Total Hemispherical Emissivity Research Articles

Mathematical Models for Pulse-Heating Experiments

Accurate measurements of thermophysical properties at high temperatures (above 1000 K) have been obtained with millisecond pulse-heating techniques using tubular specimens with a blackbody hole. In the recent trend toward applications, simpler specimens in the form of rods or strips have been used, with simultaneous measurement of the normal spectral emissivity using either laser polarimetry or integrating sphere reflectometry. In these experiments the estimation of the heat capacity and of the hemispherical total emissivity is based on various computational methods that were derived assuming that the temperature was uniform in the central part of the specimen (long thin-rod approximation). The validity of this approach when using specimens with large cross sections (rods, strips) and when measuring temperature on the specimen surface must be verified. The application of the long thin-rod approximation to pulse-heating experiments is reconsidered, and an analytical solution of the heat equation that takes into account the temperature dependence of thermophysical properties is presented. A numerical model that takes into account the temperature variations across the specimen has been developed. This model can be used in simulated experiments to assess the magnitude of specific phenomena due to the temperature gradient inside the specimen, in relation to the specimen geometry and to the specific thermophysical properties of different materials.

Measurements of IR Spectra and Thermophysical Properties of Tetragonal Zirconia by Thermal Radiation Calorimetry

Normal emission spectra from a clear surface of tetragonal zirconia ceramic stabilized by 5.3 wt% yttria are measured in the frequency range of 200–4500 cm-1. The values of the hemispherical total emissivity are derived from a spectral analysis based on virtual mode equations and Kramers-Krönig relations. With these values, thermal conductivity data are obtained from measurements of temperature gradients in the sample at temperatures between 400 K and 850 K. Heat capacity measurements are carried out by quasi-static thermal radiation calorimetry. The present results are consistent with data in the literature obtained by other methods. Based on the comparison of the spectral reflectivity with the emission spectrum, it is suggested that the effects of radiation heat transfer in the ceramic sample may be compensated for by the scattering process of grain boundaries.

Total hemispherical emissivity of oxidized Inconel 718 in the temperature range 300–1000°C

Total hemispherical emissivities were measured for Inconel 718 as a function of sample temperature. Measurements were made for both unoxidized and oxidized samples. The oxidation temperatures were 1000°C, 1100°C and 1142°C and the oxidation times were 15, 30 and 60 min, respectively. The oxidized samples showed a significant increase in emissivity over the unoxidized one which was in an as-received condition. No apparent pattern was observed in the change of emissivity as a function of oxidation time at a given oxidation temperature. In some cases, emissivity measurements made with increasing temperature were greater than those made with descending temperature. One possible explanation for this is the spalling of the oxide layer as the sample area contracted with descending sample temperature.

Optical Properties of Plasma-Sprayed ZrO2-Y2O3 at High Temperature for Solar Applications

Experimental determinations of the absorptivity and of the total hemispherical emissivity of plasma-sprayed ZrO2-7wt%Y2O3 layers were performed in the range of temperature 1000 K–2300 K. For this purpose, solar facilities of CNRS-IMP laboratory were used. The experimental set-up is described and the obtained results are presented and discussed. The spectral absorptivity at 0.65 μm ranges from 0.10 below 1000 K to 0.75 at 2300 K, and the total hemispherical emissivity increases from 0.25 at 1300 K up to 0.60 at 2200 K. [S0199-6231(00)00301-4]

Thermophysical Properties of Molten Germanium Measured by the High Temperature Electrostatic Levitator

Thermophysical properties of molten germanium have been measured using the high-temperature electrostatic levitator at the Jet Propulsion Laboratory. Measured properties include the density, the thermal expansivity, the hemispherical total emissivity, the constant-pressure specific heat capacity, the surface tension, and the electrical resistivity. The measured density can be expressed by ρ liq=5.67×103−0.542 (T−T m ) kg·m−3 from 1150 to 1400 K with T m=1211.3 K, the volume expansion coefficient by α=0.9656×10−4 K−1, and the hemispherical total emissivity at the melting temperature by e T, liq(T m)=0.17. Assuming constant e T, liq(T)=0.17 in the liquid range that has been investigated, the constant-pressure specific heat was evaluated as a function of temperature. The surface tension over the same temperature range can be expressed by σ(T)=583−0.08(T−T m) mN·m−1 and the temperature dependence of the electrical resistivity, when r liq(T m)=60μμΩ·cm is used as a reference point, can be expressed by r e, liq(T)=60+1.18×10−2(T−1211.3)μΩ·cm. The thermal conductivity, which was determined from the resistivity data using the Wiedemann–Franz–Lorenz law, is given by κ liq(T )=49.43+2.90×10−2(T−T m) W·m−1·K−1.

Thermophysical Properties of Bulk Metallic Glasses in the Stable and Undercooled Liquid – A Microgravity Investigation

AbstractThe thermophysical properties in the stable and undercooled liquid phase of a series of Zr- based metallic glass forming alloys have been investigated under the condition of reduced gravity on board spacelab. Properties measured included the specific heat capacity and the enthalpy of fusion, the electrical resistivity, the total hemispherical emissivity, and the thermal transport properties. Results for the bulk metallic glass forming alloys indicate a pronounced change in chemical short range order in the liquid phase as function of temperature.

102 非定常熱量法による板ガラスの全半球放射率測定(熱工学)

By an improved transient calorimetric technique, total hemispherical emittance ε_h of glass sheets of borosilicate and fused silica has been measured. The temperature inside the glass-sheet specimen by the technique is analyzed, and it is confirmed that the effect of the temperature distribution inside the specimen on the measured ε_h is very little. Local total emittance ε_h on the specimen surface is also analytically obtained, and the effect of ε_h on the measured ε_h is examined.

Non-contact measurements of thermophysical properties of titanium at high temperature

Four thermophysical properties of both solid and liquid titanium measured using the high-temperature electrostatic levitator at JPL are presented. These properties are density, thermal expansion coefficient, constant pressure heat capacity, and hemispherical total emissivity. For the first time, we report the density, the thermal expansion coefficient, and the ratio of the constant pressure heat capacity to the hemispherical total emissivity of undercooled titanium over a wide range of temperatures. Over the 1650 K to 2000 K temperature span, the liquid density can be expressed asρ (T)/(kg · m−3)= 4.208 · 103− 0.508 · (T−Tm)/K withTm= 1943 K, and the corresponding volume expansion coefficient asα= 1.169 · 10−4K−1. Similarly, over the 1540 K to 1940 K temperature range, the measured density of the solid can be expressed asρ (T)/(kg · m−3)= 4.321 · 103− 0.212 · (T−Tm)/K, giving a volume expansion coefficient α= 4.76 · 10−5K−1. The constant pressure heat capacity of the liquid phase could be estimated as Cp, m(T)/(J · K−1· mol−1)= 45.5 − 3.21 · 10−3· (T−Tm)/K if the hemispherical total emissivity of the liquid phase ϵTremains constant at 0.34 over the 1650 K to 2000 K temperature range. Over the 1540 K to 1940 K temperature span, the hemispherical total emissivity of the solid phase could be rendered asϵT (T) = 0.297 + 5.952 · 10−5· (T−Tm)/K. The enthalpy of fusion has also been calculated as 14.3 kJ · mol−1.

Evaluation of Hemispherical Total Emissivity for Thermal Radiation Calorimetry

An attempt to derive the hemispherical total emissivity from the normal emission spectrum is proposed for Vycor and fused silica glasses. The normal emission spectrum from a clear surface has been measured at steady state in the temperature range from 400 to 750 K. The sample is heated on one metal-backed face by thermal radiation from a heater. Temperatures inside the sample were monitored by thermocouples at two points near the surfaces. Evaluation of the hemispherical total emissivity from the normal emission spectrum is determined by means of Kramers–Kronig analysis and virtual mode equations. Assuming a linear temperature distribution within the sample, the thermal conductivities of silicate glasses were obtained at elevated temperatures. The results are comparable with those obtained by previous investigators. The effect of radiation heat transfer in a sample is also discussed.

A measurement technique for hemispherical total emissivity using feedback-controlled pulse current heating

A new measurement technique for hemispherical total emissivity of electrical conductors at high temperatures (above 1500 K) which uses feedback-controlled pulse heating has been developed. In this technique, a specimen is heatedrapidly in vacuum by resistive self-heating up to a preset high temperature in a short time (about 200 ms), and then is maintained at that temperature under brief steady-state conditions (about 500 ms). In order to achieve fast feedback temperature control, a computer-controlled system with a solid-state switch, which regulates the current through the specimen, is employed. The radiance temperature of the specimen is measured with a high-speed radiation thermometer. In order to determine the true temperature, the normal spectral emissivity of the specimen is measured with a laser ellipsometer. Hemispherical total emissivity is determined at the plateau of temperature by the use of a steady-state heat balance equation based on the Stefan-Boltzmann law. Preliminary experiments were conducted on strip-shaped specimens of tantalum and molybdenum in the temperature range approximately 1900 to 2800 K.

Thermophysical properties measurement of molten silicon by high-temperature electrostatic levitator: density, volume expansion, specific heat capacity, emissivity, surface tension and viscosity

Thermophysical properties of molten silicon measured by the high-temperature electrostatic levitator at JPL are presented. The properties include the density, the volume expansion, the constant pressure specific heat capacity, the hemispherical total emissivity, the surface tension and the viscosity. Over the temperature range investigated (1350–1850 K), the measured liquid density showed a quadratic nature expressed by ρ( T)=2.58−1.59×10 −4 ( T− T m)−1.15×10 −7 ( T− T m) 2 g/cm 3 with T m=1687 K. The volume expansion with respect to the melting point could be expressed by V( T)/ V m=1+6.18×10 −5 ( T− T m)+4.72×10 −8 ( T− T m) 2. The hemispherical total emissivity of molten silicon at the melting temperature was 0.18, and the constant pressure specific heat was determined over a 500 K around the melting point. The measured surface tension and the viscosity could be expressed respectively by σ( T)=765−0.016 ( T− T m) mN/m, and η( T)=0.75−1.22×10 −3 ( T− T m) mPa s.

Thermophysical properties of zirconium at high temperature

Six thermophysical properties of both solid and liquid zirconium measured using the high-temperature electrostatic levitator at the Jet Propulsion Laboratory are presented. These properties are density, thermal expansion coefficient, constant pressure heat capacity, hemispherical total emissivity, surface tension, and viscosity. For the first time, we report the densities and the thermal expansion coefficients of both the solid as well as liquid Zr over wide ranges of temperatures. Over the 1700–2300 K temperature span, the liquid density can be expressed as ρ1(T) = 6.24 × 103 – 0.29(T – Tm) kg/m3 with Tm = 2128 K, and the corresponding volume expansion coefficient as α1 = 4.6 × 10−5/K. Similarly, over the 1250–2100 K range, the measured density of the solid can be expressed as ρs(T) = 6.34 × 103 – 0.15(T – Tm), giving a volume expansion coefficient αs = 2.35 × 10−5/K. The constant pressure heat capacity of the liquid phase could be estimated as Cpl(T) = 39.72 – 7.42 × 10−3(T – Tm) J/(mol/K) if the hemispherical total emissivity of the liquid phase εT1 remains constant at 0.3 over the 1825–2200 K range. Over the 1400–2100 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as εTs(T) = 0.29 – 9.91 × 103 (T – Tm). The measured surface tension and the viscosity of the molten zirconium over the 1850–2200 K range can be expressed as ς(T) = 1.459 × 103 – 0.244 (T – Tm) mN/m and as η(T) = 4.83 – 5.31 × 10−3(T – Tm) mPa s, respectively.

Cryogenic testing of several parts of the meteosat second generation payload

The first Meteosat Second Generation (MSG) satellite is planned to be launched in 2000. As the first generation of the Meteosat satellites, it is mainly an Infra-Red and Visible Imager. The cooling of the experiment is provided by a passive radiator facing the deep cold space. In the frame of this program, CSL is in charge of the testing of four different parts of the payload, with different thermal environments and cooling systems. For SFIM-ODS REOSC, the I/R detectors are calibrated at 85 and 95 K, before and after cycling them from 333 down to 70 K. The visible detectors are calibrated at 248 K and cycled from 333 down to 228 K. They are cooled by conduction. For Fokker Space b.v., the Passive Cooler Assembly, including the sunshield and the two-stage radiator, is tested with regards to its mechanical stability and thermal performances. As in space, it is cooled radiatively by means of a helium plate. The main specifications for this plate are a total hemispherical emittance greater than 0.95 at 20 K, and a temperature lower than 25 K with a heat load of 200 W. Liquid nitrogen shrouds surround the set-up. For Matra Marconi Space, the Passive Cooler Assembly and the detectors are calibrated at their nominal temperature. The cooling power is provided by the same helium plate simulating the deep cold space. Again for Matra Marconi Space, the whole payload, called SEVIRI, is thermally and optically calibrated. Up to 19 temperature regulated channels are used, and the space is once again simulated by the helium plate. Three of these tests have already been performed successfully. From the facility point of view, we present here the relevant testing philosophies used to meet the specifications, the related problems, solutions and results.

Thermal properties of tantalum between 300 and 2300 K

A subsecond pulse heating method was applied to measure the specific heat capacity, electrical resistivity, total hemispherical emissivity, and normal spectral emissivity of 99.9% pure tantalum in the form of 2-mm-diameter wire. W/Re thermocouple thermometry was applied from 300 to 2300 K, with emissivity measurements above 1300 K involving pyrometric measurements. The maximum uncertainties in the specific heat capacity and electrical resistivity were less than 3 and 1 % respectively. The uncertainty of emissivity measurements was estimated as ±5%. The results are compared with literature values.

Thermophysical and Thermal Optical Properties of Vanadium by Millisecond Calorimetry Between 300 and 1900 K

A variant of millisecond-resolution pulse calorimetry in use at the Institute of Nuclear Sciences “Vinca” since 1983 involves measuring the specific heat and electrical resistivity of electrical conductors from room temperature to 2500 K and the hemispherical total emissivity and normal spectral emissivity from about 1300 K to the same maximum operating temperature. The method was applied successfully to different materials: pure metals, ferrous and nickel-base alloys, reactor materials, and refractory metals in thermal characterization of candidates for thermophysical property standard reference materials. This paper presents and discusses new data obtained in the study of thermophysical and thermal optical properties of vanadium.

Thermophysical properties by a pulse-heating reflectometric technique : Niobium, 1100 to 2700 k

Pulse-heating experiments were performed on niobium strips, taking the specimens from room temperature to the melting point is less than one second. The normal spectral emissivity of the strips was measured by integrating sphere reflectometry, and, simultaneously, experimental data (radiance temperature, current, voltage drop) for thermophysical properties were collected with sub-millisecond time resolution. The normal spectral emissivity results were used to compute the true temperature of the niobium strips; the heat capacity, electrical resistivity, and hemispherical total emissivity were evaluated in the temperature range 1100 to 2700 K. The results are compared with literature data obtained in pulse-heating experiments. It is concluded that combined measurements of normal spectral emissivity and of thermophysical properties on strip specimens provide results of the same quality as obtained using tubular specimens with a blackbody. The thermophysical property results on niobium also validate the normal spectral emissivity measurements by integrating sphere reflectometry.

Hemispherical total emissivity of niobium, molybdenum, and tungsten at high temperatures using a combined transient and brief steady-state technique

The hemispherical total emissivity of three refractory metals, niobium, molybdenum, and tungsten, was measured with a new method using a combined transient and brief steady-state technique. The technique is based on rapid resistive self-heating of a solid cylindrical specimen in vacuum up to a preset high temperature in a short time (about 200 ms) and then keeping the specimen at that temperature under steady-state conditions for a brief period (about 500ms) before switching off the current through the specimen. Hemispherical total emissivity is determined at the temperature plateau from the data on current through the specimen, the voltage drop across the middle portion of the specimen, and the specimen temperature using the steady-state heat balance equation based on the Stefan–Boltzmann law. Temperature of the specimen is determined from the measured surface radiance temperature and the normal spectral emissivity; the latter is obtained from laser polarimetric measurements. Experimental results on the hemispherical total emissivity of niobium (2000 to 2600 K), molybdenum (2000 to 2700 K), and tungsten (2000 to 3400 K) are reported.

Minimum temperature in electromagnetic levitation melting under terrestrial gravity

Recently, the existence of minima for the dissipated power in electromagnetic levitation melting has been reported. Such results will be used in order to estimate the temperature of the surface of the sample, when the only allowed mechanism for energy transport from the levitated drop is radiation cooling. It is also possible to estimate the difference in temperature, between the bulk and the surface of the specimen, when the principal mechanism for energy transport inside the liquid metal is thermal conductivity. The results suggest that even in experiments under terrestrial gravity, it should be possible to reach large undercooling for some metals of a high melting point, high total hemispherical emissivity, relatively low resistivity and density, without any need for a cooling gas atmosphere.

Une nouvelle méthode calorimétrique périodique de mesure de l'émissivité des matériaux opaques à température ambiante

This paper presents a new calorimetric periodic technique for measuring emissivity of opaque materials at room temperature, without surface temperature measurement. The sample is in a vacuum chamber, so the thermal losses are only radiative. The presented technique requires thermal modulation of one side of the sample (the front side). The measured signals are the sample's front side temperature and the infrared flux of the other side (back side). Experimental data and a heat conduction model are compared, yielding the Biot number which characterizes the thermal losses. Using the identified value of the Biot number, it is possible to access the total hemispherical emissivity. Measurements have been carried out on a PVC sample (5 mm thick) coated with black paint and on a PVC sample coated with aluminium paint. The results are concordant with the expected ones ; the repeatability error is about 3 %.

A Method for Calculating Thermal Radiation Properties of Multilayer Films from Optical Constants

A calculation method for the incident angle dependence of the solar absorptance αS and the temperature dependence of the total hemispherical emittance eH of multilayer films is proposed. The method is based on calculation of αS and eH from optical constants in the wavelength region from 0.25 to 100 μm for thin polymer films and deposited metal. In this paper we provide values of αS in the incident angle region from 0 to 90° and eH in the temperature range from 173.15 to 373.15 K for two-layer samples of aluminum-deposited polyimide film. The results obtained for αS and eH by the present method are compared with experimental results measured by both spectroscopic and calorimetric methods. The calculated results of αS and eH agree well with the experimental results.