Emissivity measurements on reflective insulation materials

The development and use of new thermal insulation products in many industrial sectors, ranging from building insulations to power generation or satellite applications, requires the accurate knowledge of the radiative properties of the investigated material, i. e. its emissivity. A major objective of the research project “Improvement of emissivity measurements on reflective insulation materials” within the framework of the European Metrology Programme for Innovation and Research was to improve and validate reference techniques for the measurement of the total hemispherical emissivity of low emissivity foils with an absolute measurement uncertainty below 0.03. The calibration and measurement procedures developed within this project shall lead to a significant benefit for industrial manufacturers of reflective foils as well as for the end-users of the industrial instruments used to characterize them.

Total hemispherical emissivity of reactor pressure vessel candidate materials : SS 316 L, SA 508, and A 387 Grade 91

Future nuclear Reactor Pressure Vessels (RPV) require high emissivity materials and alloy for efficient heat decay removal under normal and accidental events. Emissivity is the material property that determines the heat radiation rate from the material surface. The total hemispherical emissivity of candidate materials, 316L stainless steel, A508/A533B steel, and A387 Grade 91 steel were measured in the present work. Standard ASTM C835-06 was used to measure the total hemispherical emissivity of structural materials of RPV. The bell jar emissivity was measured using 304 stainless steel strips coated with black paint (Aremco HiE-Coat 840 M). Testing these strips verified that the internal bell jar emissivity is greater than 0.8 as required by the ASTM standard. Emissivity of 316L stainless steel was measured for the following conditions: (1) "as-received" from the supplier (un-oxidized) and; (2) oxidized in air at 573 K for up to 3000 h. The emissivity of as-received test strips varied from 0.24 at 434 K to 0.34 at 1026 K. The total hemispherical emissivity of 316 L stainless steel oxidized in air for 500 h or more ranged from 0.28 at 429 K to 0.38 at 1096 K. Increasing the sample oxidation time beyond 500 h at 573 K did not have any significant observable effect on the total hemispherical emissivities of any of the test strips studied. It is suspected that the emissivity ceased to increase during the additional oxidation time because of the protective chromium oxide layer formed on 316L stainless steel inhibiting further oxidation. The total hemispherical emissivity of A508/A533B steel was measured in the temperature range from 400 K to 900 K for the following surface conditions: (1) mirror polished finish; (2) mirror polished and oxidized in air at 573 K and 773 K for various durations; (3) after Electric Discharge Machining (EDM) cutting process; and (4) EDM and oxidized in air at 573 K and 773 K up to 1000 h. The emissivity of polished A508/A533B (un-oxidized) strips varied from 0.16 to 0.24 within the temperature range from 552 K to 1180 K. Emissivity values for polished samples oxidized in air for 10 h at 573 K were increased from 0.18 at 543 K to 0.28 at 815 K. While the emissivity of polished samples oxidized in air for 100 h at 573 K increased from 0.16 at 460 K to 0.33 at 815 K. The total hemispherical emissivity of EDM cut and oxidized in air at 573 K for 500 h has experienced an increase from 0.75 to 0.8 in the range of temperature from 418 K to 625 K. No further significant change in emissivity was observed following an increase in oxidation time from 500 h to 1000 h. Polished and oxidized strips at temperature 773 K for 10 h has increased from 0.74 at 464 K to 0.81 at 838 K. This strong effect of temperature has not been noticed for EDM-cut oxidized samples at 773 K for 500 h due to detach of the oxide layer from the A508/A533B substrate. The emissivity of A387 Grade 91 was measured for the following conditions: (1) EDM cut; (2) EDM cut and oxidized in air at 873 K and 1023 K for up to 5 h; (3) sandblasted with 320 grit size; and (4) sandblasted with 320 grit size and oxidized in air at 1023 K for 5 h. The emissivity of EDM cut A387 Grade 91 varied from 0.57 at 444 K to 0.62 at 1026 K, while the emissivity of A387 Grade 91 sandblasted with 320 grit increased from 0.49 at 464 K to 0.57 at 1048 K. The total hemispherical emissivity of A387 Grade 91 oxidized in air at 873K for 1 h has increased from 0.75 to 0.8 in the temperature range of 420 K to 831 K. The emissivity of A387 Grade 91 oxidized in air at 1023 K for 1h increased from 0.69 at 429 K to 0.8 at 889 K. Oxidation of A387 Grade 91 in air at 873 K and 1023 K did not result in significant difference in emissivity values among various test strips for oxidation durations of 1 h, 3 h, and 5 h. The emissivity of A387 Grade 91 sandblasted with 320 grit and oxidized at 1023 K for 5 h increased from 0.63 at 440 K to 0.74 at 944 K.