Filtered Rayleigh Scattering for Thrust (FRST) measurement was successfully demonstrated on a test engine at the Pratt & Whitney Center of Excellence at Virginia Tech. (Image: Pratt & Whitney)

In an application first, the physics of why the sky is blue is used to measure gas flows without obstructive sensors. A longstanding industry partnership between Virginia Polytechnic Institute and State University (Virginia Tech) and Pratt & Whitney has resulted in a new laser-optical technology that aims to revolutionize in-flight thrust measurement.

The pioneering technology calculates thrust using lasers to enable high fidelity measurement of key gas turbine engine parameters including velocity, temperature, and density. Known as Filtered Rayleigh Scattering for Thrust measurement (FRST), this new optical instrumentation technique offers significant advantages compared with traditional sensors and probes, which will support the development of more efficient engine core technologies and could enable the measurement of non-carbon dioxide particulate emissions in flight.

Leveraging FRST, researchers at the Pratt & Whitney Center of Excellence at Virginia Tech, directed by Virginia Tech Professor Todd Lowe, successfully measured engine thrust on a research engine in a test stand, recording similar accuracy to that of traditional sensors and probes. The teams are working toward flight testing the technology.

Long Known Physics Intersect With New Photonics Technologies

The key measurement principle relies on the physics of molecular Rayleigh scattering. “Though the principle of Rayleigh scattering has been known for over a century, Pratt & Whitney and Virginia Tech engineers have harnessed recent advancements in computing power, laser, and camera technology to demonstrate the first successful application on a turbofan engine,” said Lowe, professor in the Kevin T. Crofton Department of Aerospace and Ocean Engineering. “As we work toward in-flight demonstrations of FRST, we expect the technology will have other applications in the development and certification of aircraft engines.”

When photons pass near gas molecules, several types of interactions can occur. Rayleigh scattering is one such interaction, classified as “elastic” in nature since the photons scattered by the interacting molecule have the same energy as the incident photons of the light source. This interaction has several fascinating properties. One is that scattering is more efficient at shorter wavelengths, explaining why the sky is blue rather than green or red when air molecules scatter sunlight in the atmosphere. Importantly, in this application, Rayleigh scattering encodes the gas properties into the scattered light spectrum. This includes light frequency shifts explained by the Doppler effect due to both thermal motion and bulk motion of molecules. Interpreting these frequency shifts can be done using advanced spectroscopic techniques to convert the spectral content of scattered light signals to quantitative gas flow measurements.

In the most basic filtered Rayleigh scattering implementation, a laser is projected into a region of interest in the gas, and one or more cameras are used to image the light scattered by molecules in the flow. The cameras peer through optical notch filters that provide spectroscopic sensitivity to the features of the Rayleigh scattering spectrum. These filters also serve another function, filtering out corruptions from the laser scattering off engine or aircraft surfaces. Only recently has optical technology enabled this measurement technique to transition from highly controlled laboratory settings to industrial applications. Two technological advances have been critical for seeing this technology mature. Perhaps most important is the emergence of extremely high dynamic range scientific CMOS cameras. This technology allows sensing of very faint scattering signals in the presence of other undesirable light sources. The second advancement has been the availability of high power, tunable, single-line-mode lasers.

Students Rose Stanphill and Eszter Anna Varga in Prof. Todd Lowe’s junior level Aerothermodynamics and Propulsion course inspect the PW6000 engine that was donated to the university by Pratt & Whitney in 2022. (Image: Jama Green for Virginia Tech)

FRST leverages these recent technological advances, integrating the fundamental physics of the filtered Rayleigh scattering measurement principles with the requirements for gas turbine engine performance measurements. Spectroscopy is used at every pixel in the camera to measure the flow properties encoded in the laser light scattered by the gas. The approach takes advantage of a tunable UV laser in the region of the spectrum where molecules will scatter more light compared with visible wavelengths. The measured light spectrum is post-processed, taking into account the physics of Rayleigh scattering to return flow properties.

The implications for gas turbine measurements are profound. FRST optical instrumentation potentially eliminates the need for traditional sensors and probes, which can be difficult to install and cause flow blockage, particularly on smaller engine cores where space is limited. Historically, performance measurements are made by placing physical instrumentation hardware in high-speed, sometimes very hot, streams of the gas flow. This physical instrumentation is laboriously designed at great expense to ensure structural integrity, minimal impact on the gas turbine system, and measurement accuracy. Even with all this effort, conventional sensors and probes are known to create flow disturbances that change the performance of the engine and return results at a small number of points. FRST instrumentation in contrast returns planes of flow data with 100,000 or a million data points in a plane and does so with no hardware in the gas path.

A schematic depicting a typical Filtered Rayleigh scattering system’s optical components and arrangement. (Image: MetroLaser, Inc.)

The Future of FRST is Bright

“The ability to use lasers and optical sensors represents a major step forward in engine instrumentation technology and is a testament to the longstanding collaboration within Pratt & Whitney’s Center of Excellence at Virginia Tech,” said Geoff Hunt, senior vice president of engineering and technology at Pratt & Whitney. Hunt continues, “FRST provides a less intrusive and more cost-effective method for measuring a range of engine metrics. We see exciting potential for FRST to help advance gas turbine propulsion technologies, particularly involving smaller and more thermally efficient engine cores, which are key to our next generation military and sustainable commercial engines.”

FRST applications are not limited to thrust measurements. FRST presents opportunities to measure non-carbon dioxide particulate emissions, which could contribute to industry wide efforts to understand and mitigate the environmental impact of those emissions, particularly with regard to contrail formation. FRST could also be an enabling technology in the development process of next generation aircraft concepts. In many of the proposed advanced aircraft concepts, the aerodynamics of the aircraft and propulsion engine are essential.

Pratt & Whitney and Virginia Tech have a longstanding collaboration in propulsion technology development with a focus on advanced instrumentation. The collaboration enables multiple graduate-level projects and internships at Virginia Tech and Pratt & Whitney, and research developed at the center is often transitioned to practice, directly impacting Pratt & Whitney products. The recently established English-2-Engineering undergraduate program, housed under the center and focused on sustainability in aerospace propulsion, is another example of the joint research between P&W and VT.

This article was written by Todd Lowe, Professor of Aerospace & Ocean Engineering, Virginia Tech. For more information, visit here .