High-speed CARS in use at AEDC

  • Published
  • By Vicki Peters
  • AEDC/PA
ARNOLD AIR FORCE BASE, TENN. --  As gas-turbine and hypersonic propulsion systems advance, so must the diagnostic measurement systems used in testing them. The Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy System is a development of the Small Business Innovative Research (SBIR) program that may be used to make point source temperature measurements in extreme environments, such as in the flowfield of an operating turbine engine.

CARS simultaneously measures temperature and multiple gas concentrations with a high degree of accuracy. The CARS technique uses three laser beams, one red and two green, and focuses the three beams into one location within a gas flowfield. Whether the hot gas flow is exiting a jet engine, or entering the nozzle of a hypersonic wind tunnel, it typically contains nitrogen and oxygen. These gases are optically "excited" by the focused CARS beams, and the gases create a fourth, new laser beam that originates from the focal point of the original three.

The new beam is weaker than the ones that create it, but by sending this new signal beam into sensitive light detection equipment, the color content, or optical spectrum, of the new beam can be determined. This information can be used to define the amount of nitrogen and oxygen present at the CARS beam focal point, along with the temperature of the gas at that location.

Andrew Alexander, an engineer with the Aerospace Testing Alliance (ATA) Non-Intrusive Instrumentation and Diagnostics Group, is working with the CARS system that was recently delivered to Arnold Engineering Development Complex (AEDC) through the SBIR Program. He explained the CARS operational principal as relying on an optical property of gas molecules called the third-order nonlinear susceptibility, a property that wasn't known to exist until the advent of high power lasers in the 1960s.

Since then, Alexander explains, CARS systems have been used to measure temperatures and gas species concentrations in many research labs. There have been relatively few applications of the CARS measurement technique in aerospace testing and evaluation, mainly because of the harsh testing environments. A potential game-changer is the use of fiber optics to transfer the CARS laser beams from the relative paradise of a temperature-controlled laser lab to a nearby test cell, with its harsh, extreme conditions.

The AEDC picosecond CARS system, delivered by Spectral Energies LLC, enables the use of fiber optics for laser beam transport because of the short duration (100 picoseconds, or 100 trillionths of a second) of its laser pulses. The unique feature of the picosecond CARS technique for fiber delivery is that it requires two orders of magnitude less energy per laser pulse compared to the conventional nanosecond CARS (with a pulse duration of 10 nanoseconds, or 10 billionths of a second) and this energy reduction should allow solid core fiber optics to carry the laser pulses without suffering optical damage.

AEDC also received a nanosecond-based CARS system developed by OptoKnowledge Systems, Inc. This system uses pulsed-laser propagation through hollow-core fibers. Use of the hollow-core fiber allows CARS to be remotely located from the test rig, thus reducing the impact of vibration from the high-speed engine.

According to Dr. Joseph Wehrmeyer, an engineer with the ATA Non-Intrusive Instrumentation and Diagnostics Group, the CARS system provides new diagnostic capabilities that enable AEDC to provide test customers with data that address the challenges associated with high-speed reacting and non-reacting flows in gas-turbine systems. These new capabilities are critical for the development and long-term health of high-performance, military propulsion systems as well as commercial systems.

Alexander added that quantitative measurements are critical for validating numerical models of reacting and non-equilibrium phenomena affecting modern gas-turbine and hypersonic propulsion systems. Such experimental and numerical tools are extremely valuable in the analysis of gas-turbine combustors, as well as applications with limited optical access such as internal combustion engines and stationary power generation systems.

Initial use of AEDC's picosecond CARS system is planned for the J85 test rig at the University of Tennessee Space Institute (UTSI) Propulsion Research Center.

Availability of advanced diagnostics is crucial to the AEDC mission and it is vital that these technologies continue to be explored and developed. With decreasing defense budgets, AEDC is leveraging external funding sources whenever possible to meet these mission critical technology development needs.

The SBIR program is a funding source that serves a dual purpose explained Will Mallory, AEDC SBIR program manager. Mallory said one purpose is to develop innovative solutions and technologies for the warfighter and the other is to develop a commercial product that is beneficial to small business.

AEDC solicits topics internally once a year and works hard to identify capability gaps that could be filled with technologies developed under this program. Many current and future mission critical needs have been addressed through this program that would have not been addressed otherwise due to budget constraints.