宽带k分布模型与其他气体辐射特性计算模型精度比较

气体辐射特性计算模型的选择对于辐射热传递的计算至关重要。对采用宽带k分布模型和其它气体辐射特性计算模型计算的于两无限大平板间非等温气体辐射传递的精度进行了比较结果显示:宽带k分布模型与逐线计算结果吻合较好,比总体模型的计算精度高得多。计算结果说明:对于非等温介质,关联k假设在较大的光谱区间内相对有效,且能通过将单一气体的k分布吸收系数相加获得水蒸气和二氧化碳混合气体的k分布吸收系数的,该处理方法简单易行,宽带k分布模型很适合气体辐射换热问题的求解。     

高温气体辐射特性计算模型

高温气体辐射特性的准确计算在燃烧、红外探测等工程应用中有重要意义。对国内外气体辐射特性研究现状进行了介绍,分析了3类气体辐射特性计算方法的基本原理和主要特征,着重介绍了新发展起来的全光谱k分布(FSK)模型。列表归纳总结了各辐射特性计算模型的适用条件、计算精度及计算速度。计算了平行平板间水蒸气和二氧化碳混合气体层壁面热流,给出了各模型计算结果与逐线计算间的相对误差比较图。对不同情况下气体辐射特性计算方法的选择提出了建议,针对现有模型的缺陷对今后的研究方向作了展望。     

二氧化碳气体辐射特性宽带k分布模型

建立了一种新的宽带k分布模型,从高温气体数据库HITEMP得到二氧化碳气体光谱辐射特性参数,用关联式拟合了其主要谱带的吸收系数.用该模型计算了二氧化碳的辐射热流,与逐线计算、统计窄带模型、窄带k分布模型和全光谱k分布模型作了比较,结果表明:对等温气体,提出的宽带k分布模型和逐线计算结果吻合很好,比全光谱k分布模型更准确.如果积分格式选取合适,宽带k分布模型比统计窄带模型精度高,和窄带k分布模型的精度相当.对灰壁面平板间的非等温气体,宽带k分布模型和逐线计算结果相比误差在10％左右,大大提高了气体辐射特性的计算精度和计算速度.     

宽带k分布模型用于亚声速飞行器尾焰探测

气体辐射特性计算在大气遥感和温室气体气候效应等研究中占有重要地位。利用工程经验公式计算尾喷焰流场的温度、浓度和压力分布,采用一种对应于12点Gauss-Lobatto积分点的k分布吸收系数可由简单的拟合多项式计算得到且混合物辐射特性处理简单的宽带k分布模型计算气体辐射特性,对亚声速飞行器尾喷焰在一些探测器工作谱带区间的远程探测辐射信号进行了模拟计算,并与逐线计算作了比较。结果表明:宽带k分布模型的计算结果和逐线计算相比误差较小,可用该模型对亚声速飞行器尾喷焰的远程探测进行计算。     

Radiation Characteristic Study of Flue Gas from Oxygen-enriched Combustion Using WBCK Model and Discrete Ordinate Method

针对富氧燃烧方式下由高体积分数水蒸气和CO2混合构成的烟气辐射传热难题,推导出描述非灰气体辐射特性的宽带关联k分布模型与离散坐标法相结合的计算公式.以文献中O2/CO2燃烧方式下炉内水蒸气和CO2的生成为例,采用宽带关联k分布模型计算了不同算例下混合气体的辐射强度和壁面热流量,并与统计窄谱带模型、全光谱关联k分布模型以及灰体加权和模型的计算结果进行了比较.结果表明：宽带关联k分布模型与统计窄带模型的计算结果吻合较好;与文献中的两个模型进行了对比,其计算精度有所提高,并且对混合气体的处理更加简便灵活,适用于O2/CO2燃烧方式下烟气辐射特性的计算.     

基于宽带关联k模型的气体辐射特性分析与计算

针对富氧燃烧方式下烟气中高浓度H2O和CO2混合气体的辐射换热问题,建立了一种改进的宽带关联(IWBCK)分布模型.为了验证IWBCK模型计算的有效性,首先选取不同配比的H2O/CO2混合气体进行计算分析,并与逐线计算和基于指数的宽带关联k分布模型进行对比,表明IWBCK模型对H2O/CO2混合气体的处理较合理,与逐线计算的结果更加接近;以某电站300MW燃煤锅炉为例,采用IWBCK模型计算了空气燃烧方式下三原子气体的辐射特性,并与前苏联锅炉机组热力计算标准的计算模型进行了对比分析,结果表明:两种模型的计算结果基本吻合,说明IWBCK模型对空气燃烧方式也是有效的.基于分区段热力计算的方法,采用IWBCK模型分别计算了该锅炉在空气燃烧方式和富氧燃烧方式(30%O2/70%CO2)下各区段三原子气体的吸收系数和发射率,结果表明:在不同温度下,富氧燃烧方式下三原子气体的吸收系数和发射率有不同程度的提高,辐射换热加强.     

箱带模型结合DOM在非灰气体辐射换热计算中的应用

在总结现有计算非灰气体辐射特性的各种模型与方法的基础上，利用箱带模型与离散坐标法(DOM)相结合的方法计算了一维无限大平行平板间气体介质在均匀型、边界层型及抛物型三种温度分布下的辐射换热。研究表明，箱带模型与常用的统计窄带模型或指数宽带模型的灰带近似计算结果相比具有较好的准确性，且由于箱带模型具有简单、易于编程、计算速度快等优点，对一些较为复杂的工程计算问题是一种较好的可供选择的模型。     

基于伪洛伦兹线型的气体发射率计算模型

为了提高气体总发射率计算的准确性及效率,为高温燃烧中辐射换热的估算等工程应用提供可靠的发射率计算工具,针对CO₂、H₂O、CO 3种气体,基于伪洛伦兹线型重新生成了0.1～80 bar范围内更为准确的气体发射率查找表。基于新生成的气体发射率查找表的数据,训练了机器学习气体发射率预测模型,用于对气体发射率进行快速、准确地计算。结果表明,伪洛伦兹(pseudo-Lorentz)线型对高压气体光谱的计算明显优于Alberti等开发的谱线截断模型,能够更好地模拟CO₂、H₂O、CO 3种气体的谱线混合效应;基于伪洛伦兹线型开发的机器学习气体发射率计算模型,计算精度与逐线法直接积分算法相近,便于工程应用。     

缸内对流换热与气体流动的计算分析

摘要本文将内燃机燃烧室简化成轴对称的二维空间,将计算缸内流动的二维模型与边界层模型相结合,分析了缸内气体的迁移特性与对流换热.文中介绍了在内燃机工作过程中缸内气体边界层的分布与变化、边界层对对流换热的影响,给出了对流挟热系数沿燃烧室表面的分布与变化.与实测结果的比较表明,本文的模型具有较高的精度.     

高温隔热纤维结构材料绝热特性分析

高温隔热纤维结构的绝热特性是决定金属热防护系统工作性能的关键因素。文中应用有限元法及有限容积法对理想化的纤维结构模型进行了数值研究,得出了纤维结构内部稳态下的温度场、热流场及速度场分布,并在此基础上计算得到纤维结构的等效导热系数,分析了温度、内部气体压力以及纤维结构密度对等效导热系数的影响。所建物理模型和分析方法可行,计算结果与实验结果吻合较好。     

New weighted‐sum‐of‐gray‐gases model for typical pressurized oxy‐fuel conditions

Pressurized oxy‐fuel combustion technology has received considerable attention due to its ability to improve the overall system efficiency and to control CO₂ emissions. The characteristics of radiation heat transfer are significant for pressurized oxy‐fuel gas mixture and different from those under atmospheric conditions. Therefore, to calculate the radiation characteristics of pressurized oxy‐fuel gas mixture quickly and accurately, new weighted‐sum‐of‐gray‐gases (WSGG) model for pressurized oxy‐fuel conditions was first presented in this paper, which was applied in 3 typical high pressure conditions: 5, 10, and 15 bar. The new WSGG model correlations were suitable for pressurized conditions with a molar ratio range of 0.125‐2, temperature range of 400‐2500 K, and path length range of 0.1‐20 m. Calculations for a variety of typical pressurized oxy‐fuel combustion cases showed that the new WSGG model can accurately predict the radiation characteristics and heat transfer characteristics of the gas mixtures compared with the SNB model benchmark. In addition, the application of the previous atmospheric WSGG models yielded non‐ideal results under pressurized conditions. Consequently, the new model can provide efficient and accurate radiation heat transfer results for pressurized oxy‐fuel conditions and can be used to design pressurized oxy‐fuel combustion furnaces or boilers.     

Mixed Convection and Non-Gray Radiation in a Horizontal Rectangular Duct

Numerical studies for fluid flow and heat transfer in a horizontal rectangular duct are carried out. The flow is considered to be laminar, hydrodynamically and thermally developing. Heat transfer by both forced and natural convection is taken into account. The radiation from the gas is modeled with weighted sum of gray gases (WSGG) model. While considering non-gray radiation with WSGG, the fluid is considered to be a mixture of CO₂ and H₂O. Simulations are carried out with lower wall temperature than the inlet temperature of the gas. The effect of buoyancy and radiation on bulk mean temperature and Nusselt number are studied. The effects of temperature dependent properties are discussed. Comparative studies are carried out among forced convection, mixed convection, gray and non-gray gas radiation. It is found from the simulations that the assumption of gray gas can produce an error of ±10% over a non-gray model with WSGG for the cases studied.     

Account for variations in the H2O to CO2 molar ratio when modelling gaseous radiative heat transfer with the weighted-sum-of-grey-gases model

This work focuses on models suitable for taking into account the spectral properties of combustion gases in computationally demanding applications, such as computational fluid dynamics. One such model, which is often applied in combustion modelling, is the weighted-sum-of-grey-gases (WSGG) model. The standard formulation of this model uses parameters fitted to a wide range of temperatures, but only for specific ratios of H₂O to CO₂. Then, the model is limited to gases from fuels with a given composition of hydrogen and carbon, unless several sets of fitted parameters are used. Here, the WSGG model is modified to account for various ratios of H₂O to CO₂ concentrations. The range of molar ratios covers both oxy-fuel combustion of coal, with dry- or wet flue gas recycling, as well as combustion of natural gas. The non-grey formulation of the modified WSGG model is tested by comparing predictions of the radiative source term and wall fluxes in a gaseous domain between two infinite plates with predictions by a statistical narrow-band model. Two grey approximations are also included in the comparison, since such models are frequently used for calculation of gas radiation in comprehensive combustion computations. It is shown that the modified WSGG model significantly improves the estimation of the radiative source term compared to the grey models, while the accuracy of wall fluxes is similar to that of the grey models or better.     

WSGG models for radiative heat transfer calculations in hydrogen and hydrogen-mixture flames at various pressures

Radiative heat transfer strongly influences pollutant emission prediction in combustion systems. In this work, the weighted sum of gray gas (WSGG) models have been developed for calculating radiative heat transfer in hydrogen and hydrogen-mixture flames. The total pressure effect on cut-off width of the Lorentz line profile is analyzed and properly considered in the line by line (LBL) calculations. Based on the LBL benchmark results, two sets of WSGG model correlations have been proposed for H₂O and its mixture with CO₂ at a molar ratio (Mr) of 3, representing the typical combustion products of the hydrogen and a hydrogen-rich mixture (e.g., 50% hydrogen and 50% methane). The WSGG models are applicable and accurate with a total pressure ranging from 1 to 60 atm. Partial pressure is explicitly applied as an independent variable in the model coefficients to account for its nonlinear effect on gas emissivity, which is particularly important for a participating gas medium with a large amount of H₂O at a total pressure below 5 atm. Detailed studies are carried out to solve radiative heat transfer in non-isothermal and non-homogeneous gas media at different conditions. Results show improvement over the existing WSGG models at the atmospheric pressure and have good agreement with LBL solutions under various conditions.     

A new weighted‐sum‐of‐gray‐gases model for oxy‐combustion scenarios

A new weighted‐sum‐of‐gray gases (WSGG) model that is based on the statistical narrow band model (SNB) RADCAL is proposed for use in computational fluid dynamic (CFD) simulations of air and oxy‐combustion. When employed in conjunction with the discrete ordinates (DO) method, the model predictions compare well against line‐by‐line benchmark data that have been made available recently that are based on the latest spectroscopic databases. Furthermore, the model compares well against the EM2C SNB model calculations that have served as benchmark data in three‐dimensional geometries. Radiative transfer calculations in these prototypical problems therefore confirm recent experimental observations that SNB RADCAL and EM2C SNB serve as good model databases to develop approximate radiative property models. To achieve an optimum balance of speed and accuracy in computationally intensive CFD simulations, non‐gray formulations of the WSGG model are also employed with the P1 model and solutions are compared against those generated by the DO model. While the P1 model gave favorable comparisons when cold, black walls were present, the errors in the surface incident radiative flux predictions increased in the presence of hot, reflecting walls. Finally, in fully coupled simulations of natural gas combustion under air‐firing and oxy‐firing modes, the predicted incident radiative flux profiles were distinctly different between the gray and non‐gray calculations at regions of high temperature gradients, while the centerline temperature predictions were comparatively unaffected. The effects of turbulence radiation interactions were also accounted for through the temperature self‐correlation term. However, the magnitudes of the temperature fluctuations were small and localized within this furnace and did not significantly alter our predictions. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

槽式太阳能真空集热管的热损失研究

建立了真空集热管中吸收管与玻璃管之间热辐射和残余气体热对流、玻璃管与外界环境之间热对流和玻璃管对天空热辐射的数学模型,提出了模型的计算方法,并通过和实验数据的比较验证了模型的准确性。同时利用模型分析了几种影响热损失的主要因素,分析结果表明:吸收管温度越高,热损失越大;环境温度越低,风速越大,热损失越大,但影响很小;选择性吸收涂层的发射率是影响热损失的主要因素;真空度对热损失也有很大影响。     

相变材料在长期驻空平流层飞艇上的应用探讨

建立平流层飞艇在平飞过程中的热力学综合模型,详细分析太阳辐射、地面反射、红外辐射、对流换热及热传导对内部气体温度变化的影响。分析结论表明飞艇内部气体的昼夜温差较大是影响长期驻空平流层飞艇性能的关键因素。为此,提出将相变材料应用于平流层飞艇,分析相变传热问题的特点,建立相变传热的数学模型,对相变传热问题的求解方法进行分析和比较,并设计实施地面和高空试验,试验结果与分析结果基本一致,即将相变材料应用于平流层飞艇能有效降低飞艇内部气体的日间最高温度,此研究结果可为改善平流层飞艇昼夜温差提供新思路。     

Simplified Analysis of Radiation Heat Exchange in Boiler Superheaters

This article describes the determination of the radiation heat transfer coefficient in radiant platen superheaters and on convective heating surfaces. A new simple formula for determination of the heat transfer coefficient is derived on the basis of a diffusivity model of radiation heat exchange. The radiation heat transfer coefficients are determined on the tube surface in a convective evaporator, in a second stage convective heat superheater, and in a platen superheater of a pulverized coal-fired boiler. The calculations were carried out applying the method presented in this article, the Central Institute for Boilers and Turbines method, and formulas resulting from the analysis of heat exchange in an enclosure containing a gas of a constant temperature. In order to assess the accuracy of the achieved results, the flow of flue gas and the heat exchange were modeled using a commercial computational fluid dynamics program.     

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.