收系数。给出了适用于煤粉升温着火及炉内辐射传热过程数值计算使用的煤粉粒子、煤灰粒子辐射特性的计算公式。     

Modeling the 3-D radiation of anisotropically scattering media by two different numerical methods

An original model and code for 3-D radiation of anisotropically scattering gray media is developed where radiative transfer equation (RTE) is solved by finite volume method (FVM) and scattering phase function (SPF) is defined by Mie Equations (ME). To the authors’ best knowledge this methodology was not developed before. Missing the benchmark, another new 3-D model and code, which solve the same problems, based on a combination of zone method (ZM) and Monte Carlo method (MC), as a solution of RTE, is developed. Here SPF is also calculated by Mie Equations. The conception ZM + MC is numerically expensive and is used and recommended only as a benchmark. The 3-D rectangular enclosure and the spherical geometry of particles are considered. The both models are applied: (i) to an isotropic and to four anisotropic scattering cases previously used in literature for 2-D cases and (ii) to solid particles of several various coals and of a fly ash. The agreement between the predictions obtained by these two different numerical methods for coals and ash is very good. The effects of scattering albedo and of wall reflectivity on the radiative heat flux are presented. It was found that the developed 3-D model, where FVM was coupled with ME, is reliable and accurate. The methodology is also suitable for extension towards: (i) mixture of non-gray gases with particles and (ii) incorporation in computational fluid dynamics.     

Particle–gas reacting flow under concentrated solar irradiation

A transient heat transfer model is developed for a reacting flow of CH₄ laden with carbon particles directly exposed to concentrated solar radiation and undergoing thermal decomposition into carbon and hydrogen. The unsteady mass and energy conservation equations, coupling convective heat and mass transfer, radiative heat transfer, and chemical kinetics for a two-phase solid–gas flow, are formulated and solved numerically for both phases by Monte Carlo and finite volume methods using the explicit Euler time integration scheme. Parametric study is performed with respect to the initial particle diameter, volume fraction, gas composition, and velocity. Validation is accomplished by comparing temperatures and reaction extent with those measured experimentally using a particle-flow solar reactor prototype subjected to concentrated solar radiation. Smaller particles and/or high volume fractions increase the optical thickness of the medium, its radiative absorption and extinction coefficients, and lead to higher steady-state temperatures, reaction rates, and consequently, higher extent of chemical conversion.     

飞灰复燃对炉内颗粒沉积状况的影响分析

实际工程表明,采用飞灰复燃技术对锅炉进行改造,可以减少飞灰所带走的燃料损失,提高锅炉效率,但飞灰回收复燃给壁面颗粒沉积状况也带来了影响.采用FLUENT模拟了SZL15-1.25-AⅡ型双筒链条蒸汽锅炉炉内燃烧,对比分析了采用飞灰复燃技术前后炉内壁面颗粒沉积状况.模拟结果表明,飞灰复燃对锅炉顶墙、前墙及后墙的颗粒沉积速率影响较大,其中飞灰复燃提高了顶墙和前墙的颗粒沉积速率,降低了后墙颗粒沉积速率,而对锅炉前后拱的影响很小可以忽略.减小飞灰入射质量流量或调整飞灰入射角度为水平偏下,均可以降低颗粒在水冷壁的沉积速率,有利于炉膛与水冷壁间的传热.     

高温宽粒径氧化铝颗粒辐射特性研究

基于Mie散射理论研究了温度分布为400～1 000℃,粒径分布为0.1～20.0 mm的单个氧化铝颗粒的辐射特性参数。研究发现,当温度处于400～1 000℃时,直径为3.0 mm的氧化铝颗粒已基本满足大粒子辐射特性,而0.1～3.0 mm的氧化铝颗粒衰减效率因子、散射效率因子、吸收效率因子随粒径的增加而急剧衰减,散射反照率随粒径的增加而急剧增加。基于最小二乘法对温度分布为400～1 000℃,粒径分布为0.1～3.0 mm的氧化铝颗粒辐射特性效率因子进行拟合,拟合优度达到0.999以上,表明可以利用此非线性回归方程对此温阈、粒阈氧化铝颗粒辐射特性效率因子进行较精确的计算。     

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.     

Inverse radiation analysis of simultaneous estimation of temperature field and radiative properties in a two-dimensional participating medium

An inverse radiation analysis is presented for simultaneous estimation of temperature field and radiative properties including absorption and scattering coefficients in a two-dimensional rectangular, absorbing, emitting and scattering gray medium from the knowledge of the exit radiative energy received by charge-coupled device (CCD) cameras at boundary surfaces. The backward Monte Carlo method was introduced to describe the radiative heat transfer for its efficiency. The inverse problem is formulated as an optimization problem and solved by the least-square QR decomposition (LSQR) method. The effects of measurement errors, optical thickness and search step length on the accuracy of the estimation were investigated and the results show that the temperature field and radiative properties can be reconstructed accurately for the exact and noisy data.