Total Hemispherical Emissivity Research Articles

Software solution for control and data acquisition in the pulse calorimetry method

This work presents a software solution for adjusting, controlling, displaying, and acquiring parameters and data in the pulse calorimetry experimental technique for specific heat capacity, electrical resistivity, total hemispherical emissivity, and normal spectral emissivity measurements. The software has been developed under the LabVIEW platform, V.7.11, and an example of its application with measurement results is presented in a separate section. The total expanded uncertainty of obtained results for the specific heat capacity and electrical resistivity of palladium was 5% and 1 - 2%, respectively.

Spectral emissivity measurements of liquid refractory metals by spectrometers combined with an electrostatic levitator

A spectral emissivity measurement system combined with an electrostatic levitator was developed for high-temperature melts. The radiation intensity from a high-temperature sample was measured with a multichannel photospectrometer (MCPD) over the 700–1000 nm spectral range, while a Fourier transform infrared spectrometer (FTIR) measured the radiation over the 1.1–6 µm interval. These spectrometers were calibrated with a blackbody radiation furnace, and the spectral hemispherical emissivity was calculated. The system's capability was evaluated with molten zirconium samples. The spectral hemispherical emissivity of molten zirconium showed a negative wavelength dependence and an almost constant variation over the 1850–2210 K temperature range. The total hemispherical emissivity of zirconium calculated by integrating the spectral hemispherical emissivity was found to be around 0.32, which showed good agreement with the literature values. The constant pressure heat capacity of molten zirconium at melting temperature was calculated to be 40.9 J mol−1 K−1.

Measurement of Total Hemispherical Emissivity Using Vacuum Guarded Hot Plate

In this paper, a new measurement method for the total hemispherical emissivity of surfaces is introduced. A vacuum guarded hot plate apparatus is developed and it is employed for the emissivity measurement. Al, Ag, Au, and Ni coatings and STS 304 plate are used as the specimen. Measured total hemispherical emissivities are 0.05–0.24 with 8.1% uncertainty at most. This proves the reliability of current method even for highly reflective surfaces.

Transient Calorimetric Measurement Method for Total Hemispherical Emissivity

The transient calorimetric technique was used to measure the total hemispherical emissivity of conductive materials. The emissivity was measured in a small central region of a thin strip heated electrically in a vacuum chamber. The axial heat transfer along the sample and the heat losses from the wires were considered in the transient heat transfer calculations. An appropriate time interval for the hot sample cooling rate is needed to improve the emissivity solution accuracy. Two ways were used to analyze the data, based on the known specific heat and the assumed functions of the emissivity and the specific heat. Comparisons with steady-state data showed that their results are very similar with a maximum difference of only 13% (944 K). Therefore, the transient method based on the function assumption is a good choice for measurements when there are inaccurate or insufficient specific heat data at the desired temperatures. Since ferromagnetic materials have Curie points at higher temperatures, this study also investigated the applicability of the transient calorimetric technique for high temperature emissivity measurements. For higher temperatures above the Curie point, the steady-state method is more accurate than the transient method. These analyses provide a comprehensive understanding of the transient method for measuring the total hemispherical emissivity.

Hemispherical Total Emissivity of Potential Structural Materials for Very High Temperature Reactor Systems: Haynes 230

The hemispherical total emissivity of Haynes 230, which is regarded as a leading candidate material for heat exchangers in very high temperature reactor (VHTR) systems, was measured with various surface conditions using the ASTM C 835-06 protocol. The emissivity increased from 0.18 at 627 K to 0.235 at 1356 K for Haynes 230 as received sample. The emissivity increased significantly when its surface roughness was increased, or it was oxidized in air, or it was coated with graphite dust, as compared to the as-received material. Higher emissivity has a positive impact on high-temperature reactor operations, particularly for safety, since higher emissivity implies faster decay heat removal in postaccident VHTR environments.

Measurement of Total Hemispherical Emittance of Spacecraft Thermal Control Coatings at Low Temperatures

Spacecraft payloads that operate at cryogenic temperatures often use passive thermal control systems that incorporate a variety of thermal control coatings to reject heat to space. The thermal performance of these systems depends significantly on the emittance of their thermal control coatings. This paper presents total hemispherical emittance measurements carried out at the ISRO Satellite Centre (ISAC) for thermal control coatings: Aeroglaze Z307 absorptive conductive polyurethane black coating (from 8 0K to 150 K) and PUC conductive black polyurethane coating (from 70 K to 200 K). The experiments were conducted in vacuum using a steady-state calorimetric method. This paper highlights the importance of obtaining a steady state for accurately estimating the emittance at cryogenic temperatures and the criterion for achieving this. The results indicate that the emittance in the cryogenic temperature range increases with temperature for the coatings considered in this paper. Determination of the uncertainty in the experimental results is also presented. Further, the sensitivity of emittance estimates to chamber wall emittance is discussed.

Thermophysical properties of solid phase Ti-6Al-4V alloy over a wide temperature range

Abstract This paper presents experimental results on thermophysical properties of the most commonly used titanium alloy, Ti-6Al-4V, in its solid phase. The subsecond pulse calorimetry has been applied for measuring heat capacity, specific electrical resistivity, and hemispherical total and normal spectral emissivity, and the laser flash method for measuring thermal diffusivity. Specific heat capacity and specific electrical resistivity were measured from 250 to 1700 K, thermal diffusivity from 190 to 1530 K, hemispherical total emissivity from 780 to 1670 K, and normal spectral emissivity at 900 nm from 1300 to 1730 K. Thermal conductivity and Lorentz function were computed from experimental data in the range from about 190 to 1530 K. For necessary corrections literature data on thermal linear expansion were used. The results obtained are compared with available literature values.

Thermophysical and Radiative Properties of Conductive Biopolymer Composite

Usual plate solar collectors, based on a metal absorber (Cu, Al) selectively coated are technologically very sophisticated, expensive to produce and they are great consumer of fossil raw material. Polymeric materials are considered as a promising alternative for many interesting properties; easy moldability, corrosion resistance, they also offer a significant cost-reduction for solar thermal collectors, and a mass production may thus benefit to a broader utilization of solar energy. Most drawbacks of polymers are their low thermal properties; essentially thermal conductivity coefficient may strongly affect the solar absorber efficiency and deteriorate the collector performance. Polymers used in solar collectors are mainly petroleum-derivative product and mass use of them is not a response to environmental concern. That is why the laboratory chose to explore the potentialities of bio-polymers for the production of absorbers. This group of material presents the same properties as ordinary polymers. It is on the other hand possible to modify the thermal properties of the basic matrix by the addition of loads, such as carbon black, graphite or carbon nanotubes. The thermal performance of a solar collector is closely related to the thermal properties of the absorber. Within this framework, many measurements are necessary, more particularly the conductivity, but also emissivity and absorptivity to solar radiation. The aim of this paper is to study the thermal properties of the PLA bio-polymer charged of exfoliated graphite and/or CNT. Thereafter, the total hemispherical absorptivity, an estimation of the total hemispherical emissivity and the thermal conductivity coefficient were measured for different load rates, we will conclude on the interest and the potentialities of tested materials.

Hemispherical total emissivity of Hastelloy N with different surface conditions

The hemispherical total emissivity of Hastelloy N (a candidate structural material for Next Generation Nuclear Plants (NGNPs), particularly for the molten fluoride cooled reactors) was measured using an experimental set-up that was constructed in accordance with the standard ASTM C835-06. The material surface conditions included: (i) ‘as received’ (original) sample from the supplier; (ii) samples with increased surface roughness through sand blasting; (iii) oxidized surface, and (iv) samples coated with graphite powder. The emissivity of the as received samples varied from around 0.22 to 0.28 in the temperature range of 473K to 1498K. The emissivity increased when the roughness of the surface increased compared to an as received sample. When Hastelloy N was oxidized in air at 1153K or coated with graphite powder, its emissivity increased substantially. The sample sand blasted with 60 grit beads and sprinkled with graphite powder showed an increase of emissivity from 0.2 to 0.60 at 473K and from 0.25 to 0.67 at 1473K. The oxidized surface showed a similar behavior: an increase in emissivity compared to an unoxidized sample. This increase in emissivity has strong favorable safety implications in terms of decay heat removal in post-accident environments. The data were compared with another Hastelloy family member, Hastelloy X.

Experimental research on the influence of surface conditions on the total hemispherical emissivity of iron-based alloys

The influences of surface conditions, including surface roughness and oxidation state, on the total hemispherical emissivity were experimentally investigated using an improved steady-state calorimetric technique for iron-based alloy samples. The investigated samples were prepared using rough polishing grits and high-temperature oxidization. The surface micrograph, roughness and composition of the oxide layer were analyzed using scanning electron microscope (SEM) and auger electron spectroscope (AES) to quantitatively describe the surface characteristics. Results of two iron-based alloy samples with different surface roughnesses showed that the measured total hemispherical emissivity increased with the roughness. However, the effect of surface roughness on total hemispherical emissivity gradually became smaller at high temperatures above 1200K for the investigated roughness conditions of iron-based alloys. Another six iron-based alloy samples were pre-oxidized in a high-temperature air muffle furnace with three oxidized at 400°C and the other three oxidized at 600°C for various times. The results showed that the emissivities of samples oxidized at 400°C for various times were similar to each other and the un-oxidized substrate, while the emissivities of samples oxidized at 600°C increased with oxidation time. This distinction arose from the different oxide layer thickness as the oxidation time increased at 600°C, while the thickness was almost constant at 400°C. The characteristics of the oxide layer grown in different oxidation temperatures were different as well. For the same oxidation time, the emissivities of samples oxidized at the higher oxidation temperature were larger than the values of samples oxidized at lower oxidation temperature. At high temperature (600°C) oxidation process of iron-based alloys, the oxide layer might be compact and its density was large which would strength the capacity of radiation heat transfer of iron-based alloy, comparing with the oxide layer grown in low temperature (400°C) oxidation process at the same oxidation time.

A steady-state measurement system for total hemispherical emissivity

A steady-state calorimetric technique was developed for measuring the total hemispherical emissivity of a conductive material. The system uses a thin strip of the conductive sample electrically heated by alternating current to high temperatures in a vacuum chamber. The emissivity was measured in a central region of the sample with an approximately uniform temperature distribution. Considering the influences of the gray body assumption, wire heat losses, effects of residual gas and conductive heat loss from the region to the rest of the strip, the emissivity was accurately determined by solving the inverse one-dimension steady-state heat transfer problem. The emissivities of various metal samples (nickel and 45# steel) were measured to verify the system accuracy. And the results were then analyzed to estimate the relative errors of emissivity arising from the gray body assumption, wire heat losses, effects of residual gas, non-uniform temperature distribution and the measurement uncertainty of emissivity. In the temperature range from 700 to 1300 K, the accuracy is acceptable for practical applications within the total measurement uncertainties of 1.1%. To increase the system applicability, some issues related to sample specifications, heating power control and temperature uniformity of sample test section were discussed. Thus, this system can provide accurate measurements of the total hemispherical emissivity of conductive samples at high temperatures.

Multiple pulse-heating experiments with different current to determine total emissivity, heat capacity, and electrical resistivity of electrically conductive materials at high temperatures

A modified pulse-heating method is proposed to improve the accuracy of measurement of the hemispherical total emissivity, specific heat capacity, and electrical resistivity of electrically conductive materials at high temperatures. The proposed method is based on the analysis of a series of rapid resistive self-heating experiments on a sample heated at different temperature rates. The method is used to measure the three properties of the IG-110 grade of isotropic graphite at temperatures from 850 to 1800 K. The problem of the extrinsic heating-rate effect, which reduces the accuracy of the measurements, is successfully mitigated by compensating for the generally neglected experimental error associated with the electrical measurands (current and voltage). The results obtained by the proposed method can be validated by the linearity of measured quantities used in the property determinations. The results are in reasonably good agreement with previously published data, which demonstrate the suitability of the proposed method, in particular, to the resistivity and total emissivity measurements. An interesting result is the existence of a minimum in the emissivity of the isotropic graphite at around 1120 K, consistent with the electrical resistivity results.

Numerical investigation and experimental verification of the Joule heating effect of polyacrylonitrile-based carbon fiber tows under high vacuum conditions

Joule’s first-law dictates that when electric current passes through a conductor heat is generated. The scope of this work is to investigate the Joule heating effect of two types of carbon fiber tows in dry form. The tows are used as primary heating elements on various preform structures that require accurate and uniform temperature control, using a DC power supply. The investigated temperatures, ranged from 300 K up to 650 K. A finite difference scheme was developed, in order to predict and quantify the transient phenomenon and is verified against experimental results. In order to exploit in-depth the phenomenon and achieve very good agreement between numerical and experimental results, temperature dependence of thermal and electrical properties (specific heat capacity, total hemispherical emissivity, and volume resistivity) was examined under controlled vacuum environment, ensuring no fiber oxidation.

Thermal radiative properties of carbon materials under high temperature and vacuum ultra-violet (VUV) radiation for the heat shield of the Solar Probe Plus mission

The Solar Probe Plus (SP+) mission will approach the Sun as close as 9.5 solar radii in order to understand the origin of the solar corona heating and the acceleration of the solar wind. Submitted to such extreme environmental conditions, a thermal protection system is considered to protect the payload of the SP+ spacecraft. Carbon-based materials are good candidate to fulfill this role and critical point remains the equilibrium temperature reached at perihelion by the heat shield. In this paper, experimental results obtained for the solar absorptivity α, the total hemispherical emissivity ɛ and its ratio α/ɛ, conditioning the equilibrium temperature of the thermal protection system, are presented for different kinds of carbon materials heated at high temperatures with or without vacuum-UV (100<λ<200nm) radiation. The synergistic effect of ion bombardment is also presented for one kind of material.

A new apparatus for measuring total hemispherical emittance of surfaces at room temperature

AbstractRadiation heat transfer is an important mode of heat transfer even in our life space at room temperature. Such radiation heat transfer is evaluated mostly by calculating the radiation energy exchange among surfaces in an enclosure. For this evaluation, knowledge of the hemispherical emittances of the constituent life surfaces is needed. However, there has not been a suitable technique for measuring the emittance of each surface in the life space at room temperature. In the present work we develop a new apparatus for measuring total hemispherical emittances of real surfaces at room temperature. The apparatus consists of a total radiation flux meter and a Peltier element cooler to cool the flux meter surface from the back, and measure the net total radiation flux from the specimen surface to the surrounding surfaces to determine the total hemispherical emittance of the specimen surface. This apparatus is effective to measure total hemispherical emittances of surfaces at room temperature on site. We apply the developed apparatus to measure the total hemispherical emittances of various surfaces in the life space, such as metals, human skin, cloth, floor mat, wood, glass, brick tile, stone, cement, plant leaves, etc., and demonstrate the feasibility of the apparatus. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20349

A New Apparatus for Measuring Total Hemispherical Emittance of Surfaces at Room Temperature

A New Apparatus for Measuring Total Hemispherical Emittance of Surfaces at Room Temperature

Microstructure and variable emittance property of annealed La-Sr-Mn-O films

The La-Sr-Mn-O films were deposited on Si(100) substrates by DC magnetron sputtering and followed by air annealing at 973 K for 1 h. The microstructure and temperature dependence of total hemispherical emittance (ɛ H) of the annealed La-Sr-Mn-O films prepared at various processing parameters were investigated. The results indicated the films deposited at lower sputtering pressure and lower O 2/(O 2+Ar) volume proportion ( R O) were present in rhombohedral perovskite structure and the length of Mn-O bond was shorter. The metal-insulator transition temperature ( T MI) was higher. All of the annealed films showed the unique feature of variable emittance based on metal-insulator transition. The films showed low emittance at low temperature but high emittance at high temperature. Moreover, the ɛ H significantly changed in the vicinity of T MI. The variability of total hemispherical emittance (Δɛ) and the temperature range with obvious emittance change could be adjusted by changing the processing parameters. The Δɛ could be 0.45 and Δɛ/ɛ 355 (ɛ 355 is the ɛ H at 355K) exceeded 50% for the annealed La-Sr-Mn-O films. Therefore, the annealed La-Sr-Mn-O films showed much potential for thermal control applications as smart thermochromic variable emittance materials.

Total Hemispherical Emissivity of Potential Structural Materials for Very High Temperature Reactor Systems: Hastelloy X

An experimental system was constructed in accordance with the standard ASTM C835-06 to measure the total hemispherical emittance (emissivity) of structural materials of interest in very high temperature reactor (VHTR) systems. First, data were acquired for 304 stainless steel as well as for oxidized and unoxidized nickel, and good reproducibility and agreement with the literature was found. Emissivity of Hastelloy X was then measured under different conditions that included (a) “as received” (original sample) from the supplier, (b) with increased surface roughness, (c) oxidized, and (d) graphite coated. Measurements were made over a wide range of temperatures. Hastelloy X, as received from the supplier, was cleaned before additional roughening of the surface and coating with graphite. The emissivity of the original samples (cleaned after received) varied from ~0.18 to 0.28 in the temperature range of 473 to 1498 K. The apparent emissivity increased only slightly as the roughness of the surface increased (without corrections for the increased surface area due to the increased surface roughness). When Hastelloy X was coated with graphite or was oxidized, however, its emissivity was observed to increase substantially. With a deposited graphite layer on the Hastelloy, increases from 0.2 to 0.53 at 473 K and from 0.25 to 0.6 at 1473 K were observed—a finding that has strong favorable safety implications in terms of decay heat removal in postaccident VHTR environments. Initial oxidation of Hastelloy X surfaces was observed to notably increase the emissivity of the Hastelloy X but was not observed to progress significantly beyond the initial oxidation even with more prolonged exposure. Since there is likely to be initial surface oxidation of any Hastelloy X used in the construction of VHTRs, this represents an essentially neutral finding in terms of the safety implications in postaccident VHTR environments.

Emissivity of microstructured silicon

Infrared transmittance and hemispherical-directional reflectance data from 2.5 to 25 microm on microstructured silicon surfaces have been measured, and spectral emissivity has been calculated for this wavelength range. Hemispherical-total emissivity is calculated for the samples and found to be 0.84 before a measurement-induced annealing and 0.65 after the measurement for the sulfur-doped sample. Secondary samples lack a measurement-induced anneal, and reasons for this discrepancy are presented. Emissivity numbers are plotted and compared with a silicon substrate, and Aeroglaze Z306 black paint. Use of microstructured silicon as a blackbody or microbolometer surface is modeled and presented, respectively.

Measurements of Heat Capacity of Pure Titanium and Zirconium by Electromagnetic Levitation

The heat capacity of titanium and zirconium was measured in the temperature range from approximately 1,300 K to 1,800 K . The measurement technique is based on the modulated power method originally proposed by Fecht and Johnson. The heat capacity of a sample can be derived from the sample’s temperature response to the modulated electromagnetic heating power if the sample’s total hemispherical emissivity is known. The experiments were performed using an electromagnetic levitator. The solid titanium and zirconium samples were suspended in the center of an induction coil with a very small diameter (0.15 mm) Pt (87 %)-Rh (13 %) wire. The heat capacity measurements of titanium and zirconium are presented, and the uncertainty is estimated to be ±3 %.