Multispectral Radiation Thermometry Research Articles

Multispectral pyrometry for surface temperature measurement of oxidized Zircaloy claddings

Non-contact temperature measurement in a nuclear reactor is still a huge challenge because of the numerous constraints to consider, such as the high temperature, the steam atmosphere, and irradiation. A device is currently developed at CEA to study the nuclear fuel claddings behavior during a Loss-of-Coolant Accident. As a first step of development, we designed and tested an optical pyrometry procedure to measure the surface temperature of nuclear fuel claddings without any contact, under air, in the temperature range 700–850°C. The temperature of Zircaloy-4 cladding samples was retrieved at various temperature levels. We used Multispectral Radiation Thermometry with the hypothesis of a constant emissivity profile in the spectral ranges 1–1.3µm and 1.45–1.6µm. To allow for comparisons, a reference temperature was provided by a thermocouple welded on the cladding surface. Because of thermal losses induced by the presence of the thermocouple, a heat transfer simulation was also performed to estimate the bias. We found a good agreement between the pyrometry measurement and the temperature reference, validating the constant emissivity profile hypothesis used in the MRT estimation. The expanded measurement uncertainty (k=2) of the temperature obtained by the pyrometry method was ±4°C, for temperatures between 700 and 850°C. Emissivity values, between 0.86 and 0.91 were obtained.

Spectral emissivity model of steel 309S during the growth of oxide layer at 800–1100 K

This work strived to determine the analytical relationships between the spectral emissivity and the wavelength at different temperatures during the growth of oxide layer on the specimen surface of steel 309S. In the experiment, the spectral emissivity was measured at eight wavelengths, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, and 2.1μm, at temperatures from 800 to 1100K in increments of 20K by multispectral radiation thermometry. To accurately measure the normal spectral emissivity, the detector employed in the thermometry should be perpendicular to the surface of specimens as accurately as possible. The temperature of specimen surface was measured by the two thermocouples, which were symmetrically welded onto the front surface of specimens. The average of their readings was regarded as the true temperature. With the spectral emissivity measured here, the variation in the spectral emissivity with wavelength was evaluated at different temperatures and different heating times. Ten emissivity models were evaluated. The effect of number of the parameters used in the models on the fitting accuracy was studied. Both the five-parameter LLWE and LWE models were very suitable for fitting the spectral emissivity and could farthest relieve the effect of heating time on the accuracy of temperature prediction. The uncertainties in the temperature prediction of steel 309S specimens were basically within 10K by thermometry using the two models over the present temperature and wavelength ranges.

Normal Spectral Emissivity Models of Steel 304 at 800–1100 K with an Oxide Layer on the Specimen Surface

This work investigated the normal spectral emissivity characteristics of Steel 304 over a temperature range from 800 to 1100 K and a wavelength range from 1.4 to 2.1 μm. In the experiments, specimens were heated in air for 6 h at a certain temperature. Two platinum–rhodium thermocouples were symmetrically welded onto the front surface of the specimens near the measuring area for accurate monitoring of the surface temperature. The temperatures measured by the two thermocouples had an uncertainty of 1 K. The average of their readings was regarded as the temperature of the specimen surface. The radiance stemming from the specimens were measured at temperatures from 800 to 1100 K in increments of 20 K by multispectral radiation thermometry. Variation in the normal spectral emissivity with respect to wavelength and temperature was studied for different heating times. The effect of surface oxidization on the accurate prediction of the specimen surface temperature was discussed. Similar variation in the normal spectral emissivity for different heating times showed that the emissivity followed a certain rule. Nine emissivity models were examined for accuracy in the temperature prediction. The results showed that the four-parameter log-linear wavelength and line wavelength emissivity models generated best overall temperature prediction. We concluded that the effect of surface oxidization on the emissivity models of Steel 304 could be dismissed; and that the same models could be used to predict its surface temperature from 800 to 1100 K.

Application of multispectral radiation thermometry in temperature measurement of thermal barrier coated surfaces

Abstract Ceramics coatings are materials widely used in gas turbines to provide thermal shielding of superalloy materials against excessive turbine temperatures. However, measurement of their surface temperatures using conventional radiation thermometers, more so in the presence of high ambient radiation and low emissivity is quite challenging. A multispectral method employing curve fitting technique to measure the temperature of such targets in the range of 800–1200 K and ambient temperature of 1273 K is implemented in this paper through simulation. Several simulated experiments were carried out to identify emissivity models best suited for multispectral radiation thermometry applicable to ceramic coatings. The best emissivity model applicable to yttria-stabilized zirconia of coating thickness of 330 μm in the wavelength range of 3.5–3.9 μm was found to predict temperature with an error of less than 1.5% in the presence and absence of background noise.

Effect of oxidation on aluminum alloys temperature prediction using multispectral radiation thermometry

Experiments were first conducted to measure the spectral intensity values under an open-air heating system and a high-vacuum heating system. Eight emissivity models were used to examine the multispectral radiation thermometry (MRT) in order to understand the effect of surface oxidation on temperature determination and find the best MRT emissivity model as well. Emissivity spectra are found similar in shape but not in magnitude under oxidized and unoxidized conditions. Surface oxidation causes the increase in emissivity and the difference at varied temperatures. Overall, one model shows the highest accuracy in temperature prediction for oxidized and unoxidized aluminum alloys.

Temperature Determination Using Multispectral Radiation Thermometry Algorithms for Aluminum Alloys

Multispectral radiation thermometry (MRT) was applied to predict the aluminum surface temperature. Experiments were conducted to measure the spectral intensity values for five different aluminum alloys, AL1100, AL2024, AL5083, AL6061, and AL7005, at 600 K, 700 K, and 800 K. The experimental work is coupled with six MRT emissivity models encompassing mathematical and analytical functions to infer surface temperature. Assessment of the MRT emissivity model is subject to parametric effects of number of wavelengths, alloy composition, and temperature. Results show that increasing wavelength number does not significantly improve measurement accuracy while applying MRT. If the emissivity model can represent well the real emissivity behaviors, a more accurate inferred temperature can be achieved. Overall, most models achieve high accuracy in temperature prediction, except two emissivity models. One particular emissivity model provides the best compensation for the aforementioned parametric influences.

Experimental investigation of emissivity of aluminum alloys and application of multispectral radiation thermometry

Abstract Experiments were first conducted to measure the emissivity values of a variety of aluminum alloys at 600, 700, and 800 K. The effects of wavelength, temperature, alloy composition, and heating time on emissivity were investigated. Multispectral radiation thermometry (MRT) with linear emissivity models (LEM) and log-linear emissivity models (LLE) were then applied to predict surface temperature. Parametric influences of wavelength number, heating time and order of emissivity models were examined. Results show that the spectral emissivity decreases with increasing wavelength and increases with increasing temperature. A stronger alloy effect is evident at higher temperature. The spectral emissivity reaches steady state after the first hour heating due to the surface oxidation becoming fully-developed. Half of the temperature predictions by MRT emissivity models provide the absolute temperature error under 10% and quarter of the results are under 5%. Increasing the order of emissivity model and increasing the number of wavelengths cannot improve temperature measurement accuracy. Overall, LLE models show higher accuracy than LEM models. The first-order and second-order LLE models and the first-order LEM model give good results most frequently and provide the best compensation for different alloys, the number of wavelengths, and temperatures.

Examination of multispectral radiation thermometry using linear and log-linear emissivity models for aluminum alloys

Multispectral radiation thermometry (MRT) using two commonly used emissivity models, linear emissivity models (LEM) and log-linear emissivity models (LLE), was used to predict the aluminum surface temperature. Experiments were conducted to measure the spectral intensity values for five different aluminum alloys at three different temperatures. Overall, three emissivity models give good results most frequently and provide the best compensation for different alloys, the number of wavelengths, temperatures, and heating time.

The Assessment of Multispectral Radiation Thermometry Using Linear and Log-Linear Emissivity Models for Steel

Experiments were performed to measure the spectral intensity values of a variety of steel samples at 700, 800, and 900 K. The experimental work is coupled with two commonly used emissivity models, linear emissivity models (LEMs) and log-linear emissivity models (LLEs), to examine multispectral radiation thermometry (MRT) on inferring surface temperature. Overall, the first-order LEM and the first-order LLE show the best compensation for different alloys, number of wavelengths, temperatures, and heating time. The results reveal that the better the emissivity model suitably represents real surface emissivity behavior, the more accurate is the inferred temperature by MRT.

Suitability of Multispectral Radiation Thermometry Emissivity Models for Predicting Steel Surface Temperature

a0, a1 = unknown coefficients in the emissivity model c1 = first thermal radiation constant c2 = second thermal radiation constant L ;b = spectral intensity of blackbody radiation L ;gen = generated spectral intensity of radiation L ;meas = measured spectral radiation intensity m = number of unknown coefficients of emissivity model N = total number of wavelengths available in the examined wavelength range n = required minimum number of wavelengths T = surface temperature T = spectral radiance temperature (equivalent blackbody temperature of the measured spectral intensity) = spectral emissivity = wavelength 2 = least-squares error

Study of Steel Emissivity Characteristics and Application of Multispectral Radiation Thermometry (MRT)

Experiments were first conducted to measure the emissivity values of a variety of steel samples at 700, 800, and 900 K. The effects of wavelength, temperature, alloy composition, and heating time on emissivity were investigated. Multispectral radiation thermometry (MRT) with linear emissivity models (LEMs) and log-linear emissivity models (LLEs) were then applied to predict surface temperature. Parametric influences of the number of wavelengths and order of emissivity models were examined. Results show that the spectral emissivity decreases with increasing wavelength and increases with increasing temperature. Steel with higher chromium content has lower emissivity value because of the chromium oxide protection layer. The spectral emissivity reaches steady state after the third hour heating due to the surface oxidation becoming fully developed. Increasing the order of polynomial and increasing the number of wavelengths cannot improve temperature measurement accuracy. Overall, the first-order LEM and the first-order LLE showed the best accuracy for different alloys, the number of wavelengths, and temperatures.

Investigation of steel emissivity behaviors: Examination of Multispectral Radiation Thermometry (MRT) emissivity models

Steel emissivity behaviors were investigated in this study. Experiments were conducted to measure emissivity. Six emissivity models were then applied to examine Multispectral Radiation Thermometry (MRT) on inferring surface temperature. The data show that emissivity decreases with increasing wavelength. For steel containing high chromium, emissivity is usually lower than others because of the chromium oxide protection layer. Two emissivity models provide the best overall compensation for different alloys, number of wavelengths, and temperatures. The results reveal that if the emissivity model can well represent the real emissivity behaviors, the more accurate inferred temperature can be achieved.

Modeling the effects of surface roughness on the emissivity of aluminum alloys

This study explores the relationship between the emissivity of aluminum alloy surfaces and surface roughness. Two methods are discussed which yield good overall predictions of the emissivity of rough surfaces. One method consists of using a mathematical multispectral radiation thermometry (MRT) model for the emissivity and determining both the surface temperature and the empirical constants in the emissivity model from radiance measurements. This method requires new emissivity constants to be determined for each surface topography. This study also presents an alternative method for determining the emissivity of rough surfaces. This method relies on determining the emissivity characteristics of a single reference surface and inferring the emissivity of any other rough surface of the same material by relating a surface roughness function (determined by surface topography instrumentation) of the rough surface to that of the reference surface. Using data for AL 7075 with various degrees of surface roughness, this method is shown to yield better accuracy than the first method.

Mathematical determination of emissivity and surface temperature of aluminum alloys using multispectral radiation thermometry

A relatively simple emissivity model has recently been shown to effectively capture the parameteric effects of wavelength, temperature, alloy and surface roughness on the emissivity of aluminum surfaces. In the present study, a mathematical method is developed for determining both the empirical constant in the emissivity model and the surface temperature based on spectral radiance measurements. This study proves the relationship between reflectance ratio and optical roughness is by no means universal, and the complex effects of wavelength, temperature, alloy and surface roughness are more accurately captured with the aid of a multispectral emissivity model.

Emissivity characteristics of polished aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models

Emissivity characteristics were measured for several polished aluminum alloy samples over the spectral range of 2.05–4.72 μm and temperatures of 600–800 K. Overall, aluminum alloys buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. Only AL 7150 follows the expected trend, while the emissivity of the other alloys decreases between 600 and 700 K and increases between 700 and 800 K, and the emissivity of commercially pure aluminum (AL 1100) decreases monotonically with increasing temperature. The experimental results are used to assess the accuracy of popular multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions are achieved using the simplest form of MRT emissivity models and minimum number of wavelengths required by the model. Two relatively simple models are identified for best overall predictions for different alloys and temperatures. Despite the relative success of these two models, this study clearly demonstrates that improvements are required in both instrumentation and emissivity models to achieve acceptable accuracy in the implementation of radiation thermometry in the aluminum industry.

Emissivity characteristics of roughened aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models

Experiments were performed to examine the emissivity characteristics of aluminum alloy samples over the spectral range of 2.05 to 4.72 μm and temperatures of 600, 700 and 800 K. AL 1100, 7150, 7075 and 2024 samples were tested which possessed polished, 6- and 14-μm surface finishes. Additionally, extruded and saw-cut samples were tested to examine the effects of extreme roughness on emissivity. For the polished, 6- and 14-μm samples, the emissivity decreased appreciably between 2.05 and 3.5 μm, and increased slightly between 3.5 and 4.72 μm. Spectral variations were far less pronounced for the extruded and saw-cut surfaces. Eighteen MRT emissivity models were examined for accuracy in temperature measurement. Drastic changes in the emissivity distribution precluded the use of a single function to accurately represent every band of the measured spectrum. Overall, two relatively simple models provided the best overall predictions for different alloys and temperatures. These are the same models that yielded the best overall results for polished aluminum surfaces in a previous study by the authors. Despite the relative success of these models, this study points to a need to greatly enhance the measurement accuracy of radiation thermometers to meet the needs of the aluminum industry.

Experimental Investigation of Emissivity of Aluminum Alloys and Temperature Determination Using Multispectral Radiation Thermometry (MRT) Algorithms

Experiments were performed to measure the emissivity spectra for aluminum (Al) surfaces that are subject to variations in alloy, temperature, heating time, and surface finish. The linear emissivity model (LEM) and log-linear emissivity (LLE) model were tested against thermocouple measurements to explore the accuracy of these models at inferring surface temperature. The data show emissivity decreases with increasing wavelength for λ<3.5 µm, but the trend is reversed at higher wavelengths. Except for AL 1100 (commercially pure Al), the emissivity of all alloys tested decreased 600–700 K and increased 700–800 K. The increased emissivity at 800 K was closely associated with a discoloration of surfaces from light gray into black. Surface roughness produced a 2- to 3-fold increase in emissivity compared with polished surfaces. Overall, the third-order LEM model showed the best overall accuracy for different alloys, temperatures, and surface roughness. Nonetheless, this study points to a need for more accurate models that could handle the diverse operating environment of Al processing plants.