A steady-state measurement system for total hemispherical emissivity

Abstract

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.

Steady state calorimetric measurement of total hemispherical emittance of cylindrical absorber samples at operating temperature

“…Vacuum pressure is measured and kept at p < 10 -4 mbar during heat-up and measurement. Heat loss through gas gas P is calculated according to [5] based on the pressure measurements. The inner surface of the artificial sky of 50 cm in diameter and 70 cm in height is coated with Senotherm UHT 600 with an emittance of 0.8 ± 0.1at room temperature in order to achieve a high total hemispherical absorptance of surface 2 A .…”

“…The counter heaters are clamped on the lead wires of heaters and thermocouples and hence heat the lead wires to minimize heat loss via the lead wires. Heat loss through lead wires wires P is calculated according to [5]: T is the temperature of the homogenization tube (K) and ch T the temperature of the counter heater inner surface (K).…”

“…Steady state calorimetry is another method commonly used in measurements of the total hemispherical emittance of coatings on substrates, metals other solid materials [5], [6]. As the steady state calorimetry does not rely on external reference samples, is not sensitive to angular dependence of reflectance and the heating of the sample is an integral part of the measurement, a test bench for the application in solar thermal was realized at DLR`s QUARZ Center, Cologne, Germany.…”

Emissivity Measurements on Shape Memory Alloys

“…Pressure inside the vacuum bell jar is kept at 3 x 10 -3 Pa during the experiments. Under these conditions, assuming that the heat loss throughout the sample due to conduction and convection are insignificant when compared to the loss caused by radiation, the emissivity is [16]:…”

Correlations between High-Temperature Oxidation Kinetics and Thermal Radiation Characteristics of Micro-Structured Nickel Surfaces Oxidized at 1173 K

“…The surface thermal radiation properties of high-temperature metal oxides are also important in high temperature applications. For example, Teodorescu et al [9] studied the spectral-directional emittance of high purity (99.99%) nickel oxidized in air at temperatures between 673 to 873 K. Fu et al [10][11][12][13] measured the total hemispherical emissivity of oxidized nickel at high temperatures using a steady-state calorimetric technique.…”