The inaccuracy of heat transfer characteristics of insulated and non-insulated circular duct while neglecting the influence of heat radiation

The non-insulated and insulated ducts are commonly applied in the industries and various buildings, because the heat radiation equation contains the 4th order exponential of temperature which is very complicate in calculations. Most heat transfer experts recognized from their own experiences that the heat radiation effect can be ignored due to the small temperature difference between insulated and non-insulated surface and surroundings. This paper studies in detail to check the inaccuracies of heat transfer characteristics non-insulated and insulated duct by comparing the results between considering and neglecting heat radiation effect. It is found that neglecting the heat radiation effect is likely to produce large errors of non-insulated and thin-insulated ducts in situations of ambient air with low external convection heat coefficients and larger surface emissivity, especially while the ambient air temperature is different from that of surroundings and greater internal fluid convection coefficients. It is also found in this paper that using greater duct surface emissivity can greatly improve the heat exchanger effect and using smaller insulated surface emissivity can obtain better insulation.     

多谱段温度测量方法

提出了基于谱段的温度测量方法———多谱段温度测量方法。与基于波长的温度测量方法相比 ,这种辐射测温方法可以采用容易实现的光学滤色技术和简单的数学处理手段     

Heat loss factor of evacuated tubular receivers

Tubular receivers with an evacuated space between the absorber and concentric glass cover to suppress convection heat loss are employed as absorbers of linear concentrators in the intermediate temperature range. A knowledge of their heat loss factor is important for a study of the thermal performance of such solar concentrating systems. The heat loss factor of a collector can be calculated by solving the governing heat transfer equations or estimated from an empirical equation, if available. The governing equations must be solved simultaneously by iterations, but this is tedious and cumbersome. Although several correlations exist for determining the heat loss factor for flat-plate collectors and non-evacuated tubular absorbers of linear solar collectors, there is no available correlation for predicting the heat loss factor of evacuated receivers.

A correlation to calculate the heat loss factor (U_L) of evacuated tubular receivers as a function of variables involved (absorber temperature, emittance, diameter and wind loss coefficient) has been obtained. The correlation developed by a least square regression analysis predicts the heat loss factor to within ±1.5% of the value obtained by exact solution of the simultaneous equations in the following range of variables: wind loss coefficient, 10–60 W/m²°C; emittance, 0.1–0.95; and absorber temperature, 50–200°C.     

Emissivity of spinel and titanate structures aiming at the development of industrial high-temperature ceramic coatings

High-emissivity coatings have been pointed out as a promising energy-saving solution for industrial furnaces since their development in the aerospace area. However, due to a lack of in-depth knowledge of radiation heat transfer and thermal-optical interactions, as well as the struggle to obtain proper emissivity values, commercial versions of such coatings have not yet been widely established. In this work, the evaluation of some fundamentals on thermal-optical properties, and their potential applications, led to the development of engineered structures of usual inexpensive oxides, such as the titanate and spinel ones, which presented improved emissivity values, even at high temperatures (1500 °C), when compared to commercial references. The findings of these compounds indicated a considerable cost-effective saving potential for large industrial furnaces.     

低红外发射率水性聚氨酯的研究

为满足红外隐身涂料成膜物的要求,采用在715～1250cm^-1波段红外透明性好的聚碳酸酯二醇（PCDL）为软段,而以异佛尔酮二异氰酸酯（IPDI）为硬段,合成了低红外发射率水性聚氨酯。该树脂在8～14μm红外窗口红外光谱吸收峰少,红外发射率低,涂膜物理综合性能优良。     

水泥熟料的黑度及其与温度关系的研究

本文根据热辐射的基本理论,建立测定高温硅酸盐水泥熟料黑度的试验设备,对国内若干厂家回转窑生产的硅酸盐水泥熟料的单色黑度及其与温度的关系进行了较深入的研究。采用0. 65μm 红光的精密光学高温计测量水泥熟料和人工黑体的温度,用回归分析法,导出水泥熟料在各种亮度温度 t_0℃时的单色黑度ε_(?)与温度之间的经验公式。所得结论对实际生产有较大的实用意义。该方法也适于用作教学试验。     

8～14μm光谱区平均比辐射率计算方法的研究

就８～１４μｍ红外光谱区内利用测得的多个窄波段的平均光谱比辐射率计算该光谱区的平均比辐射率进行了理论分析和实验研究；并与相应的算术平均值进行了比较。结果表明，用较简单的算术平均值代替较繁复的理论计算值，其相对误差不超过１％。     

The contribution of the thermal radiation of the inner surfaces of a room on its thermal losses

The temperature difference between the surfaces of the internal and the external walls of a room causes a heat flux from the internal to the external walls through radiation and finally to the environment through conduction. This thermal flux, which is different to the thermal losses due to the conductance of the external walls, is a function of the thermal and the structural characteristics of the walls. In the present work a simple analytical expression is given, which relates the thermal losses due to radiation and the total thermal losses due to radiation and convection directly to the structural characteristics of the room, that is the reflectivity of the internal surfaces, the conductance of the external walls, the ratio of the external to the internal wall surfaces and the temperatures of the internal space of the room and the environment. This relationship has been verified by experiments carried out on an experimental arrangement which simulates this phenomenon.     

Canopy directional emissivity: Comparison between models

Land surface temperature plays an important role in many environmental studies, as for example the estimation of heat fluxes and evapotranspiration. In order to obtain accurate values of land surface temperature, atmospheric, emissivity and angular effects should be corrected. This paper focuses on the analysis of the angular variation of canopy emissivity, which is an important variable that has to be known to correct surface radiances and obtain surface temperatures. Emissivity is also involved in the atmospheric corrections since it appears in the reflected downwelling atmospheric term. For this purpose, five different methods for simulating directional canopy emissivity have been analyzed and compared. The five methods are composed of two geometrical models, developed by Sobrino et al. [J. A. Sobrino, V. Caselles, & F. Becker (1990). Significance of the remotely sensed thermal infrared measurements obtained over a citrus orchard. ISPRS Photogrammetric Engineering and Remote Sensing 44, 343-354] and Snyder and Wan [W. C. Snyder & Z. Wan, (1998). BRDF models to predict spectral reflectance and emissivity in the thermal infrared. IEEE Transactions on Geoscience and Remote Sensing 36, 214-225], in which the vegetation is considered as an opaque medium, and three are based on radiative transfer models, developed by François et al. [C. François, C. Ottlé, & L. Prévot (1997). Analytical parametrisation of canopy emissivity and directional radiance in the thermal infrared: Application on the retrieval of soil and foliage temperatures using two directional measurements. International Journal of Remote Sensing 12, 2587-2621], Snyder and Wan [W. C. Snyder & Z. Wan (1998). BRDF models to predict spectral reflectance and emissivity in the thermal infrared. IEEE Transactions on Geoscience and Remote Sensing 36, 214-225.] and Verhoef et al. [W. Verhoef, Q. Xiao, L. Jia, & Z. Su (submitted for publication). Extension of SAIL to a 4-component optical-thermal radiative transfer model simulating thermodynamically heterogenous canopies. IEEE Transactions on Geoscience and Remote Sensing], in which the vegetation is considered as a turbid medium. Over surfaces with sparse and low vegetation cover, high angular variations of canopy emissivity are obtained, with differences between at-nadir view and 80° of 0.03. Over fully vegetated surfaces angular effects on emissivity are negligible when radiative transfer models are applied, so in these situations the angular variations on emissivity are not critical on the retrieved land surface temperature from remote sensing data. Angular variations on emissivity are lower when the emissivity of the soil and the emissivity of the vegetation are closer. All the models considered assume Lambertian behaviour for the soil and the leaves. This assumption is also discussed, showing a different behaviour of directional canopy emissivity when a non-Lambertian soil is considered.     

宽带辐射高温计定标黑体炉的辐射特性

本文绐出了标定宽带辐射高温计,特别是红外辐射温度计的黑体炉辐射特性的精密解。定义了波段(或带宽)有效发射率作为其主要特性参数,建立了数学模型,并以工厂实际使用的锥一筒非等温定标黑体炉为例在计算机上求得了典型解。从与相应的光谱有效发射率解的比较中可见:采用本法将较大地提高宽带辐射高温计的定标精度。