Thermal Conductivity of Evacuated Highly Transparent Silica Aerogel

The thermal loss coefficient k of evacuated load bearing transparent silica aerogel tiles is determined for temperatures between 280 K and 400 K. The measurements were performed with the large guarded hot plate high vacuum system LOLA I, the reference plates of which can be pressed onto the samples with an adjustable load (0... 1 bar). For a 22mm layer of aerogel (density ρ ≈ 100...110g/liter) the loss coefficient varied between about k = 0.36 W//m² K) at 280 K and k = 1.45 W/(m².K) at 400 K, corresponding to a pseudo-conductivity λ = 8. 10^?3 W/(m. K) and λ = 32. 10^?3 W/(m. K), respectively. Thus at typical winter temperatures (inside 20 °C, outside ?10 °C to 0 °C), evacuated transparent aerogel could provide excellent thermal insulation as a spacer in double pane windows. It also could be used in passive solar systems, for example as a cover for Trombe walls or as a transparent insulating structure around house walls. The dramatic increase of thermal losses at higher temperatures is due to a transmission window for infrared radiation between 3 and 7 μm, Additional data reveal the influence of water adsorption and of air pressure on the thermal conductivity of aerogel.     

用改进的离散坐标法计算炉内三维辐射传热

采用离散坐标法进行炉内三维辐射传热的计算,首先在正方体炉膛内验证了该法的精确性,计算结果与区域法进行比较,表明离散坐标法算法可靠,计算工作量小,适合于炉内辐射传热的计算。然后针对长方体炉膛计算了吸收－发射－散射介质的传热问题,表明传统的离散坐标法不适合计算具有复杂相函数曲线的辐射传热问题,因此采用改进的离散坐标法,并得到了合理的结果。最后,对于煤粉燃烧炉膛将辐射传热问题和炉内流动、燃烧过程耦合起来进行计算,表明离散坐标法是一种很有工程应用价值的炉内辐射传热计算方法。     

A comparative study of solar irradiation models on various inclined surfaces for India

This paper presents a statistical approach for the estimation of the diffuse/global irradiation on various inclined surfaces from the measured data of horizontal surface. In fact diffuse solar radiation on an inclined plane consists of two components: sky diffuse radiation and reflected radiation from the ground. For analyzing estimation of the daily tilted sky diffuse component from the daily horizontal diffuse irradiance, we have considered six models Badescu, Circumsolar, Skartveit and Olseth, Hay, Klucher and Liu and Jordan (Isotropic). All these models except Badescu adopted the same methodology for estimating the ground-reflected radiation component, therefore, only sky diffuse component was analyzed at Lucknow (latitude 26.75°, longitude 80.50°), India location. Statistical analysis showed that the Skartveit and Olseth model gives good prediction for the low inclination angle however; Klucher model gave better performance for highly inclined south-facing surfaces. The Root Mean Square Errors (% RMSE) value varies from 3.45% to 24.15% except for Badescu and Circumsolar model which predict worse results. In general, Klucher’s model provides close agreement with the measurements.     

A brief review of models for computing solar radiation on inclined surfaces

Modelling of solar energy systems requires estimation of the hourly radiation incident on surfaces of different tilts and orientations. Most meteorological stations report radiation values on a daily rather than hourly basis. Again, the value of the diffuse component may or may not be reported. Hence, different strategies may be encountered, for each location, with the common goal of computing hourly radiation on inclined surfaces.

In this study, five different schemes are presented to achieve this goal. In each scheme, certain correlations are required which are discussed in detail. The study is divided into five sections, each dealing with a specific type of correlation. These sections are: (i) correlations between daily diffuse and global radiation; (ii) correlations between hourly/daily global radiation; (iii) correlations between hourly/daily diffuse radiation; (iv) correlations between hourly diffuse and global radiation; and (v) models for computing diffuse sky radiation on inclined surfaces. The last section deals extensively with the anisotropic nature of sky diffuse radiation.

The important aspects of all correlation studies are highlighted, and the relative merits and demerits of their results are brought to light.

Mathematical expressions, where available, for models/correlations are provided so that the reader will have access to a comprehensive study. This information should be useful for modelling purposes in which computation of radiation on surfaces of different orientations and tilts is required.     

The Influence of Friction Factor on the Convective and Radiative Heat Transfer in Inclined Cylindrical Annulus

The effect of friction factor on the unsteady state mixed convective-radiative heat transfer in an inclined cylindrical annulus is investigated from continuity, momentum and energy equations. The outer cylinder is kept at a constant temperature while the inner cylinder is heated with constant heat flux. The governing equations are normalized and solved using the vorticity-stream function and the BFC （body fitted coordinates） methods. The two heat transfer mechanisms of convection and radiation are treated independently and simultaneously. A computer program （Fortran 90） was built to calculate Nusselt number （Nu） and friction factorffor unsteady state condition for fluid Prandtl number fixed at （Pr = 0.7） （for air as working fluid） with radius ratio （/~ = 2.6）, Rayleigh number （0 〈 Ra 〈 103）, Reynolds number （50 〈 Re 〈 2,000）, conduction-radiation parameter （0 〈 N 〈 10）, optical thickness （0 〈 l＂ 〈 10） and different annulus inclination with horizontal plane （0~ _〈 d 〈 90~） for concentric cylindrical annulus. For the range of parameters considered, results show that radiation enhance heat transfer. It is also indicated in the results that as 3 increasefwill be decrease and also when Re increasefwill be decrease for any value of Ra causing increase in heat transfer. The maximum value off can be recognized at ~ = 90~ and the minimum value at 6 = 0~ for low Re. There is an optimum value of annulus inclination that gives maximum value of Nu, this maximum value appears at 90~ of annulus inclination comparison of the result with the previous work shows a good agreement.     

Model evaluation and optimum collector slope for a tropical country

Various models proposed in the literature, viz. the Beam method, proposed by Morse & Czarnecki; two methods proposed by Liu & Jordan, using hourly values and average daily values of solar radiation: and Klucher's method are compared in this paper using measured hourly values of global and diffuse radiation on a horizontal surface and the total radiation on a tilted surface. It was observed that the model proposed by Klucher estimates the radiation more consistently with the experimental observations for clear as well as cloudy sky conditions.

This method was used for predicting the year round radiation availability on (i) a fixed collector inclined at an optimum tilt, (ii) a partially tracking collector in which the slope is fixed but following the sun's azimuth angle, and (iii) a fully tracking collector. In this analysis, measured values of hourly global and diffuse solar radiation on a horizontal plane for four widely separated Indian stations, viz. Delhi (28°38′N), Poona (18°29′N), Calcutta (22°36′N), and Madras (13°8′N) as per India Meteorological Department classification for climatic analysis, have been used.

With the model proposed, the radiation availability on different slopes for all the twelve months of the year for a fixed and partially tracking collector have been evaluated and optimum tilts for various seasons of a year for different solar energy applications for all the four Indian stations have been determined. For a typical winter month at New Delhi in the semi-arid zone of India, a fixed collector, a partially tracking collector and a fully tracking collector receive 60, 80 and 95% more radiation compared to that received on a horizontal plane respectively. Another observation is that the variation of the insolation received on inclined surfaces is not at all sensitive to the angle of tilt in the neighbourhood of the optimum angle of operation.     

炉内辐射换热过程的有限体积法

简要分析了含吸收散射性介质的三维空腔内辐射传递方程的有限体积法求解过程,应用该方法对四角切圆炉膛内的辐射换热过程进行模拟计算,得出了炉膛内温度分布,并将计算结果与实测值进行了比较。通过数值计算表明：有限体积法计算速度快,对不规则边界适应性强,具有很高的工程可用性。     

Experimental testing of models for the estimation of hourly solar radiation on vertical surfaces at Arcavacata di Rende

More than 55,000 data of hourly solar radiation on a horizontal surface and on vertical surfaces exposed to the south, west, north and east, measured at Arcavacata di Rende (CS), were compared with hourly radiation data calculated by various calculation models.Erbs, Reindl et al. and Skartveit et al. correlations for the split of hourly global radiation in the diffuse and beam components were used together with the isotropic sky model and three anisotropic sky models.The agreement between experimental and calculated data is generally good.     

地热换热器孔外传热模型研究进展

总结了以往地热换热器孔外的理论传热模型，并将其归为两大类：有限长热源模型和无限长热源模型。在简要对其进行介绍和总结之后，进一步比较了他们之间的差别和联系。指出无限长模型不能适用于地热换热器长时间的传热分析，只能用来分析短时间内非稳态的传热分析。而有限长的传热模型由于考虑了地面作为传热边界对地下传热过程的影响，能够用来分析长时间的传热分析。     

透明热反射薄膜研究与开发——上海市能源研究所廿年研究工作的回顾

透明热反射薄膜（Transparent Heat Mirror，以下简称THM膜）是一种光谱选择性材料，在太阳能开发、建筑节能，以及家用电器等领域节能有着广泛的应用前景，上海市能源研究所（SIER）在近20年中从基础研究和材料开发等方面均进行了一些有意义的研究，现就有关该材料的基础性研究和某些方面的应用试验综述介绍。     

计算辐射换热的Monte—Carlo—Zone方法

本文提出了Ｍｏｎｔｅ－Ｃａｒｌｏ方法和Ｚｏｎｅ方法相结合的一种新的辐射传热的计算方法，简称为Ｍｏｎｔｅ－Ｃａｒｌｏ－Ｚｏｎｅ方法，并用此方法对充满均匀和非均匀吸收辐射灰介质的无限大平板间的辐射换热进行了计算，得出平板间的无量纲辐射热流及无量纲“跳跃”温度，并与有关文献的分析计算结果进行了比较，具有很高的精度度。     

太阳能热水器传热传质和强化传热研究

在低温太阳能光热光伏联合应用试验台的基础上,结合GB/T 17049—2005,利用Gambit、Ansys Fluent和Tecplot软件,对全玻璃真空管太阳能热水器进行传热传质和强化传热分析。结果表明:所建立的二维数值计算模型,能准确反映同一条件下,全玻璃真空管太阳能热水器的变化趋势;在数值模拟基础上,确定了单面受热时的最佳安装角度为51°,加装反光板类似双面受热的最佳安装角度为38°;在粗略估算和细化分析的基础上,确定了不同真空管结构的最佳导流板长度及安装位置;通过实验和数值模拟,确定了58mm×1 800mm为优化的全玻璃真空管结构。     

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。     

Solar radiation modelling in the urban context

This paper describes alternative methods for predicting surface irradiance in the urban context. In this the focus is on means of accounting for the effects of nearby obstructions on reducing direct sky radiation and on contributing reflected radiation. The first two methods involve abstracting the urban skyline into an effective canyon using isotropic and anisotropic tilted surface irradiance models. The third predicts the irradiance contribution from two hemispheres which are discretised into patches––given the radiance of the sky and dominant obstructions (if these exist) and associated view factors––so that we have a new simplified radiosity algorithm (SRA). Results from the three methods (isotropic canyon (IC), anisotropic canyon (AC) and simplified radiosity algorithm (SRA)) are compared with a ‘truth model' under the following circumstances: (i) unobstructed sky, (ii) sky obstructed by black surfaces, (iii) sky obstructed by grey diffusely reflecting surfaces. Results show conclusively that the SRA offers superior accuracy at comparable speed to the canyon models. The SRA also compares well with a ray tracing program, it can handle urban scenes of arbitrary geometric complexity and is readily amenable for inclusion into standard computer programs that require surface irradiance as an input.     

Increase in distillate productivity by inclining the flat plate external reflector of a tilted-wick solar still in winter

This paper presents a theoretical analysis of a tilted-wick solar still with an inclined flat plate external reflector on a winter solstice day at 30°N latitude. The daily amount of distillate of a still with an inclined reflector would be about 15% or 27% greater than that with a vertical reflector when the reflector’s length is a half of or the same as the still’s length.     

流化床气固传热模型

文中建立了计算流化床气固有效传热系数的简单模型，并且依据气固传热实验得到的稳态工况活动段区气体温度分布数据进行了计算，计算结果和经验数值非常吻合。本模型为研究流化床气固传效提供了参考。     

柴油机气缸内热辐射研究方法的探讨

作者从气缸内热辐射模型和实验两个方面探讨了气缸内辐射研究的方法。综述了气缸内热辐射理论和实验技术的进展,提出了气缸内辐射研究朝着多维化方向发展。     

流化床煤燃烧与气化过程中的辐射换热

流化床反应器中颗粒与颗粒之间的传热在一定程度上决定了化学反应的速率及反应的中间历程。本文通过对气固流化床乳化相中颗粒群结构的进一步认识，建立了颗粒间的辐射换热模型，比较了不同颗粒直径、不同床层温度水平及不同流化工况下颗粒间辐射换热与通过气膜导热份额的大小，并预测了流化床反应器中反应颗粒与惰性床料之间的温差，对于流化床反应器选择合理的运行工况和进行操作参数优化具有参考价值     

用离散坐标法求解矩形炉内三维辐射换热

叙述了有介质参与的三维空腔内辐射换热的离散坐标解法，并编制程序对三维矩形炉膛内辐射换热进行了数值模拟，计算结果与区域法吻合，是炉内辐射传热数值模拟的一种较好方法。最后展望并指出求解辐射传递方程的离散坐标法在非正交坐标系下应用的重要性。     

Unstructured Polygonal Finite-Volume Solutions of Radiative Heat Transfer in a Complex Axisymmetric Enclosure

Radiative heat transfer in a complex axisymmetric enclosure with participating medium is investigated by using the finite-volume method (FVM). In particular, an implementation of the unstructured polygonal meshes is adopted by connecting each center point of the unstructured triangular meshes rather than joining the centroids of the triangular elements to the midpoints of the corresponding sides to form a polygonal element. Also, typical considerations regarding application of the present polygonal mesh system to axisymmetric radiation are discussed. After a mathematical formulation and corresponding discretization equation for the radiative transfer equation (RTE) are derived, the final discretization equation is introduced with the conventional finite-volume approach by using the directional weights. For validation and comparison, three test examples with complex axisymmetric geometries have been accomplished. The present study shows that not only is the method flexible in treating radiative problems with complex geometries, it is also accurate and efficient for the analysis of axisymmetric radiative heat transfer.