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UV laser scanner measures turbofan thrust

20 Jun 2023

Collaboration between Pratt & Whitney and Virginia Tech researchers said to represent 'major step forward' in engine metrology.

Jet engine maker Pratt & Whitney says that its long-standing research collaboration with Virginia Tech has resulted in a new technology for calculating engine thrust, using ultraviolet lasers and Rayleigh scattering.

Revealing details of the work during this week's Paris Air Show, the partners hailed the result as a “major step forward” in engine instrumentation technology, enabling high-fidelity measurement of key gas turbine engine parameters including velocity, temperature, and density.

They refer to the patent-pending technique as Filtered Rayleigh Scattering for Thrust measurement (FRST), saying it offers significant advantages over traditional sensors and probes.

It is expected to help with the development of more efficient engine core technologies, and could also be used for in-flight measurement of non-CO2 particulate emissions.

UV scattering
Geoff Hunt, Pratt & Whitney’s senior VP of engineering and technology, commented: “The ability to use lasers and optical sensors represents a major step forward in engine instrumentation technology.

“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.”

The technique operates in the ultraviolet region, using Rayleigh scattering from gas molecules in the air passing over a laser-illuminated area.

“Applied to a turbofan engine, the light is provided by a laser beam directed across the turbine's gas path, while the resulting scatter is recorded by a high-performance camera and filtered for corruptions in the signal,” explains the engine firm.

It adds that FRST could potentially eliminate the need for traditional sensors and probes used to calculate engine thrust, which can be difficult to install and cause flow blockage - particularly on smaller engine cores where space is limited.

Contrail mitigation
The ability to measure particulate emissions could also contribute to industry-wide efforts to better understand and mitigate their environmental impact - perhaps even reducing the generation of contrails in the future.

Virginia Tech professor of aerospace engineering Todd Lowe said in a release that the collaborative effort had harnessed recent advancements in computing power, lasers, and camera technology and applied it to the well-known technique of Rayleigh scattering to demonstrate the first successful application on a turbofan engine at a Virginia Tech test stand.

“As we work towards in-flight demonstrations of FRST, we expect the technology will have other applications in the development and certification of aircraft engines,” he added.

At the American Institute of Aeronautics and Astronautics (AIAA) AVIATION 2023 Forum, held in San Diego last week, Lowe was scheduled to present on the feasibility of an FRS system operating with 387 nm lasers - instead of the more typical 532 nm wavelength - and using cesium vapor as s molecular filter.

The non-intrusive and seedless optical measurement technique is said to provide time-averaged, planar measurements of three-component velocity, static temperature, and static density of aerodynamic flows.

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