Total hemispherical emissivity of oxidized Inconel 718 in the temperature range 300–1000°C☆
Introduction
Heat transfer in high vacuum systems occurs primarily by thermal radiation. In order to calculate such radiation transfers, it is necessary to know the radiation properties of the surfaces which are emitting and absorbing radiation. In general, these properties must be determined experimentally and the surface properties of the test materials must be as close as possible to those under actual operating conditions.
Unfortunately, such measurements are not always simple to accomplish, particularly when the surface properties change with time as is often the case. Thus, one must make radiation property measurements over a range of surface conditions to attempt to bracket these property values and to better understand their influence on the radiation heat transfer processes.
The present investigation considers thermal radiation heat transfer under vacuum conditions, where the radiating and absorbing surfaces are Inconel 718. The purpose of this research was to develop highly precise data of the total hemispherical emissivity of oxidized Inconel 718 for application to the analyses of accident conditions in proton spallation targets under conditions in which the target clad material is simultaneously oxidized and overheated to high temperatures. The crucial parameter in calculating the peak temperatures of such targets during thermal transients is the radiative emissivity of the Inconel 718 clad. Radiation heat transfer calculations can only be as accurate and precise as the values of the radiative emissivity that go into the calculations; using emissivities which are either too low or too high will drive the calculated peak temperatures to extremes which renders parametric calculations of little value. Best-estimate accident calculations require precise and accurate thermal radiative emissivity data. A search of the literature failed to find any previous measurements of the radiation properties of this material precise enough for the present application [1], [2], [3]. Of particular interest are the effects of surface oxidation and temperature on the emissivity. Therefore, the present investigation was conducted, the purpose of which was the measurement of total hemispherical emissivities over a range of temperatures (200°C⩽T⩽1000°C) and a variety of surface oxidation conditions.
The above discussion can be illustrated by reference to Fig. 1 from [3]. In the figure, the total hemispherical emissivities under a variety of surface and temperature conditions are measured. It is seen first of all that the emissivities increase with temperature under all surface conditions. In addition, curve A whose surface is in the as-rolled condition has the lowest emissivity of the three samples. Curve B which is in the as-rolled condition but which has been oxidized in air at a temperature of 815°C for 15 min has a significantly larger emissivity than curve A. Finally, curve C which has been sand blasted to increase its surface roughness and also oxidized has the largest emissivities of all three samples. That these differences are significant is illustrated by the figure where at 400°C, the thermal radiation emitted by surface C is more than 2.5 times the radiation of surface A.
Section snippets
Theory
Fig. 2 illustrates the basic theory of the experimental apparatus. If A1 is the experimental sample and A2 represents the surroundings, then the radiosity J is related to the blackbody emissive power Eb and the irradiation of surface A1 by G as follows:Since ρ1=(1−α1) and if the surface A1 is opaque and the system is either gray or close enough to thermal equilibrium so that α=ϵ, then Eq. (1) becomesHowever, the net heat flow from surface A1
Test sample preparation and experimental apparatus
Samples were prepared from Inconel 718 alloy material by successively rolling and annealing bar stock to a material thickness of 0.005 in. (0.127 mm). Strips of this material were cut to a rough size of 6 in. (152.4 mm) long by 0.125 in. (3.175 mm) wide using tin snips. A fixture was designed and fabricated to allow the long edges of the samples to be surface ground parallel and to a uniform width. Final strip widths were nominally 0.105 in. (2.667 mm). Sample thickness and width dimensions
Measurement errors
Eq. (7b), the operative equation of the apparatus, can be utilized to approximately predict the experimental errors of the emissivity measurements. Both Q1−2 and A1 are implicit functions of other variables as shown in Eq. (10). The RMS uncertainties of the measured emissivities can be computed directly from this equation. The measurements of ΔV and i were the result of measurements of AC voltage with a Hewlett Packard 3455A digital voltmeter; the unit is a five-digit precision voltmeter with
Experimental results and discussion
As previously mentioned, the emissivity samples were Inconel 718. Table 3 lists the constituents for this particular alloy. Ten experimental runs were made on Inconel 718 samples under various surface oxidation and surface temperature conditions. Table 4 lists these temperature and oxidation conditions for nine of the runs ( and and to ). Run number was for an unoxidized sample in an as-rolled condition from the manufacturer. Run number was an oxidation in a CO2 atmosphere
Practical significance
The performance of systems at elevated temperatures frequently relies on the ability of thermal radiative heat transfer to reject heat from the system when other heat transfer mechanisms become ineffective. The system's ability to function normally at elevated temperatures or to recover from transient off-normal conditions will usually depend upon not exceeding some threshold temperature, usually determined by some material property or other failure criteria. This can be the case for materials
Conclusions
Total hemispherical emissivities were measured for Inconel 718 under a variety of surface temperatures and surface oxidation conditions. The following results were obtained:
- 1.
Unoxidized Inconel 718 in an as-received condition had emissivities that increased from approximately 0.24–0.33 over a surface temperature range of 200–1000°C.
- 2.
When the surface was oxidized in air at atmospheric pressure with temperatures of 1000°C, 1100°C and 1142°C, the emissivity increased slightly with temperature in the
Recommendations and future research needs
This study only considered the thermal radiative emissivities of pre-oxidized Inconel 718 at temperatures up to 1000°C. It is the expectation of the authors that other Inconel alloys will perform qualitatively similarly if pre-oxidized in air, however, additional studies with other alloys (i.e., Inconels and stainless steels) would be useful to expand the available database and to confirm the present trends. In addition, the data reported in this paper were truncated at 1000°C due to the
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Equipment and procedures for evaluation of total hemispherical emittance
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Under contract number DE-AC02-98CH10886 with the United States Department of Energy.