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AEDC Spark Tank: AEDC engineer proposes new high-temperature measurement, calibration techniques

  • Published
  • By Deidre Moon and Bradley Hicks
  • AEDC Public Affairs

ARNOLD AIR FORCE BASE, Tenn. – To say temperatures produced by some of the test facilities within the Arnold Engineering Development Complex are hot is a bit of an understatement.

Such test cells, like the arc-heated facilities at Arnold Air Force Base, headquarters of AEDC, can produce temperatures up to 3,000 degrees Celsius. That’s hotter than the earth’s thermosphere, the center of a campfire, the surfaces of both Venus and Mercury, and lava.

With the innovation funding received as part of the Arnold Engineering Development Complex 2022 Spark Tank event earlier this year, AEDC technology engineers at Arnold Air Force Base are exploring new techniques in measuring these high temperatures.

 David Plemmons, a senior scientist with the AEDC Technology Innovation Branch, who presented the proposal, explained that the current measurement system is lacking some capabilities but being able to accurately measure these high temperatures is important.

“Hypersonic vehicles that fly through the earth’s atmosphere are heated to extreme temperatures, ranging up to thousands of degrees Fahrenheit,” he said. “These temperatures are generated by the friction of the air flow over the vehicle’s surface. Loosely speaking, the heat generated by the friction is proportional to its speed. The vehicle speed is specified by its Mach number. A Mach number of 1 is the speed of sound. Hypersonic speeds are at or above Mach 5.

“At hypersonic speeds, surface temperatures on flight vehicles exceed the limits of conventional contact temperature sensors such as thermocouples and RTDs [resistance temperature detectors] that must be in physical contact with the surface where the temperature is measured.”

Plemmons mentioned that because of this, hypersonic ground test facilities at Arnold have been using other techniques to measure temperatures on the surface of hypersonic models.

“[An] optical pyrometer works by measuring and analyzing light that is emitted from a heated surface and does not require contact with the surface,” he said.

Plemmons noted that extreme heat can cause a test article, support structure and even the facility itself to undergo thermal expansion, which can cause the model to move.

“This can cause erroneous temperature measurements,” he said. “But using the new technique, we offer a solution to both challenges: non-contact temperature measurement and compensation for model movement. In my opinion, it will be an asset in any high-temperature hypersonic ground test facility.”

According to Plemmons, using the measurement technique will also help increase data accuracy, allowing AEDC to deliver better quality data to its test customers.

“Accurate temperature measurements are critical for thermal protection systems testing,” he said.

Calibrating new high-temperature measurement devices

Because the pyrometers used to measure these high temperatures can only be calibrated up to a temperature of 3,000 degrees Celsius, the blackbodies used for calibrating pyrometers cannot reach those temperatures either.

With temperatures in the high-enthalpy arc heater facilities exceeding this limit, the extension of available technology to meet a high temperature range is also being researched by AEDC team members.

This effort is another one of the eight projects that received funding through the AEDC Spark Tank. Like with the high-temperature technique exploration project, Spark Tank money was awarded through several sources. The high-temperature calibration proposal received AEDC Innovation Grant funding.

With this funding, a proof-of-concept system was developed to demonstrate how an increase in measurable temperatures can be achieved.

The Spark Tank proposal and the proof of concept borne from it centered on the use of a blackbody light source, which is a thermal light source that produces a broad spectrum of white light. The shape and intensity of a blackbody spectrum is temperature dependent.

“Calibrated blackbodies are available that can be operated up to a temperature of 3,000 degrees Celsius,” said Plemmons, who, along with principal investigator Joseph Braker, an optical engineer for AEDC, worked on the proposal. “We test systems at AEDC that exceed these temperatures. These higher temperatures are primarily observed in the arc heater. However, hypersonic testing in general produces elevated temperatures because of the high speeds.”

The ultimate purpose of the Spark Tank project is the development of a synthetic blackbody source that will be used to calibrate the optical pyrometers and allow for measurements above the current 3,000-degree threshold. This artificial high-temperature blackbody can be made by properly combining the output of multiple LED lights at various wavelengths.

The synthetic blackbody will reproduce the spectral shape of a thermal source at temperatures above 3,000 degrees Celsius.

“A blackbody source emits an optical spectrum that is well-defined and accurately modeled by the Planck function,” Braker said, adding the Planck function describes how much of a particular wavelength is emitted at a certain temperature and can be used to find radiance as a function of wavelength for a given temperature. “By using LEDs to generate an optical spectrum that is also defined by the Planck function, we can emulate a blackbody source very precisely without needing to reach comparable temperatures. We can then use this emulated blackbody source to calibrate spectrum-based measurement devices with no theoretical upper bound.”

Plemmons said if the project is brought to fruition, AEDC could gain a new capability that will increase the accuracy and quality of data delivered to test customers.

And while the arc-heated facilities are expected to be the primary user of this capability, Plemmons said it would prove useful in other high-temperature and hypersonic test facilities.

“This will benefit AEDC by reducing uncertainty in temperature measurements in hypersonic test facilities and improving test data quality,” he said.

With the proof-of-concept system now complete, the next step is the production of a prototype that can be implemented in a test facility. Additional funding would be needed to complete this.

“If sufficient funding is obtained, I would expect that a prototype system could be fielded in fiscal year 2023,” Plemmons said.