Characterization of the thermal radiative properties of heterogeneous porous materials up to T= 2 500 K by a spectroscopic device (original) (raw)
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International Journal of Thermal Sciences, 2019
The influence of the texture of fused silica glasses − containing unconnected spherical bubbles − on their thermal radiative properties was investigated by combining experimental measurements and numerical simulations. Exact numerical replicas of the porous samples were obtained from X-ray microtomography and image analysis showed that their bubble populations include two lognormal distributions of radii. The complex refractive index of the silica matrix was extracted from emittance measurements acquired at 1200 K on two reference silica samples. The comparison between direct measurements of the normal spectral emittance of the porous samples and those generated by applying Monte-Carlo ray tracing, performed on the numerical replicas, validated the use of geometric optics. The OH content of the porous samples was evaluated to be around 210 ppm. Ray tracing simulations carried out on a set of virtual samples having a similar texture proved to be a flexible means to address experimental limits when characterizing semi-transparent materials. The simulations also provided a natural framework to test the ability of the modified two-flux approximation model to predict the thermal radiative response of the porous silica investigated.
Spectroscopic research on infrared emittance of coal ash deposits
Experimental Thermal and Fluid Science, 2009
This paper deals with thermal radiation characteristics of ash deposits on a pulverized coal combustion boiler of an electric power plant. Normal emittance spectra in the near to medium infrared (2.5-25 lm) region and total normal emittances were measured on four kinds of ground ash deposits. Measurements were conducted in the 570-1460 K temperature range which is common for boiler furnaces, by both heating and cooling the ash samples, with the aim to study the effect of their thermal history. Dependence of emittance on wavelength, temperature and chemical composition was studied, too. Samples were tested for transparency (opacity) to verify the accuracy of results. It was determined that the thicknesses used for the ash powders are opaque for infrared radiation for thicknesses in the order of a millimeter. Tests have shown that spectral emittance increases with an increase of wavelength with a characteristic pattern common for all samples. Spectral normal emittance increases strongly with temperature at shorter wavelengths and remains high and unchanged at longer ones. Emittance spectra are not very sensitive to chemical composition of ashes especially beyond k % 5 lm. With an increase of temperature, total emittance of the powdered sample decreases to a minimum value around 1200 K. Further temperature rise induces an increase of total emittance due to sintering in the ash. On cooling, the emittance increases monotonically following the hysteresis. Quantitative directions for evaluating thermal radiation characteristics of ash deposits for the merits of the safety design of boiler furnaces were proposed. That comprises correlating the experimentally obtained emittance spectra with curves of simple analytical form, i.e., a continuous function of minimum emittance vs. wavelength. The proposed method can be extended to other specimens from the same furnace and used to determine correlations for thermal calculation of old and design of new furnaces -with similar geometry and combusting similar coal. The method is potentially applicable to completely different boiler furnaces combusting different coal, and the authors recommend running the tests with new deposit samples. The data will then be applicable to the thermal design of a whole new class of furnaces, having similar geometry and combusting similar coal. This is expected to greatly enhance the accuracy and precision of thermal calculation as well as the efficiency of thermal design of steam boilers.
Textural parameters influencing the radiative properties of a semitransparent porous media
International Journal of Thermal Sciences, 2011
In the field of thermal radiation, a current key challenge is to understand the role played by the texture of semitransparent heterogeneous materials on their thermal radiative properties. This paper is restricted to the case of any porous materials endowed with spherical pores and for which their mean pore size is greater than the incident wavelength. In this work, an algorithm had been developed to elaborate porous silica glass with prescribed textural properties. The design of virtual pore's geometries was based on the textural investigation of a real porous glass, beforehand characterized by synchrotron x-ray m-tomography. Then a Monte Carlo Ray Tracing code is carried out to predict the thermal radiative properties of the numerical samples. In this work, the solid phase of each sample is composed of silica of high chemical purity for which the optical functions are known. Lastly, for the spectral range where the numerical glasses are transparent, the roles played by both the porosity and the volumetric surface on the scattering behavior are discussed.
International Journal of Heat and Mass Transfer, 2013
Radiative heat transfer to a solid is a key mechanism in fire dynamics, and in-depth absorption is especially of importance for translucent fuels. The sample-heater interaction for radiative heat transfer is experimentally investigated in this study with two different heaters (electric resistance and tungsten lamp) using clear PolyMethylMethAcrylate (PMMA) samples from two different formulations (Plexiglass and Lucite). First, the significant effects of the heater type and operating temperature on the radiative heat transfer are revealed with broadband measurements of transmittance on samples of different thicknesses. Then, the attenuation coefficient in Beer-Lambert's law has been calculated from detailed spectral measurements over the full wavelength range encountered in real fires. The measurements present large spectral heterogeneity. These experimental results and calculation of in-depth absorption are used to explain the reason behind the apparent variation of the fuel absorbance with the sample thickness observed in past studies. The measurement of the spectral intensity emitted by the heaters verifies that the common assumption of blackbody behavior is correct for the electric resistance, whereas the tungsten lamp does not even behave as a greybody. This investigation proofs the necessity of a multi-band radiation model to calculate accurately the fire radiative heat transfer which affects directly the in-depth temperature profiles and hence the pyrolysis process for translucent fuel.
Measurement of band emissivity of porous spheres at high temperatures
The Canadian Journal of Chemical Engineering, 1987
A relatively simple technique has been developed to measure the band emissivity of spherical solids at high temperatures. The technique involves measuring the surface temperature of a sphere when in thermal equilibrium with a furnace enclosure, followed by isolating it from the enclosure by means of a shield. The emissivity is evaluated from the drop in energy flux from the pellet consequent upon changing the enclosure from an effectively black-body to a freely radiating one. The value of the band emissivity is validated by simulating the experimental cooling curve using independently measured values of the effective thermal conductivity and total emissivity of the solid to obtain internal consistency. In this way highly accurate values of band emissivity are obtained. The technique was applied to pellets of both reacted and unreacted mixtures of calcined-dolomite and silicon, as used in the commercial production of magnesium. On a mis au point une technique relativement simple pour mesurer I'kmissivitk de bande de solides sphkriques h des tempkratures tlevkes. La technique fait appel h la mesure de la temp6rature de surface d'une sphbre en tquilibre thermique avec I'enceinte d'un four, puis h son isolation de l'enceinte au moyen d'un &ran protecteur. L'kmissivitk est 6valuCe B partir de la diminution du flux d'knergie de la pastille rksultant du changement d'une enceinte constituant un vkritable corps noir pour une enceinte irradiant librement. La valeur de l'kmissivitk de bande a Ctt confirm& en simulant la courbe de refroidissement exp6rimentale en utilisant des valeurs mesurkes indkpendamment de la conductivitk thermique effective et de I'Cmissivitk totale du solide afin d'obtenir une certaine cohkrence interne. On obtient de cette manibre des valeurs trbs prkcises de I'kmissivitC de bande. Cette technique a t t t appliquke h des pastilles de mtlanges de dolomite calcink et de silicium avant et aprhs rkaction, telles que celles utiliskes dans le production commerciale de magnesium.
Investigation on thermal radiation spectra of coal ash deposits
International Journal of Heat and Mass Transfer, 2009
This paper deals with thermal radiation properties of ash deposits on a pulverized coal boiler of an electric power plant. Normal emittance spectra in the 2.5-25 lm interval, and total normal emittance, were measured on 4 kinds of ash layers of a mm magnitude order thickness, at 560 ? 1460 ? 560 K in heating and cooling. It was found that ash powder layers are opaque for infrared radiation. The emittance increases with ash radiation wavelength and temperature. Ash powder is sintered and fused above 1200 K. The emittance of the sintered layer is above that of the unsintered layer. The authors propose, and explain by an example, correlating the experimentally obtained emittance spectra of ash deposits with a continuous curve, the formula of which defines the dependence of emittance on wavelength and temperature, i.e. e = e(k,T). Use of this formula, with parameter values determined by the proposed methodology, may greatly simplify the practical application of the experimentally determined emittances in the thermal design of existing and new steam boiler furnaces.
Spectral Emittance Characteristics of Powdery and Molten Coal Ashes
Spectral emittances of two lignite and two bituminous coal ashes have been measured in the spectral range between 980 nm and 17020 nm at temperatures from about 500°C to 1100°C in order to investigate experimentally th e effect of heat treatment. From each type of ash two samples have been prepared, one from untreated laboratory ash and one from heat-treated ash which has been heated up to a temperature of 1400°C for one hour. The emittance measurements of these eight specimen show that heat treatment may have manifold effects on both the physical structure and the mineralogical composition of ashes and thus on their emittance characteristics. The underlying mechanisms of these modifications and their impact on the heat transfer in an industrial boiler will be discussed.
Thermoluminescence Spectra of Minerals
Mineralogical Magazine, 1993
Thermoluminescence (TL) studies of insulators, including crystals of minerals such as calcite, quartz or zircon, have resulted in the development of a wider range of applications in the fields of radiation dosimetry and archaeological and geological dating. Most conventional TL measurements are made by recording light emission during heating by means of broad-band optical filters and blue-sensitive photomultiplier tubes. Much more information may be gained by monitoring the details of the emission spectrum during thermoluminescence. TL spectra of minerals exhibit changes as a result of crystal purity, radiation dose, dose rate and thermal history. This paper exemplifies the additional information available as a result of spectral studies, and focuses on investigations undertaken by the University of Sussex TL group. Examples include work on calcite, fluorite, zircon and feldspars.
Fuel, 2014
Ash residues that arise during combustion of solid fuels form deposits on heat exchanger surfaces which hinder the heat transfer to the working fluid steam. The thermal conductivity and -especially within the combustion chamber -the optical properties of the deposits determine the transferred amount of heat. Thus, knowledge of the optical properties, here represented by the normal emittance, are crucial for the design and operation of a steam generator.