Spectral, Directional Emittance at Elevated Temperatures for Various Materials

Abstract

Material samples of stainless steel and titanium with different surface roughness values were investigated to determine their spectral directional emissivity at elevated temperatures up to 1,200 K in the visible to near infrared wavelength range (VIS/NIR) and in the infrared (IR) around 4 m. This work supports experiments for the investigation of transition from laminar to turbulent flow and the resulting heat flux augmentation with particular respect to isolated and distributed surface roughness in the Ballistic Range Facility at NASA Ames Research Center. The measurements of spectral directional surface emissivity were accomplished by measuring the radiation from an appropriately designed test specimen in a furnace heating facility with optical access. The blackbody radiation, needed to determine the emissivity, was generated through a cavity in the specimen itself. During the time of measurement, the sample was shielded from the furnace radiation through a retractable cold radiation shield. The potential effect of sample emission reflected from this radiation shield was investigated in a separate setup using Joule heated samples. No significant influence of this irradiation on the measured sample emission was seen so far. In the VIS/NIR, a 500 mm focal length spectrometer was used for detection. The specimen surface was imaged on the spectrometer entrance slit enabling the simultaneous measurement of a sample normal to the viewing direction, the blackbody cavity, and a tilted sample. In the IR, a FLIR camera with a band pass filer transmitting between 3.8 and 4 m was used to image the tests specimen. In general, surface roughness was found to increase emissivity significantly in comparison to polished samples of the same material. The actual roughness values seem to have only a minor effect. The measured emissivities show only a weak variation with temperature.