Evaluating the effect of long-term forest fire retardants on thermal properties: Fuel heat content and flame emissivity
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Low emissivity
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Emissivity of a metal is fairly low in general. Due to an oxidation film formed on the metal surface, however, its emissivity changes rapidly, and large errors in the temperature measurement can be caused. This is a serious problem for the radiation thermometry of metals. In order to eliminate the error due to surface conditions of the targets, the authors propose a new method based on their findings that the apparent emissivity of a metal surface depends on the direction from which the target is looked at. This means that the radiance of the target does not obey Lambert law. The proposed method uses the ratio of radiance measurements taken at two different angles to compensate the emissivity or the emissivity ratio. This method is promising for the in-line use at metal manufacturing processes such as steels or aluminums.
Low emissivity
Thermal Radiation
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Many studies of anomalous microwave emission (AME) have computed an AME emissivity to compare the strength of the AME detected in different regions. Such a value is usually defined as the ratio between the intensity of the AME at 1 cm and the thermal dust emission at 100 μ m. However, as studies of Galactic dust emission have shown, the intensity of the thermal dust emission at 100 μ m is strongly dependent on the dust temperature, which has severe implications for the AME emissivity defined in this way. In this work, we illustrate and quantify this effect and find that the AME emissivity decreases by a factor of 11.1 between dust temperatures of 20 and 30 K. We, therefore, conclude that computing the AME emissivity relative to the 100 μ m emission does not allow for accurate comparisons between the AME observed in different environments. With this in mind, we investigate the use of other tracers of the dust emission with which to compute the AME emissivity and we ultimately conclude that, despite the difficulty in deriving its value, the column density of the dust would be the most suitable quantity with which to compute the AME emissivity.
Thermal Emission
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Low emissivity
Thermal Radiation
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The emissivity is a key parameter to measure the surface temperature of materials in the radiation thermometry. In this paper, the surface emissivity of metallic substrates is measured by the multi-wavelength emissivity measurement apparatus developed by the Harbin Institute of Technology (HIT). The measuring principle of this apparatus is based on the energy comparison. Several radiation thermometers, whose emissivity coefficients corrected by the measured emissivity from this apparatus, are used to measure the surface temperature of stainless steel substrates. The temperature values measured by means of radiation thermometry are compared to those measured by means of contact thermometry. The relative error between the two means is less than 2% at temperatures from 700K to 1300K, it suggests that the emissivity of stainless steel substrate measured by the multi-wavelength emissivity measurement apparatus are accurate and reliable. Emissivity measurements performed with this apparatus present an uncertainty of 5.9% (cover factor=2).
Low emissivity
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Experiments were first conducted to measure the spectral normal emissivity values of a variety of aluminum alloys at 600, 700, and 800 K. Multispectral radiation thermometry (MRT) using linear emissivity models (LEM) and log-linear emissivity models (LLE) were then applied to predict surface temperature. Results show that the spectral emissivity decreases with increasing wavelength and increases with increasing temperature. Alloy effect becomes evident at higher temperature. The surface oxidation becomes fully-developed after the first hour heating and results in constant emissivity. Half of temperature predictions by MRT emissivity models provide the absolute temperature error under 10% and a quarter if the results are under 5%. The better emissivity model to suitably represent the real surface emissivity behaviors the more accurate inferred temperature by MRT can be achieved. Increasing the order of emissivity model and increasing the number of wavelengths cannot improve temperature measurement accuracy. More accurate temperature measurement by MRT can be achieved at higher temperature. Overall, three emissivity models give good results most frequently and provide the best compensation for different alloys, the number of wavelengths, and temperatures.
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The determination of emissivity is crucial in any temperature measurement using radiation thermometry. Without this knowledge, large measurement uncertainties result. There is a lack of information on emissivity for common materials. Where there are databases, these databases often give emissivity for a specific material in a range or give emissivity for different conditionings of the material. This information may not apply to certain uses of the material. This creates quite a bit of doubt for anyone making measurements in the field. What is needed is a method to determine emissivity for a material object in the field. In 2011, Yamada and Ishii discussed a method that was set up in a fixed geometry to determine the emissivity of a specular object. In this paper, a method is discussed to determine the emissivity of both specular and diffuse objects using a thermal radiation source. The theory is presented. Then, practical measurements which were made are discussed. These measurements are compared to emissivity determined by other methods.
Low emissivity
Thermal Radiation
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Determination of temperature by IR thermal cameras depends significantly on the knowledge about the emissivity of solids in question. Solids emissivity data can be found in handbooks but can also considerably differ from real values. It is therefore necessary to obtain emissivity data by measurements taken on real materials. Solids emissivity can be determined by the same IR system used for temperature determination. A mathematical model for emissivity determination is derived in the paper as well as the function of error in determining solid materials emissivity by the 870SWB Agema IR THERMOVISION camera with 2-5 μm spectral range. The standard relative error and partial errors in determining emissivity of solid surfaces are analyzed and the obtained results point out to the possibility to model conditions necessary for acquiring demanded accuracy in emissivity determination.
Low emissivity
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Low emissivity
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Under precondition of high emissivity,the thickness of high emissivity ceramic coatings should be reduced to satisfy the requirement for mechanical property,which can also save raw materials and decrease the weight of the apparatus.So the relationship between emissivity and thickness of the coatings is important.In this paper,the coatings were prepared from Al2O3 sols and high emissivity fillers by spin and spray processes,respectively.The relationship between emissivity and thickness of coatings was examined,and the critical thickness of coatings was determined while the emissivity changes with the thickness rapidly.Based on Maxwell-Garnett theory,a new theoretical model about the relationship between emissivity and thickness of composite coatings is established.The emissivity values from the theoretical calculation are agree with the experimental data,which shows that the model can be used to predict the emissivity or the critical thickness of the composite coatings.
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