AEDC hypersonic tests to benefit from improvements to emissivity measurement accuracy Published Dec. 21, 2020 By Jill Pickett AEDC/PA ARNOLD AIR FORCE BASE, Tenn. -- As hypersonic vehicle development heats up, the Arnold Engineering Development Complex Aerothermal Measurements Laboratory (ATML) is preparing to help test engineers know just how hot it is getting. When a vehicle travels at high rates of speed, friction of the air against the vehicle generates heat that can threaten the integrity of the vehicle’s surface. Aerothermal tests in AEDC ground test cells, or wind tunnels, analyze this effect on transonic, supersonic and hypersonic flight systems. “Most high-speed flight vehicle programs have required aerothermal capabilities provided by the AEDC test facilities and the technical services of the ATML,” said Nick Galyen, AEDC program manager for the ATML. “Surface temperature and heat flux are necessary to understand boundary layer transition, especially for hypersonic flight systems and vehicles during test and evaluation, science and technology applications in AEDC Tunnels A, B, C, 16T (transonic), 16S (supersonic) and the Aerodynamic and Propulsion Test Unit,” he said. In order to improve measurement accuracy, a Small Business Innovation Research effort by Advanced Fuel Research is underway to provide the ATML with a new spectral emissometer with enhanced capabilities to replace an older legacy system. The ATML fabricates and installs temperature and heat flux sensors in flight system models, but there are limitations on the number and locations these can be placed. Infrared cameras allow for continuous measurement of the surface temperatures of a model under test, but the infrared camera measurements must be verified first. That’s where the emissometer comes into the picture. “The emissivity of the surface of a material relates to its effectiveness in emitting energy as thermal radiation, both visible radiation and infrared radiation,” said Dr. Robert Howard, AEDC subject matter expert for Instrumentation and Diagnostics. “The emissivity varies from zero, for a shiny mirrored surface, to one for an ideal black surface. For any particular wavelength and temperature, the amount of thermal radiation emitted depends on the emissivity of the object's surface. Thus, the surface temperature can be deduced from a measure of the radiation at a particular spectral region and knowledge of the emissivity. In general, the surface emissivity of materials can change with wavelength, temperature and viewing angle, making it is necessary to characterize the emissivity for materials of interest over a wide range conditions that might be encountered during test programs.” The emissometer bombards a sample with electromagnetic radiation from a blackbody simulator. A true blackbody would perfectly emit radiation of all wavelengths. The radiation from the blackbody simulator excites the atoms of the sample causing it to emit radiation. The optical spectrometer within the emissometer then detects and records the energy emitted from the material at specific wavelengths. Comparing these results to those of an infrared camera allows technicians to verify the accuracy of the camera. “High-temperature surface emissivity data is critical to the transformation of infrared images into temperature maps across a hypersonic test article,” said Dr. Joe Wehrmeyer, AEDC technical advisor for Test Technology. “These temperature maps can subsequently be transformed into surface heat transfer information, all of which is important to characterize a hypersonic vehicle’s thermal protection systems.” The enhanced emissometer will be able to heat samples above 3,000 degrees Rankine, with a stretch goal of achieving 5,000 degrees Rankine. The legacy instrument was limited to 3,000 degrees Rankine or below. It will also be able to take spectral measurements at four different viewing angles as opposed to the legacy instrument which can only take measurements at a single angle. Both variables can influence spectral emissivity and the resulting confidence in the accuracy of temperatures deduced from an infrared camera image. “Spectral directional emissivity can be angle dependent, so the best condition in your test cell is to have your thermal camera mounted to measure at the same angle as used in the emissometer,” Galyen said. “But, for example, if due to clash with other hardware, your thermal camera can only be mounted at a 45-degree angle to the target surface, it is possible that the emissivity value from the original angle is not accurate, thus increasing your temperature measurement error in the test cell.” Adding capabilities is also in the plans for the new instrument. “The new emissometer will maintain the capability to optically determine the sample’s temperature with the additional goal of simultaneous measurements of reflectance and transmittance to decrease measurement time,” Galyen said. “All of these are vast improvements to the older emissometer developed and delivered by Advanced Fuel Research in 1993.” The new emissometer has been designed, parts have been ordered and fabrication of the new system is underway. Delivery of the completed instrument is expected in 12 to 18 months. Keely Beale, who heads up the ATML, is excited for the new capabilities that the technology will bring to the lab and to AEDC.