Conventional Simultaneous Measurement of Specific Heat Capacity and Thermal Conductivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal conductivity and the specific heat capacity of an isolated solid specimen simultaneously. The system, in which a disk-shaped specimen and a flat heater are mounted in a vacuum chamber with the specimen heated on one face by irradiation, is presented. A theoretical formulation of the simultaneous measurement at quasi-steady state is described in detail. Noncontact temperature measurement of both specimen surfaces has been performed using pyrometers and a thermocouple set in the gap between the heater and the specimen. Pyroceram 9609 specimens, whose surfaces were blackened with colloidal graphite, were used in the measurement. The largest error involved in the noncontact temperature measurement is ±2°C in the range from 450 to 650°C. The resultant values of the specific heat capacity and the thermal conductivity deviate by about 10% from the recommended values for the Pyroceram specimen.     

液氮温区高温超导滤波器的漏热分析

为在液氮温区工作的高温超导滤波器的制冷机冷源选型,基于Sage 10软件对超导滤波器件的冷却装置进行了漏热分析,仿真计算了铜线和同轴线的几何参数对传导漏热量的影响,以及真空罩、冷盘的尺寸和发射率对辐射漏热量的影响,并综合上述分析计算了超导滤波系统的总漏热量。在仿真计算中发现,信号线的导热和真空罩中冷盘的辐射漏热在系统总漏热量中起主导作用。仿真计算结果表明,通过增大信号线长度、减小信号线直径的方式可将信号线漏热量降至0.72 W,约为初始导热量的1/3。与此同时,冷盘表面采用抛光镀金的方式减小表面发射率,使辐射漏热量降至原来的2/3。根据模拟计算的最优结果,选择制冷量为3 W@77 K的制冷机作为高温超导滤波器的冷源。     

An Apparatus for Specific Heat Capacity Measurement by Thermal Radiation Calorimetry

Specific heat capacity measurements of disk-shaped specimens have been performed with an apparatus based on thermal radiation calorimetry. The specimen surfaces were irradiated by two fiat heaters in a vacuum chamber so that homogeneous temperature distribution within an insulating specimen was achieved. Homogeneity was confirmed by computer simulation based on the control-volume method. The values of specific heat capacity were obtained by measurement of the specimen temperature, the time rate change of the specimen temperature, and the radiant power from the heater for heating and cooling modes. The specific heat capacities of Ni metal, A1₂O₃ ceramic, MgO ceramic, and A1N ceramic were measured in the temperature range from 220 to 500°C to confirm the validity of this calorimeter. The relative error involved in the measured values was estimated to be ±3%.     

Numerical Analysis of a Radiant Heat Flux Calibration System

Heat flux gauges are one of the devices that are used to determine the heat loads to which high-speed aerospace structures are subjected during flight. Prior to installation, these gauges are calibrated. The calibration system must be well understood if the heat flux gauges are to provide useful data during flight tests. A pseudo three-dimensional model of the radiant heat flux gauge calibration system was developed. The radiant heat flux gauge calibration system consists of a graphite plate heater and a circular foil heat flux gauge. The numerical model simulates the combined convection, radiation, and mass loss by chemical reaction on the graphite plate surface. It can be used to identify errors due to heater element erosion, and the deviations in the predicted heat fluxes due to uncertainties in various physical parameters of the system. A fourth-order finite difference scheme is used to solve the steady-state governing equations and to determine the temperature distribution in the gauge and the graphite plate, the incident heat flux on the gauge face, and the flat plate erosion. Initial gauge heat flux predictions from the model are found to be within ±5% of experimental results.     

An apparatus for noncontact measurement of thermal conductivity by thermal radiation heating

Thermal radiation calorimetry was applied to measure the thermal conductivity of insulating solid specimens. We consider the system in which a disk-shaped specimen and a flat heater are mounted in a vacuum chamber with the specimen heated on one face by irradiation. A temperature difference between two faces was observed at elevated temperatures under steady-state conditions. An apparatus was developed using a thin graphite sheet as the heater element. Disk-shaped Pyrex glass and Pyroceram specimens, whose surfaces were blackened with colloidal graphite, were used in the measurements. Noncontact temperature measurement was performed using pyrometers and a thermocouple set in the gap between the heater and the specimen. Deviations of the estimated thermal conductivities from the recommended values were about 5% in the temperature range 250 to 800°C. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

System for Measuring the Spectral Distribution of Normal Emissivity of Metals with Direct Current Heating

A system for measuring time variations of the normal spectral emissivity at wavelengths ranging from 0.55 to 5.3 m was developed and applied to metal specimens in vacuum and oxidizing environments in the temperature range from 780 to 1200° C. The specimen was heated to high temperatures by passing a direct current in a vacuum chamber, and the surface oxidation was controlled by a low-pressure oxidizing gas. The specimen temperature was measured by a single-band (0.9-m) radiation thermometer viewing at a cavity formed in the specimen from the rear side. The front surface of the specimen was observed by a multiband (112-wavelength) radiation thermometer to measure the normal spectral emissivity. The effective normal spectral emissivity of the specimen cavity was evaluated to be 0.94±0.05 at a wavelength of 0.9 m in comparison with a metal tube having a small blackbody hole on the rear. The measurement uncertainty of the normal spectral emissivitiy by the system was estimated to be 5 to 10% of the emissivity value in most of the interesting ranges of emissivities, temperatures, and wavelengths.     

Effect on the Spectral Emissivity of SPHC Steel by Surface Oxidization

Surface oxidation can dramatically change the spectral emissivity of a metallic surface that is maintained at an elevated temperature over a period of time. Many kinds of metallic specimens, in particular for steel in use in industrial conditions, are usually heated in an oxidizing environment at high temperatures for a relatively long time. For this reason, in this paper, the SPHC steel was chosen as the specimen to investigate the effect on the spectral emissivity by surface oxidization over the temperature range from 800 K to 1000 K. The experimental setup for the spectral emissivity measurement operates at a wavelength of $1.5\,\upmu $ 1.5 μ m with a bandwidth of 20 nm. The temperature of the sample surface is determined by averaging two R-type platinum–rhodium thermocouples. The radiant energy from the surface of the SPHC specimen is received by an infrared detector. On the one hand, it is found in a detailed study that the spectral emissivity varies as the temperature for a given heating time; on the other hand, it was investigated that the spectral emissivity varies as the heating time at a given temperature. We find that the relationship between the spectral emissivity and the temperature can still be fitted to an exponential function by the least-squares method. The peculiar behavior of the spectral emissivity is discussed at an early stage when the oxidation film on the surface of the SPHC sample is grown. The conclusion is drawn that the contribution to the spectral emissivity by the surface oxidation mainly comes from the oxidation growth during the first 3 h.     

Study on Relationships Between the Spectral Emissivity of DC01 Steel and Temperature in an Oxidizing Environment

Steel is exposed in air at elevated temperatures in production processes. Surface oxidization is propagating through the whole period of manufacture. To measure the temperature of a steel surface in production as accurately as possible, the relationship of the spectral emissivity versus temperature must be determined in the oxidizing environment. For this reason, in this paper, a study of the effect of surface oxidization on the spectral emissivity over the temperature range from 800 K to 1120 K by taking DC01 steel as an example is described. In this experiment, the surface temperature is measured by averaging two R-type platinum–rhodium thermocouples. The radiant energy coming from the steel surface is received by an InGaAs photodiode detector. The experimental setup works at a wavelength of 1.5  \(\upmu \mathrm{m}\) with a bandwidth of 20 nm. Two kinds of relationships between the spectral emissivity and the temperature are studied in detail in the oxidizing environment. One is that the spectral emissivity varies with the heating-duration time at a given temperature. The other is that the spectral emissivity varies with the temperature at a given heating-duration time. Resonant peaks of the spectral emissivity are observed during the whole heating period. The behavior of the spectral emissivity is discussed when the oxidization film on the specimen surface is grown, in particular at the early stages of the heating duration. Analytical formulas of the spectral emissivity versus temperature are derived for different heating-duration times: (30, 60, 90, 120, 180, and 240) min. The conclusion is gained that the coefficients of the analytical expressions between the spectral emissivity and the temperature are different for the measurements obtained at the different heating-duration times, although the same logarithm-functional form is suitable for fitting all the experimental results obtained in the present work.     

Radiative properties characterization of ZrB2–SiC-based ultrahigh temperature ceramic at high temperature

Experimental investigations of radiative property on pre-oxidized ZrB₂–SiC–15 vol.%–C ultrahigh temperature ceramic (ZSC UHTC) at high temperature range of 1100–1800 °C were performed. By Fourier transform infrared radiant (FT-IR) spectrometer, spectral emissivity was measured in the wavelength region between 3 and 18 μm. Total normal emissivity was calculated using spectral emissivity data via theoretical formula. It has been found that high emissivity for all the testing specimens was presented, and the total normal emissivity is between 0.65 and 0.92 with temperature range from 1100 to 1800 °C. Moreover, the total normal emissivity of pre-oxidized ZSC ceramic decreased non-monotonously as the temperature increased. The total normal emissivity decreased as the testing temperature increased from 1100 to 1800 °C, whereas the total normal emissivity at the testing temperature of 1600 °C was higher than that of 1400 and 1800 °C. Macroscopical surface morphology and microstructure were carried out before and after the testing.     

A microsecond-resolution transient technique for measuring the heat of fusion of metals: Niobium

A microsecond-resolution pulse-heating technique is described for the measurement of the heat of fusion of refractory metals. The method is based on rapid resistive self-heating of the specimen by a high-current pulse from a capacitor discharge system and measurement of the current through the specimen, the voltage across the specimen, and the radiance temperature of the specimen as a function of time. Melting of the specimen is manifested by a plateau in the temperature versus time function. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements gave a value of 31.1 kj · mol^–1 for the heat of fusion of niobium, with an estimated maximum uncertainty of ±5%. Electrical resistivity of solid and liquid niobium at its melting temperature was also measured.     

Apparatus for Measuring Spectral Emissivity of Solid Materials at Elevated Temperatures

Spectral emissivity measurements at high temperature are of great importance for both scientific research and industrial applications. A method to perform spectral emissivity measurements is presented based on two sample heating methods, the flat plate and tubular furnace. An apparatus is developed to measure the normal spectral emissivity of solid material at elevated temperatures from 1073 K to 1873 K and wavelengths from \(2\,\upmu \hbox {m}\) to \(25\,\upmu \hbox {m}\). Sample heating is accomplished by a torch flame or a high temperature furnace. Two different variable temperature blackbody sources are used as standard references and the radiance is measured by a FTIR spectrometer. Following calibration of the spectral response and background radiance of the spectrometer, the effect of the blackbody temperature interval on calibration results is discussed. Measurements are performed of the normal spectral emissivity of SiC and graphite over the prescribed temperature and wavelength range. The emissivity of SiC at high temperatures is compared with the emissivity at room temperature, and the influence of an oxide layer formed at the surface of SiC on the emissivity is studied. The effect of temperature on the emissivity of graphite is also investigated. Furthermore, a thorough analysis of the uncertainty components of the emissivity measurement is performed.     

Use of finite-penetration-depth method to calculate the heating of a plane plate under the action of a radiant heat flux

Using the finite-penetration-depth method, a solution is obtained to the problem of plate heating a radiant flux. The results are compared with a numerical solution.Notation t temperature - T absolute temperature - ,a thermal conductivity and diffusivity of plate material - _re reduced emissivity - x coordinate - q specific heat flux - time Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 1, pp. 138–142, July, 1980.     

Energy transfer by radiation in a lumpy layer

Energy transfer by radiation in a layer of lumpy material is examined, A formula is derived for the effective coefficient of heat conduction of the layer for radiation heat transfer.Notation I, K opposing radiant fluxes, W/m² - a _c, r_c, d_c layer absorptivity, reflectivity and transmissivity - a _p.th, d₀ transmissivity for radiant energy passing directly through the layer and after reflection from the layer material - _c emissivity - r material reflectivity - L layer thickness, m - h distance between separate interlayers, m - T temperature, °K - q_r resultant radiant flux in the layer, W/m2 - _eff effective coefficient of radiation heat conduction of the layer, W/m · deg - angular coefficient from one base of the hole to the other - f₀ magnitude of the hole area in the plate, m²/m² - p porosity - E_{incident k} magnitude of incident radiant fluxes on the plate k Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 20, No. 5, pp. 796–801, May, 1971.     

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.     

A transient (subsecond) technique for measuring heat of fusion of metals

A transient technique is described for measuring the heats of fusion of metals with melting temperatures above 1500 K. The specimen configuration consists of a strip of the metal under study sandwiched between two strips of another metal with a higher melting temperature. The basic method consists of rapidly heating the composite specimen by passing a subsecond-duration electrical current pulse through it and simultaneously measuring the radiance temperature of the containment metal surface, as well as the current through and voltage drop across the specimen. The melting of the metal under study is manifested by a plateau in the temperature versus time function for the containing metal surface. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements on several tantalum-niobium-tantalum specimens yield a value of 31.5 kJ · mor^–1 for the heat of fusion of niobium, with an estimated maximum inaccuracy of ± 5%.     

Measurement of the heat of fusion of tantalum by a microsecond-resolution transient technique

The heat of fusion of tantalum was measured using a microsecond-resolution pulse-heating technique. The technique is based on rapid (about 100-s) resistive self-heating of a specimen by a high-current pulse from a capacitor discharge system and measuring the current through the specimen, voltage across the specimen, and radiance temperature of the specimen as functions of time. Melting of a specimen is manifested by a plateau in the radiance temperature versus time function. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements gave a value of 34.8 kJ · mot^– for the heat of fusion of tantalum, with a total uncertainty of ±6%. Electrical resistivity of solid and liquid tantalum at its melting temperature was also measured.     

Simultaneous Measurement of Specific Heat Capacity, Thermal Conductivity, and Thermal Diffusivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal diffusivity of a solid specimen, along with simultaneous measurements of specific heat capacity and thermal conductivity. In this calorimeter, a disk-shaped solid specimen whose surfaces are blackened is heated and cooled slowly on one face by irradiation in a vacuum chamber. A quasi-steady-state approximation in which a linear temperature gradient within the specimen was assumed is considered in the analysis. The validity of this approximation was confirmed by the results of computer simulation based on the control-volume method. Measurements of Pyroceram 9606 and Pyrex 7740 by use of thermocouples in the temperature range between 250 and 400°C gave values consistent with those obtained by previous authors, within experimental error, for all three thermophysical properties.     

The effects of gas pressure in plasma surface alloying process on testing accuracy of the photoelectric pyrometer

In order to control the specimen temperature precisely in a plasma surface alloying process, the measurements obtained using a photoelectric pyrometer were compared with a thermocouple system. The effect of pressure on the accuracy of the photoelectric pyrometer is discussed. The results indicate that at a specific temperature, as the working pressure increases, a larger emission coefficient ε should be used to keep the pyrometer values consistent with those of the thermocouple, which is considered to give the real temperature. The value of ε increases by about 0.1 as the working pressure increases by 5-10 Pa. With this adjustment, the error of the pyrometer is within ±3°C.     

多光谱测温技术研究

辐射温度是表征辐射热源的核心参数,要准确测试辐射温度首先须准确测试热辐射体的发射率。本文根据弹药爆炸时火焰辐射温度的特性和普朗克定律的适用性,采用多光谱的方法对火焰光谱发射率进行拟合运算和绝对辐射定标,研制了适合于弹药爆炸时火焰温度的多光谱测温设备,并进行了现场测试和分析,完成了火焰辐射温度的准确测试。     

Measurement of the heat of fusion of molybdenum by a microsecond-resolution transient technique

A microsecond-resolution pulse-heating technique was used for the measurement of heat of fusion of molybdenum. The method is based on rapid resistive self-heating of the specimen by a high-current pulse from a capacitor discharge system and measuring current through the specimen, voltage across the specimen, and radiance temperature of the specimen as functions of time. Melting of the specimen is manifested by a plateau in the temperature versus time function. The time integral of the power absorbed by the specimen during melting yields the heat of fusion. Measurements gave a value of 36.4 kJ · mol^–1 for the heat of fusion of molybdenum with an estimated maximum uncertainty of±6%.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards